ADVANCES IN ION CHROMATOGRAPHY
FOR ENVIRONMENTAL APPLICATIONS
by
RIDA SADEK AL-HORR, B.S.
A DISSERTATION
IN
CHEMISTRY
Submitted to the Graduate Faculty of Texas Tech University in Partial Fulfillment of the Requirements for the Degree of
DOCTOR OF PHILOSOPHY
Approved
Chairperson of the Committee
Accepted
Dean of the Graduate School
August, 2003 ACKNOWLEDGMENTS
I would like to express my deep appreciation to my research advisor Pumendu K.
Dasgupta, P. W. Horn Professor of Chemistry. It is due to his enlightened guidance and strong support £uid encotiragement that I was able to present this work. I also would like to thank Dr. Carol Korzeniewski and Dr. John N. Marx for their assistance and valuable comments throughout my graduate studies.
I would like to acknowledge Jianzhong Li, Gautam Samanta, Charles B. Boring,
Genfa Zhang, Rahmat S. Ullah, Kevin Morris and all other research group members for their assistance in various aspects of this work.
I owe a lot to my brother Hadi Al-Horr for his help and support, and I am especially thankful to my family members for their inspiration and motivation. I also thank my fiance Yasmin Soussan for her patience and understanding. TABLE OF CONTENTS
ACKNOWLEDGEMETS ii
ABSTRACT iv
LIST OF TABLES vi
LIST OF FIGURES vii
LIST OF ABBREVIATIONS xi
CHAPTER
I INTRODUCTION 1
II TWO-DIMENSIONAL CONDUCTOMETRIC DETECTION IN ION CHROMATOGRAPHY: SEQUENTIAL SUPPRESSED AND SINGLE COLUMN DETECTION WITH PASSIVE HYDROXIDE INTRODUCTION 18
III FIELD MEASUREMENT OF ACID GASES SOLUBLE ANIONS IN ATMOSPHERIC PARTICULATE MATTER USING A PARALLEL PLATE WET DENUDER AND AN ALTERNATING FILTER-BASED AUTOMATED ANALYSIS SYSTEM 53
IV A CONTINUOUS ANALYZER FOR SOLUBLE ANIONIC CONSTITUENTS AND AMMONIUM IN ATMOSPHERIC PARTICULATE MATTER 97
V SEMI-CONTINUOUS MEASUREMENT OF MAJOR INORGANIC SOLUBLE GASEOUS AND PARTICULATE CONSTITUENTS IN SEVERAL MAJOR US CITIES 132
VI SUMMARY AND CONCLUSIONS 184
ni ABSTRACT
Ion cliromatography (IC) is a widely used analytical tool for the determination of many ionic species. Applications of ion chromatography extend over a wide range of chemical analyses. Introduction of eluent suppression in the mid-1970s extended the botmdaries of conductometric detection into trace analysis. Ctirrent state-of-the-art IC systems require only water to operate, exhibit excellent reliabilities, and provide the ability of sample preconcentration and simultaneous multiple ion measurement, making them attractive for atmospheric analysis.
Atmospheric particulate matter (PM) contains many inorganic and organic soluble ions. A number of those are weak acid anions that are largely undetectable in suppressed ion chromatography. An improved method that uses sequential suppressed and unsuppressed IC for the sensitive detection of both common anions and very weak acid anions has been investigated. After suppressed conductometric detection, the effluent is passed into a membrane device where KOH is passively introduced into the eluent stream using Donnan forbidden leakage.
High temporal resolution measurement of atmospheric gases and constituents of atmospheric particulate matter (PM) is important to understand the chemistry and sources of atmospheric pollution. New continuous collection devices coupled with IC systems for fully automated measurement of soluble inorganic gases and soluble ionic constituents of atmospheric PM have been developed. Soluble gas collection is accomplished with a parallel plate wet denuder (PPWD).
iv For particle collection, an automated alternating filter-based system was initially developed. This system uses two glass-fiber filters that alternate between sampling, and washing and drying. More recently, a continuous soluble particle collector (PC) of simpler design has been developed; this device does not use steam. Preceded by a denuder and interfaced with an ion chromatograph, this compact collector permits automated collection and continuous extraction of soluble anions and ammonium ion in atmospheric particulate matter. The systems have been deployed in a number of major field studies held in urban and suburban locations in the United States. LIST OF TABLES
3.1 Fotir states of the instmment, programmed chromatograph TTL outputs and outputs of Integrated Circuit Chips UI and U2 85
3.2 Average anion composition of day and night fime aerosol in midtown Atlanta, August, 1999 86
3.3 Organic anion composition of aerosol filter samples collect in Houston, TX, 2000 and Philadelphia, PA, 2001 and identified by IC-MS 87
4.1 Count median diameter, mass median diameter and mass median aerodynamic diameter of particle generated by VOAG with
different feed (NH4)2S04 solution doped with fluorescein 121
4.2 Loss of aerosols in the PPWD and the air-inlet nozzle of the PC 122
5.1 Sampling locations and available measurements 157
5.2 Day and night correlafion of NO3, N02', HONO, and HNO3 measured in four cities 15 8
VI LIST OF FIGURES
1.1 Schemafic of electrolytic suppressor mechanism 17
2.1 Theoretical response plots 40
2.2 Cassidy plot of response sensitivity in linear axes 41
2.3 Experimental system 42
2.4 Base introduction device designs 43
2.5 Current efficiencies observed with electrodialytic devices with different membranes 44
2.6 Background noise in electrodialytic devices with different membranes 45
2.7 Passive Dorman leakage of KOH through various sheet membranes as a function of feed KOH concentration 46
2.8 Donnan leakage of different alkali hydroxides through the RAI PTFE membrane 47
2.9 Dependence of Donnan leakage on tubular membrane dimensions 48
2.10 Detection of 0.6 |J.M borate in a sample mixture on the second detector 49
2.11 Second detector response to various analytes 50
2.12 2D ion chromatogram under standard conditions 51
2.13 2D ion chromatogram of an air filter sample extract 52
3.1 Wetted denuder shovra schematically 88
3.2 Particle collection system 89
3.3 Particle system set up 90
3.4 Schemafic ofelectronics governing instrument operation 91
VII 3.5 HN03/Nitrate, HONO/Nitrite and S02/Sulfate patterns at a midtown location in Atlanta, GA 92
3.6 HCl/Chloride, Oxalic acid/Oxalate levels at a heavily industrialized site close to the shipping chaimel in Houston, TX 93
3.7 Representative chromatograms 94
3.8 Gradient ion chromatogram of an aerosol collected during the Atlanta experiment 95
3.9 Log ?R versus log [eluent] plots 96
4.1 Particle collector 123
4.2 Field sampling and airflow schematic 124
4.3 Total particle collection/analysis system 125
4.4 Penetration curve of standard size polystyrene beads in the particle collector with a cyclone-style inlet 126
4.5 Representative system output 127
4.6 Integrated sulfate measurements versus sulfate measured by present instrtiment 128
4.7 Sulfate and nitrate concentrations 129
4.8 HCI and particulate Nitrate patterns in Tampa, FL 130
4.9 Sulfate/Ammonium equivalent ratio with sulfate and ammonium equivalent concentration patterns, Tampa, FL 131
5.1 Average minimum and maximum concentration of soluble ions in particulate matter measured in four studies 159
52 Average minimum and maximtim concentration of soluble acid
gases and ammonia measured in three studies 160
5.3 Deployment location at HRM 3 161
5.4 Sulfate/Sulfur dioxide measured patterns in Philadelphia, PA 162
vni 5.5 Sulfate/Sulfur dioxide measured patterns in Houston, TX 163
5.6 Sulfate/Sulfur dioxide measured patterns in Tampa, FL 164
5.7 Sulfate measured patterns in Lindon, UT 165
5.8 Pattern of HNO3 and HONO in Philadelphia 166
5.9 Pattern ofN02'and NO3" in Philadelphia, PA 167
5.10 Pattern of HONO and HNO3 in Houston, TX 168
5.11 Pattern of NO2" and NOB" in Houston, TX 169
5.12 Pattern of HNO3 and NO3" in Tampa, FL 170
5.13 Pattern of HONO and NO2" in Tampa, FL 171
5.14 PattemofN03' and NO2" in Lindon, UT 172
5.15 SO2, S04^", HNO3, and N0{ patterns in Philadelphia: July 10-July 11,2001 173
5.16 8O2, 804^", HNO3, and NO3 patterns in Philadelphia: July 17-July 18,2001 174
5.17 SO2, S04^', HNO3, and NO3 patterns in Philadelphia: July 21-July 26, 2001 175
5.18 Wind direction and solar radiation in Philadelphia during high PM
and trace gases episodes 176
5.19 HCI, HNO3 and NOi patterns in Tampa, FL 177
5.20 HCI, CI", and relafive humidity patterns in Tampa , FL 178
5.21 Total anion equivalents, equivalent NH4'^, and NH3 concentration in Philadelphia, PA 179 5.22 Total anion equivalents, equivalent NH4"', and NH3 concentration in Houston, TX 180
5.23 Total anion equivalents, equivalent NH4"^, and NH3 concentration in Tampa, FL 181
IX 5.24 Equivalent ammonium versus equivalent sulfate in Tampa, FL 182
5.25 Total anion equivalents, equivalent NH4'^, and NH3 concentration in Lindon, UT 183 LIST OF ABBREVIATIONS
a.c alternating current
A Ampere cm centimeter
CC concentrator column °c degree Celsius
DPM digit panel meter d.c direct current
FTF fiber trap filter
FFAH filament filled annular helical
FPD flame photometric detector
FV flame volatilization
ft feet
GF glass fiber
H height
Hz hertz
HPLC high performance liquid chromatography
hr hour
in. inch
i.d. irmer diameter
IC ion chromatography
XI Kg kilogram
L length
LOD limit of detection
LC liquid chromatography
MFC mass flow controller
MS mass spectrometry m meter
MENG microelectrodialytic NaOH generator
Heq microequivalent
^tg/m^ microgram pre cubic meter
|j,L microliter
}im micrometer
[M micromolar
^S micro Siemen mA milliampere mL milliliter mm millimeter mM millimolar min minute nL nanoliter nm nanometer o.d. outer diameter
xu PPWD parallel plate wetted denuder
PC particle collector
PCS particle collection system ppb part per billion ppm part per million ppt part per trillion
Wi/2 peak half-width
PFA perfluoroalkoxy Teflon
Pg picogram
PEEK polyether ether ketone
PVC polyvinyl chloride
PVDF polyvinylidine fluoride
RE relative humidity
RSD relative standard deviation
^R retention time
S second
S/N signal-to-noise ratio
SLPM standard liters per minute
PTFE Teflon
TTL transistor transistor logic
2DIC two-dimensional ion chromati
UV ultraviolet
Xlll V volt
W watt w width
xiv CHAPTER I
INTRODUCTION
Chromatography has become a principal tool for the rapid separation and characterization of many classes of compotmds. Although Brunschwig, a Strasbourg stirgeon, purified ethanol by a chromatographic technique (1512), and Day, an American geochemist, separated crude oil on Fuller's earth (1898-1903), it was the work of Mikhail
Tswett, a Russian botanist, who managed to separate plant pigments, that marked the first systematic study and is recognized as the beginning of chromatography.' These results were first presented as a public lecture in 1903, and this year is thus being celebrated as the centermial year for the separation sciences'' and for chromatography in particular.'*
Chromatography (chromatus = color and graphein = to write) has come a long way since it was first invented by Tswett. Chromatography is a technique for separating a multi-component sample into various purer fractions that are detected downstream with an appropriate detector. Any chromatographic process involves two mutually immiscible phases.^ These are the stationary and the mobile phase. The stationary phase could be solid or liquid attached to an inert support material. The mobile phase, also referred to as the eluent or the carrier, is the solvent that flows through the stationary phase. The mobile phase, which could be liquid or gas, mobilizes the sample through the stationary phase in a process known as migration. Separation occurs because different compounds have different migration rates, which are due to their different affinity for the stationary and the mobile phases. During the migration process each compound is present at equilibrium between the mobile and the stationary phase. The slower the migration rate of a compoimd, the higher the fraction of that compound present in the stationary phase, and vice-versa.
The original chromatographic system, now referred to as classical column chromatography, was a glass coltimn containing a packing of fine particles in which the solvent or the mobile phase flowed by gravity.^ Though this kind of chromatography is extremely flexible in that many different combinations of packing and solvents can be used, it is tedious with poor reproducibility, rendering it impractical for most of today's analyses. However, it is still practical for large scale purification of many organic substances, especially for mixtures produced in developing organic synthetic methodology and in purifying many biomolecules.
Since then the practice of chromatography has experienced many changes and improvements. The advent of paper chromatography in the 1940's and thin-layer chromatography (TLC) in the 1950s greatly simplified the practice of analytical liquid chromatography. Today column chromatography routinely produces faster separation and better resolution than TLC. Column chromatography can be divided into gas chromatography (GC), liquid chromatography (LC), and supercritical fluid chromatography (SFC) to reflect the physical state of the mobile phase.'
Modem liquid chromatography is typically operated at high pressure, several thousand psi.^ It is refen-ed to as high-pressure liquid chromatography or high performance liquid chromatography (HPLC). LC embraces several distinct types of
interaction between the liquid mobile phase and the various stationary phases. When the separation involves predominantiy a simple partition between two immiscible liquid phases, one stationary and one mobile, the process is called liquid-liquid chromatography
(LLC). In liquid-solid chromatography (LSC), also called adsorption chromatography, the retentive ability of the stationary phase is mainly due to its physical surface forces.
Ionic or charged species are usually separated in ion exchange chromatography (IC) by selective exchange with counterions of the stationary phase. Today ion exchange chromatography is practiced in almost every field of science.^
Ctirrent Technology and Svstem Requirements
Ion chromatography is the principal analytical tool used in this research. The general system components are described in this section with more focus on anion exchange chromatography. Modern IC system requirements are in many regards similar to those of an HPLC system. However, there are some components that are unique to IC.
The general components include a high pressure eluent pump, a separator column
(usually preceded by a guard column), a suppressor and finally a detector.
Ptimping and Eluent svstem
A high-pressure (up to 5000 psi) piston pump is used to pump the eluent or, in today's state-of-the-art IC systems, deionized (DI) water through the chromatography system. IC pumps may have single head or dual heads.^ Each head has its own piston and two check valves to control the direction of liquid flow. The pistons are connected to an eccentric cam whose movement controls that of the pistons. Usually all liquid transfer lines and wet system components are made of polyether ether ketone (PEEK). Stainless steel can also be used in non-corrosive environments.
Modern state-of-the-art IC systems require just water to operate. Eluents are electrolytically generated^''^online during the analysis. The process offers substantial benefits to the practice of IC. In addition to the operational simplicity of such a system, it is effective in eliminating carbonate formation in manually prepared hydroxide eluents.
Carbonate is a stronger anion eluent than hydroxide, and its presence in variable concentrations in the eluent can lead to poor separation, reproducibility and detection limits.^ In suppressed conductometric detection it increases backgrotmd levels and generates baseline shifts in gradient separations.
The eluent generator unit is placed after the pump and contains a cartridge of potassium hydroxide (KOH) or methanesulfonic acid (MSA) for anion or cation eluent generation, respectively. The cathode and anode are separated by an ion exchange membrane. For anion chromatography hydroxide is generated at the cathode according to the following reaction:
2H20 + 2e- -> 2 0H- + H2(g) (1.1)
while at the anode the feed solution contains KOH from the cartridge:
2 0H--2e-^ H2O +'/202(g) (1.2) Then K"^ is transferred across the cation exhange membrane to the cathode to form KOH.
The concentration of the eluent produced is changed by simply changing the supplied DC current
Columns of Ion Exchange Resin
The separation of cations and anions on ion exchange resin goes back many years before IC became widely accepted as an analytical tool." Ion exchange resin beads can be made of silica but more commonly of polymers such as polystyrene or polyacrylate.
The polystyrene based exchange resins are made by copolymerizing styrene with a small amotmt of divinylbenzene (DVB) for crosslinking. The amount of DVB added affects the rigidity of the beads. Microporous beads (gel type) are made with up to 25% weight of
DVB while in macroporous resins the % weight of DVB can reach 55%.^ Ion exchangers are made by introducing appropriate ionic functional groups into the polymer.
Most common anion exchangers are made of two substrate types: microporous substrates, which are mainly used as a support for latex coated microbeads; or macroporous substrates.'^ Anion exchangers are usually functionalized with quatemary ammonium groups. The polymeric benzene ring is first chloromethylated followed by a reaction with tertiary amine. Latex agglomerated ion exchangers have also been successfully used for various applications of IC. These ion exchangers are made by electrostatically attaching latex microbeads with an approximate diameter of 0.1 ^im to the surface of a relatively large core substrate (5 -30 ^m). For anion exchangers, the latex particles are fiinctionalized with quatemary ammonium groups while the surface of the core PS-DVB substrate is sulfonated. These resin are chemically and physically stable, provide moderate backpressure £md high chromatographic efficiency.'^ Dionex Corp. has made a variety of latex agglomerated resins to develop IC columns for different applications.''*
Most current cation exchangers are either strong or weak acid exchangers. Strong acid exchangers are functionalized with sulfonic acid groups.'^ Weak acid exchangers are ftmctionalized with carboxylic acid or a mixture of carboxylic and phosphonic acid groups.'^ They are basically used in applications where separation of cations of different charge is desired. Dionex Corp. has made several cation exchangers by coating their latex coated anion exchange resins described before with a second layer of sulfonated latex particles. The acidic cation exchange latex particles are attached to the aminated latex particles underneath, which are attached to the surface of a sulfonated bead.
Suppression
Introduced in 1975 by Small et al.,'^ suppression is a pre-detection step that eliminates the background eluent conductivity contribution in addition to enhancing the conductance of the analyte ion (for all but very weakly acidic analytes). As a result both sensitivity and detection limits are improved. After separation the column effluent passes through a suppressor where Na"" or K"" fromth e eluent is exchanged with H"", thus neutralizing the eluent hydroxide and changing the analyte from the Na^ or K^ salt form to the more conducting acid form. Early suppressors were simply columns of cation exchange resins that required frequent offline regeneration and caused considerable peak dispersion and broadening. Since then, the technique has passed through several refinements. In 1981 fiber suppressors were introduced '^ followed by flat membrane suppressors in 1985.'^ Basically, an ion exchange membrane was used with a constant flow of a regenerant solution. Though the devices did not require offline regeneration, they consumed a relatively large voltime of the regenerant solution. In 1989 Strong and
Dasgupta introduced the electrodialytic suppressor. Based on the same principle, in
1992 Dionex Corp introduced the Self Regenerating Suppressor (SRS).'^' Figure 1.1 shows a schematic of the mechanism of an anion SRS suppressor. Basically the SRS is composed of a cathode and an anode separated by two cation exchange membranes, thus, forming three compartments for liquid flow. The column effluent containing the eluent and eluite flows in the middle chatmel between the membranes. At the anode side, water flows between the anode and the membrane generating hydrogen ion and oxygen
Anode: 2H2O - 46" ^ 4H^ + 202(g) (1-3)
the hydrogen ions permeate through the membrane into the middle channel and replace the eluent cation (example: Na"" or K""), thus neutralizing OH" and changing the analyte from the salt to the acid form, which is then measured by conductivity in a neutral medium. The eluent cation (K^) permeates through the other cation exchange membrane into the cathode. Water flowing between the cathode and the membrane generates hydrogen gas and hydroxide ion (1.1)
Detection While developing ion exchange resins is important for the practice of ion chromatography, it is the development of appropriate detection techniquesthat has led to the rapid evolution of IC. Several detection techniques are currentiy used with IC, most commonly suppressed conductivity, UV-Vis absorption, pulsed amperometry and mass spectrometry. Suppressed conductivity is by far the most widely used detection technique associated with IC. Conductometric detection offers several characteristics that are particularly attractive for IC analysis. Conductivity is a universal characteristic of all ions, and the technique is simple and non destmctive.
For a strong acid passing through a conductivity detector, the signal Gis ()^S/cm) at any point in the eluite band is directly proportional to eluite concentration C (in Molar)
^^ according to
G,s=1000C(^H + ^x) (1.4)
where A,H and A,H are the equivalent conductances of H"^ and X', respectively. In the case of a weak acid, the conductivity signal Giw depends on the dissociation constant K of the acid.
Giw=1000C'(?LH + ^x) (1.5) where C is the concentration of X, the dissociated fraction of HX, approximated by solving the quadratic equation
K = XV(C-X). (1.6)
Hence
C'=0.5(-K+(K' + 4KC)"0l/2\ . (1.7)
the expression for C is an approximation that does not apply at very dilute conditions or in cases where K is very low, since at these conditions the dissociation of HX is affected by traces of acid present in the background suppressor effluent. Chapter II elaborates more on detection of weak acid anions.
Research Presented in this Dissertation
The overall objective of the research presented in this dissertation is to fabricate a fully automated system for the collection and sensitive analysis of soluble gases and soluble ionic constituents of atmospheric particulate matter (PM) with high temporal resolution. Such meastirement is substantially powerftal in that it can provide chemical and physical differentiation and correlate tropospheric conditions with gas particle chemical and physical interaction.^^'^"'^^ PM constitute a wide range of different kinds of particles that vary widely in chemical composition, size and toxicity. Ion chromatography provides a convenient analytical tool for measuring ionic constituents of
PM along with their soluble precursor gases. However, many constituents of PM are weak acid anions that are not detectable by suppressed IC. Chapter II describes an improved method for the conductometric detection of both common anions and very weak acid anions. Then in Chapters III and IV, fully automated systems for the collection and measurement of soluble PM constituents and gases are described. The resuhs of field meastirement in several U.S. cities are presented in Chapter V. Finally, Chapter VI emphasizes the significance of this work and presents conclusions and future directions.
The contents of Chapters II and III have been published.^^'^^ The contents of Chapter IV has been submitted for publication. The contents of Chapter V are being prepared for submission to a suitable journal.
Two-Dimensional Detection in Ion Chromatography: Sequential Conductometry after Suppression and Passive Hydroxide Introduction
An improved method that uses sequential suppressed and non-suppressed IC for the sensitive detection of both common anions and very weak acid anions is described.
After suppressed conductometric detection of an electrolytically generated hydroxide eluent and an electrolytic suppressor, the eluent is passed into a membrane device where potassium hydroxide (KOH) is passively introduced into the eluent stream using Donnan forbidden leakage. The conductivity of the stream is then measured by a second conductivity detector. The background conductance of the second detector is typically maintained at a relatively low level of 20-30 i^S/cm. The weak acids are converted to potassium salts that are fiilly ionized and are detected against a low KOH background as
10 negative peaks. The applicability of different commercially available cation exchange membranes was studied. Device configurations investigated include a planar 2-channel device, a tubular device and a filament filled helical (FFH) device. The FFH device provides more effective mixing of the penetrated hydroxide with the eluent stream resulting in a noise level < 7 nS/cm and a band dispersion value of less than 82 |j,L.
Optimal design and performance data are presented.
Meastirement of Acid Gases and Soluble Anions in Atmospheric Particulate Matter using a Parallel Plate Wet Denuder and an Alternating Filter-Based Automated Analysis System
Diffusion based collection of gases is currently the best method to discriminate between the same analyte present in the gas and particle phase. The smallest particle has a diffiision coefficient several thousand times less than that of a gas molecule. Several denuders and denuder designs have been described. ' Throughout this work, a parallel plate wet denuder (PPWD) was used to collect and remove gases.^' The collection efficiency/for a parallel plate denuder is given by
/= 1 - 0.91exp(-2.4wA/s) (1.8)
A = 7xDL/Q (1.9)
where w is the width of the plate, s is the separation between them, D is the diffusion coefficient of the gas, L is the active length of the denuder, and Q is volumetric flow rate.
11 A new fully automated instrument for the measurement of acid gases and soluble anionic constituents of atmospheric particulate matter is presented in Chapter III. The instrtiment operates in two independent parallel charmels. In one channel, a parallel plate wet denuder collects soluble acid gases; these are analyzed by anion chromatography
(IC). In a second chaimel, a cyclone removes large particles and the aerosol stream is then processed by a second wet denuder to remove potentially interfering gases. The
particles are then collected by one of two glass fiber filters, which are alternately
sampled, washed and dried. The washings are preconcentrated and analyzed by IC.
Detection limits of low to subnanogram per cubic meter concentrations of most gaseous
and particulate constituents can be readily attained. The instrument has been extensively
field-tested; some field data are presented. Resuhs for the first attempts to decipher the
total anionic constitution of urban ambient aerosol by IC-MS analysis are also presented.
A Continuous Analyzer for Soluble Anionic Constituents and Ammonium in Atmospheric Particulate Matter
A new continuous soluble particle collector (PC) is described in Chapter IV; this
device does not use steam. Preceded by a denuder and interfaced with an ion
chromatograph, this compact collector (3 in. o.d., ~5 in. total height) permits automated
collection and continuous extraction of soluble anions and ammonium ion in atmospheric
particulate matter. The PC is mounted atop a parallel plate wetted denuder for removal of
soluble gases. The soluble gas denuded air enters the PC through an inlet. One version
of the PC contained an integral cyclone-like inlet. For this device, penetration of
particles as a ftinction of size was characterized. In the simpler design, the sampled air
12 enters the PC through a nozzle and deionized water flows through a capillary tube placed close to the exit side of the nozzle by Venturi action or is forcibly pumped. The resulting water mist attaches to the aerosol, which impacts on a hydrophobic PTFE membrane filter that constitutes the top of the PC and the airfiow exit. Water drops coalesce on the filter and fall below into a purpose-machined cavity equipped with a liquid sensor. The water and the dissolved constituents are aspirated by a pump and pumped onto serial cation and anion preconcentrator columns. Ammonium captured by the cation preconcentrator is eluted with NaOH and is passed across an asymmetric membrane device which allows the ammonia from the alkaline donor stream to diffuse into a deionized water receiver stream flowing countercurrent. The conductivity of the receiver effluent is measured and provides a measure of ammonium. The anions on the anion preconcentrator column are eluted and measured by a fully automated ion chromatography system. The total system thus provides automated semicontinuous meastirement of soluble anions and ammonium. With a 15-min analytical cycle and a sampling rate of 5 L/min, the limit of detection (LOD) for ammonium is 8 ng/m^ and those for sulfate, nitrate and oxalate are <0.1 ng/m^ The system has been extensively field tested.
Semi-Continuous Measurement Of Major Soluble Gaseous And ParticulateConstituents In Several Major Us Cities
The data collected in field measurement campaigns launched at or in the vicinity of three major urban US cities and one suburban area are presented in Chapter V. All of measurements were conducted in the summertime. The chapter focuses on data collected
13 during TexAQS 2000 (Texas Air Quality Study, Houston, TX); NEOPS 2001 (North East
Oxidant and Particle Study, Philadelphia, PA); BRACE 2002 Study (Bay Region
Atmospheric Chemistry Experiment, Tampa, FL); and a measurement campaign in
Lindon, UT, a suburban location, in 2002. Incidents that highlight the importance of continuous analysis in better understanding gas-particle partitioning, heterogeneous chemistry of PM formation, relations between PM growth and precursor gases are investigated. An overview of the observed chemistry at the different sites is also presented.
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30. Dasgupta P. K., ACS Adv. Chem. Ser 232, 1993, 41 -90.
31. Simon, P. K.; Dasgupta, PK.;^i7a/. Chem., 65, 1993, 1134-1139.
32. De Santis, F.; Anal. Chem., 66, 1994, 3503 - 3504.
16 Anode + H2O O2 [H^ 3 Cation Exchange membrane K" + OH" ^ H2O H" OH" - • X" K X" ^ Cation Exchange membrane KOH H2O <- H2
Cathode
Figure 1.1. Schematic of electrolytic suppressor mechanism, X" is the analyte anion
17 CHAPTER II
TWO-DIMENSIONAL CONDUCTOMETRIC DETECTION
IN ION CHROMATOGRAPHY: SEQUENTIAL
SUPPRESSED AND SINGLE COLUMN
DETECTION WITH PASSIVE HYDROXIDE
INTRODUCTION
Introduction
Ion chromatography (IC) continues to play a leading role in many areas of analytical chemistry, with applications that range from trace analysis in semiconductor fabrication to environmental analysis. Small et al.' pioneered the technique of suppressed conductometry in 1975; it is still considered the key feature that distinguishes IC from the liquid chromatographic analysis of ions. The mainstay of IC is in the analysis of anionic analytes and we will therefore confine our attention to this area with the note that identical considerations apply to cation analysis systems.
From a standpoint of detectability, suppression is greatly beneficial in the determination of strong acid anions, and even for anions derived from weak acids, at least up to pKa values of 4. It is integral to the practice of modem IC; detection limits that result from removing the conductive eluent ions and converting the analyte to a highly conducting acid are tmsurpassed by other techniques.
However, weak acid anions are not easily detectable by suppressed IC. Anions derived from acids with pKa>7 are virtually undetectable. Hence, the concept of converting such weakly dissociated acids to more dissociated compounds was developed.
Berglund and Dasgupta published a series of papers in which the weak acid HX was converted by two sequential steps (HX^ NaX -^ NaOH) to NaOH^ or in a simultaneous cation/anion exchange step to LiF.^ The best results were, however, achieved by combining both suppressed and single column IC. Following a conventional suppressed
IC, a controlled amount of NaOH was electrically introduced into the detector effluent by a microelectrodialytic NaOH generator (MENG). With a ~0.1 mM NaOH background, the noise level was 20 nS/cm; the exact band dispersion was not measured. ^ In a subsequent more detailed paper, * the dispersion was measured to be 94 ^L for a device of 15 mm active length. Further developments led to planar MENG devices that exhibited noise levels as good as 8 nS/cm with band dispersions in the range of 78-90
^tL.^
Caliamanis et al. have developed an altogether different approach. A commercial suppressor unit bearing cation exchange membranes and an NaOH-EDTA external bathing solution is used to convert HX to NaX.'°"''' Yuan et al.'"* suggested operating a suppressor in a mode such that the eluent is just short of completely neutralized.
However, it is very difficult to maintain such a system with a constant low-noise environment background.
The work described in this chapter elaborates on previous studies that utilized base introduction after a conventional suppressed IC. It is the added and different dimensionality brought about by the additional detector that makes the overall approach attractive. ^'^ It differs from other work in that passive, rather than electrodialytic base
19 introduction is used, requiring no electronic control. Further, different commercially available membranes have been studied in different physical designs and in different thickness with different bases to determine the optimum conditions so that results as good as the best of the previous electrodialytic base introduction efforts can be realized in a simpler maimer. The recent commercial availability of electrodialytic eluent generators'^ capable of producing highly pure hydroxide eluents, which lead to nearly invariant backgrounds even with gradient elution, makes two-dimensional ion chromatography
(2DIC) more attractive than ever before.
Principles
Analytes elute from a suppressor as an acid HX (when we are concerned with weak acids, even if a given analyte may be multiprotic, consideration of ionization beyond the first proton is tinnecessary).* The suppressed conductometric signal is related to 0.5(A,H+ + ?^x-)((Ka^ + 4CKa)°^ - Ka)) where C and Ka are the eluite concentration and the dissociation constant of HX, respectively, under conditions where autoionization of water can be neglected. For most practical purposes, the presence of frace acids in the background, whether from regenerant leakage in a chemically regenerated suppressor or from omnipresent CO2, is a more meaningful concern than the autoprotolysis of water.
Figure 2.1 depicts the nature of the problem. All of these computations were carried out with the following assumptions: temperature 25°C, monoprotic acid analytes HX (with
Xx- equal to 50 and pKa ranging from strong acid to 10) and the analyte concentrations represented in the abscissae are those at the point of measurement in the detector
20 (injected concentrations would typically be an order of magnitude higher, accounting for typical cliromatographic dispersion). Numerical computations were carried out on the basis of solving the complete charge balance equation for a given system using the nonlinear curve fitting capabiHties of Microsoft Excel Solver with a numerical accuracy of seven significant digits in the computed H^ concentration. '^ Specific analyte concentrations solved were 0.1, 0.3, 1, 3, 10, 30, and 100 |j,M and the lines shovm are spline-fits through these points. Panel a shows the situation for a hypothetical pure water backgrotmd. For clarity, the first three panels are in log-log scales. The minimum ordinate value is 1 nS/cm, slightly below the current state of the art of the noise levels encotmtered in suppressed hydroxide eluent anion chromatography. Realistically 10 nS/cm is the level at which a peak could be detected by a current state-of-the-art system.
In general, at low analyte concentrations, there is little difference from a strong acid down to a pKa of about 5. Past a pKa of 7, the response begins to decrease about 1 log unit with each log unit decrease in Ka. The possibility that acids with pKa >7 can be detected at low concentrations is obviously remote. In reality, when auxiliary acids such as CO2 (in panel b assuming 10 |aM ECO2, 120 ppb total inorganic C, background 0.76 nS/cm; pure water saturated with atmospheric CO2 contains 13-17 |aM i;C02) or H28O4
(in panel c, assuming I ^iM H2SO4, typical minimum leakage from a chemically regenerated suppressor, resulting in a background of 0.86 nS/cm) are present, the detectability of weaker acids deteriorates considerably. In panels b and c, the pKa 10 case disappears from the viewing region, and in fact, it is clear that there is little hope of detecting acids weaker than pKa of 7 even at relatively high concentrations. In addition,
21 the detectability of a weak acid analyte in a real matrix that may contain other, more ionized constituents at higher concentrations, is likely to be far worse if there is any possibility of co-elution. Even when a weak acid analyte elutes on the tail of a stronger acid peak, it may never be seen, both due to the suppression of ionization of the weak acid and due to the intrinsically lower response.
The introduction of a low but constant concentration of a strong base to the effluent from the above conventional suppressed conductometric IC system prior to detection by a second conductivity detector has been proposed previously. ^"^ An analysis of the relative response behavior is noteworthy. Figtire 2. Id shows (in a linear scale) the response behavior of analytes from a strong acid to a pKa of 10 for the 10 ^M SCO2 background, as well as the responses resulting from the second detector upon introduction of 125 ]xM NaOH (no volumetric dilution or dispersion is assumed, the backgrotmd is -25 |j,S/cm, such signals have no significant dependence on whether some weak or strong acids, such as CO2/H2SO4 are present in the background). These signals appear as negative peak responses (which they are). For a strong acid HX with A,x- of 50, the response is 37% in magnitude for the base introduction system relative to that of the conventional suppressed system (increases to 48% for A,x- of 20). For the strong acid case, this represents a 2-3-foId loss of sensitivity and is not attractive. However, the base introduction system shows the same response (within ±3.8%) from a strong acid to an analyte with a pKa of 8, a response comparable in magnitude to the response of an analyte with a pKa of 5 in a suppressed IC system, but with better linearity. With analytes of pKa
>5^ the base introduction response is favored by one order of magnitude with each order
22 of magnitude decrease in Ka. With analytes of acidity weaker than a pKa of 8, the pH afforded by the introduction of 125 ^iM NaOH is insufficient to maintain full ionization.
By the time a pKa of 10 is reached, the sensitivity has decreased to 40% of that for the corresponding case of a strong acid, but it is still four orders of magnitude more sensitive than the corresponding suppressed detection response. Indeed, the response in the second detector to an analyte of pKa 10 is significantiy better than that of an analyte of pKa 6 in the first detector, with much better response linearity.
1 7
The linearity of response is best examined with a Cassidy plot, as shown in
Figure 2.2. It is interesting to note that in the absence of a strong acid in the background, theory predicts that there will be considerable nonlinearity in the response at very low analyte concentrations in the conventional suppressed conductometric detection mode.
This behavior is due to the pliant nature of the baseline, which in the limit is constituted of water, a weakly ionized acid. Appearance of an analyte peak on the baseline causes decreased dissociation of the background constituents, similar to the subsidence of soil upon erecting a stmcture. This was quantitatively probed for carbonate eluents by
Doury-Berthod et al.'^ where a large amount of carbonic acid is present as the
background but at the detection limits possible today, this behavior will be expected at
low analyte concentrations even with pure water as background. The fact that sufficient
strong acid may be present in a real eluent background (even one electrodialytically
generated) can constittite a blessing in disguise in so far as response linearity at low
concentrations is concerned. All responses shown in Figure 2.2 assume a 10 ^M CO2
background, which may be the least contaminated background that can be attained in
23 practice. In the conventional detection mode, the response per unit concentration is initially low due to the CO2 background and also decreases at the high concentration end for all but a strong acid analyte. As a result, analytes of intermediate pKa values, most notably at 4 and 5, show a peak in sensitivity as a function of concentration. The general nonlinearity of response and the drastic decrease in response at analyte pKa values >6 is apparent in this depiction, in marked contrast to the essentially uniform response for the base introduction detection mode, at least up to a pKa value of 8. The latter also shows usable response up to a pKa value of 10.
In the present system, negatively charged hydroxide ions are introduced through a negatively charged cation exchange membrane, Donnan-forbidden ion penetration'^ is the mechanism of base introduction. The relevant parameters are thus (i) the concentration gradient across the membrane (ii) the characteristics of the membrane and (iii) nature of the cotmterion accompanying OH'. The penetration rate of the forbidden ion decreases with increasing size and charge,'^ and introduction of OH" is thus easier than most other anions. The penetration rate is also inversely related to the membrane thickness and directly to the available surface area. These parameters are optimized in this work.
Experimental Section
Figure 2.3 represents the system used in this work. The base introduction device was placed between two conductivity detectors. The system temperature was controlled at all times by placing columns, detector cells, the base introduction device and all connecting tubing in a chromatographic oven.
24 Base Introduction Device
Three different devices designs were investigated (see Figure 2.4). Device A is made up of two Plexiglas blocks, each containing an inscribed channel (0.6 x 0.6 x 40 mm) with 10-32 threaded ports that connect them to the outside. Platinum wires (0.3 x
15 mm) partially fill the channels and exit through additional independent 10-32 threaded ports, as shown. These wires are used as electrodes connected to a constant current source for electrodialytic introduction of base. The cation exchange membrane is placed between the blocks and separates the two fiow channels; bolts hold the blocks together.
Several different cation exchange membranes were investigated. Donor hydroxide solution fiows through one channel while the suppressed effluent from the first conductivity detector Dl flows through the other side to detector cell D2.
The other two designs are based on perfluorosulfonate Nafion® membrane tubing.
Terminal bores of 1.5 mm OD, 0.25 mm bore PTFE tubes were enlarged by drilling.
Nafion tubes, the terminal ends of which are strengthened by PTFE or PEEK tubular inserts, can be put into the end-enlarged PTFE tubes and sealed by standard compression fittings. Each end terminates in a tee such that the donor base solution can be made to flow in a jacket that connects the two tees and surrounds the Nafion tube. Device B uses a 90 mm long Nafion tube in a linear configuration. Two membranes were tested, with respective dry dimensions of 0.35 x 0.525 and 0.30 x 0.40 mm (ID x OD). Device C represents the third design in which a 0.25 mm nylon monofilament filled Nafion tube
(250 X 0.30 ID x 0.40 mm OD) was coiled into a helical stmcture before incorporation
25 into an external jacket, following the design of a filament-filled annular helical (FFAH)
20 suppressor.
All experiments were carried out with a DX-500 ion chromatography system, consisting of a GP-40 gradient pump equipped with a degasser, an LC-30 chromatography oven, an EG-40 eluent generator, and CD-20 and ED-40 conductivity detectors. All connections utilized 0.25 mm polyether ether ketone (PEEK) tubing. For chromatography, Dionex AG 11 and AS 11 guard and separator columns were used. Data collection and analysis utilized PeakNet™ 5.1, all from Dionex Corp. (Sunnyvale, CA).
All experiments were carried out at 30°C with a chromatographic flow rate of 1 mL/min.
All conductance values are corrected to 25 °C assuming a temperature coefficient of
1.7%/°C. Except as stated, the hydroxide flow rate was 0.5 mL/min (observed values were affected at flow rates less than 0.4 mL/min) and 100 mM KOH was used as feed.
Band Dispersion Measurements
Band dispersion was calculated as the square root of the difference between the squares of the band half-widths of the first and second detector response.'^ Band dispersion calculated in this way decreases with increasing band volumes. Dispersion affects sharp narrow peaks more than it affects broad peaks. Therefore, band dispersion was computed on sharp early eluting peaks of 0.25 mM acetate (injection volume 25 ^L,
5 mM KOH eluent).
26 Results and Discussion
Electrodialytic Base Introduction through Different Membranes
Most ion exchange membranes are available in sheet form. Base introduction capabilities were therefore tested with device design A (Figure 2.4a), which allowed both electrodialytic and Donnan-forbidden passive penetration to be tested. Baseline noise was taken to be the standard deviation of the baseline over a 15 min period. Figure 2.5, shows the background conductivities generated with different membranes as a function of the current. Exact Faradaic behavior and a membrane with no zero current leakage will result in a backgrotmd conductance of 27.1 )aS/cm (100 |j,M KOH) for a drive current of
160 [lA. This ideal behavior is shovm as the thick solid line. The behavior of most of the membranes falls into one group and a collective best fit drawn through them is shown as a second line. This exhibits a small background bleed (ca. 1.1 jiS/cm, ~4 [M KOH) and a mean slope that is 78% of theoretical. One membrane, a radiation grafted PTFE cation exchange membrane, falls in a class by itself and exhibits very significant zero current penetration of 16.8 |LiS/cm (over 60 |aM KOH) and a relatively low current dependence of
KOH generation (47% of Faradaic).
The background noise levels observed with the different membranes are obviously of interest since they control the detection limits that could ultimately be attained. Figure 2.6a shows the noise levels observed as a function of background conductance. It is clear that the strong cationic Teflon membrane again falls in a class by itself by providing the lowest background noise. However, since this membrane also exhibits a very high zero current background conductance it is instmctive to look at the
27 noise as a fimction of the electrodialytic drive current; this is shown in Figure 2.6b. In this depiction, the noise appears to be largely independent of the membrane. Rather, it is linearly proportional to the electrodialytic drive current. If microbubbles of electrolytic gas, the amount of which is expected to be proportional to the drive current, is the dominant contributor to the observed noise, then this behavior is understandable.
Whether or not bubbles are specifically involved, the data strongly suggests that the observed noise in the backgrotmd conductance is directly related to the drive current, more than any other factor.
Passive Introduction of Base through Different Membranes
The foregoing experiments suggested that the simpler expedient of passive,
Donnan-forbidden introduction of base to the desired extent (ca. -100 |aM) may not only be possible but may be desirable from a standpoint of background noise. It has been suggested in previous studies'^ that when maintaining a sufficient flow rate prevents buildup on the receiver side, the Donnan penetration rate (A) of the forbidden ion is a quadratic function of the feed concentration (m) as follows:
m^ = aA^ + pA + Y (2.1)
where a and P are positive constants and y is a constant of either sign.
Figure 2.7 shows the observed concentration of KOH in the receiver (as determined from
the conductance) as a ftinction of the feed concentration for several different membranes.
28 The line through the points is the best fit for each case to eqn.2.1 above. The Dow perflurosulfonate ionomer (PFSI) membrane and the thin grafted Teflon membrane both have very high penetration rates and desired degree of Donnan leakage can be achieved with relatively low feed concentrations. The Dow PFSI was an experimental material available in very limited quantity and further work was done with the thin Teflon membrane only.
Dependence of Penetration Rate on the Nature of the Cation
Hydroxides of the alkali metals, LiOH, NaOH, KOH, and CsOH, were used individually as feed solutions and the penetration rates were measured for the thin Teflon membrane. The penetration rates, shown in Figure 2.8, are in the order
LiOH»NaOH>KOH>CsOH and directly reflect the order of the ion exchange affinities of these ions for cation exchange sites, Li"^ being the most easily replaced. This is logical since one would expect that ion exchange sites on the feed side of the membrane to be saturated with the metal ion (both because of its high concentration and high alkalinity) such that the overall rate is likely to be controlled by the rate which the metal ion leaves the membrane on the receiver side. Note that this behavior is opposite to that expected for diffusive transfer through a passive, e.g., a dialysis membrane, because the diffusivity is much lower for the large solvated Li^ ion than the Cs ion.
Regrettably, these series of experiments were performed after most other experiments described in this chapter. It is obvious that for base introduction purposes, it should be preferable to use LiOH, even though KOH was used for most of the
29 experiments in this study. For detection after base introduction, one is interested in maintaining some constant concentration of base introduced. Because LiOH has the lowest equivalent conductance among the alkali hydroxides, it also provides the least background conductance at the same concentration (the conductance due to 100 |LtM
MOH is 23.7, 24.9, 27.2 and 27.6 ^S/cm for M = Li, Na, K and Cs, respectively) and should therefore provide the least conductance noise at the same background base concentration.
Effects of Temperature on Penetration Rate
The effect of temperature was examined for KOH penetration through the thin
Teflon membrane from 25°C to 40°C. The penetration increased from 62.5 \xM to 68.4
I^M, essentially lineariy @ 0.39% /°C.
Effects of Membrane Thickness on Penetration Rate
It is intuitive that penetration rate should increase with decreasing membrane thickness and the data in Figure 2.7 already provide some support towards this.
However, the membrane types differ in that experiment and no clear conclusions can be drawn. The two tubular membranes used for the constmction of device B were identical in length but varied in radial dimensions (525 x 350 vs. 400 x 300 [im in o.d. x i.d., respectively). Compared to the first, the second tube provides a 42% lower extemal surface area but the wall thickness is also 43%) lower. The data presented in Figure 2.9 makes it clear that the wall thickness is by far the dominant factor. A complete
30 understanding of the exact dependence would have required the same membrane in different thicknesses; this was not available. In the above experiment, the decrease in inner diameter increases the flow velocity by 36% at the same volumetric flow rate, this may also have a small effect on increasing the penetration rate by decreasing the stagnant botmdary layer thickness.
Device Performance: Noise and Dispersion
As previously noted, experiments with device A showed passive penetration was superior in terms of noise performance than electrolytic introduction of base. The conductance noise level measured directly at the exit of device A fabricated with the thin
Teflon cation exchange membrane with KOH feed concentration adjusted to produce
-100 i^M KOH in the effluent was 28±2 nS/cm. It was observed also that incorporation of lengths of connecting tubing between the base introduction device and the detector reduces the noise. This suggested that mixing within the device is incomplete.
Incorporation of a 0.75 mm i.d. 750 mm long mixing coil woven in the Serpentine II design^' reduced the noise level to 7 ± 2 nS/cm. However, the band dispersion induced by the device, already at a significant value of 96 ± 8 ixL, increased by a further 55 |iL with the addition of the mixing coil.
Both versions of device B exhibited noise levels similar to that of Device A
(without mixer). However, dispersion in straight open tubes is the highest of all^' and even with the narrower membrane tube, the band dispersion was measured to be 110 ± 4
31 nL (148 ± 6 |nL for larger tube). Incorporation of a mixer to reduce noise will clearly make this even worse.
A logical solution seemed to be the incorporation of base introduction and mixing functions within the same device. The helical geometry is known to induce good mixing while minimizing band dispersion due to the development of secondary flow that is perpendicular to the axial flow. This secondary flow flattens the parabolic profile of the axial flow velocity observed in a linear tube and leads to both reduced axial dispersion and increased radial mixing inside the tube.^''^^ FFAH devices, albeit of somewhat larger dimensions, have previously been used as suppressors.'^'^''^^
Built along this design. Device C indeed exhibited the best performance. Even though the tube itself was nearly three times as long as device B, the band dispersion was measured to be 78± 4|j,L. Under isocratic elution conditions the noise level was measured to be 5 ± 2 nS/cm and 10 ± 2 nS/cm under a demanding steeply changing gradient elution condition. Because of its larger surface area relative to device B, a lower concentration of feed KOH is needed to reach a -100 i^M concentration in the receiver.
At 30 °C, a 50 mM KOH feed leads to a background conductance of 28 ).iS/cm with an eluent flow rate of 1 mL/min. Under a given feed condition, the penetration of KOH remains constant. In one experiment, the flow rate of 35 mM of electrodialytically generated KOH used as eluent was varied between 0.5 to 1.75 mL/min in 0.25 mL/min increments. The electrodialytically suppressed conductance always remained below 0.8
^S/cm. The suppressor effluent (essentially water) was passed through a FFAH device with 65 mM carbonate-free KOH (electrodialytically generated by a second
32 electrodialytic generator) acting as feed. The observed background conductance was linearly related to the reciprocal of the eluent flow rate with a linear r^ value of 0.9999.
The device showed excellent reproducibility. Taking borate, a classic weak acid analyte, the reproducibility at the 50 (xM injected level was 2.0% in RSD, the S/N= 3 limit of detection was 0.6 \iM (6.5 ppb B, 25 [iL injection, 15 pmol) with a linear r^ value of 0.9997 for response in the 5-100 |LIM range (7 mM KOH isocratic elution, XR -6.3 min).
This performance is notable because boric acid has a pKa of 9.23 and under the above conditions elutes as a relatively broad peak (w/, -40 s). Response from 0.6 [iM borate
(and several other ions at trace levels) is shown in Figure 2.10.
Base Introduction versus Ion Exchange: The Effect of Device Design
Different membrane devices are commercially available as suppressors. The purpose of such devices in anion chromatography is to exchange large concentrations of eluent cations, and as such requires significant ion exchange capacities. As a result, such suppressor devices are often designed with ion exchange screens in between ion exchange membranes;^'* these screens are particularly valuable in gradient elution because of their ability to provide reserve ion exchange capacity. While these devices can undoubtedly be used for base introduction, it is to be noted that they are capable of ion exchange on the screens, without immediate and concomitant base introduction. This process can occur in addition to the base introduction process. Note that when the sole process is introduction of the base MOH through the membrane, the reaction that occurs
33 for any analyte HX (within the limits that HX does not exist as an unionized acid at a pH of~10(-100|aMMOH))is:
MOH + HX ^ MX + H2O. (2.2)
In this case all signals are uniformly negative and the signal intensity is controlled by the analyte concentration and the difference in equivalent conductance between the analyte ion and OH". If the analyte HX is significantiy ionized, the resulting H^ can be ion exchanged for M"^ at the interior membrane surface:
J^ membrane "•" n aq —^ H membrane + M aq. (2.3)
Processes 2.2 and 2.3 cannot be distinguished in practice because the M* that is being exchanged at the membrane surface would have otherwise been introduced as MOH.
There is the apparent difference in principle that process 2.2 results in a production of an additional water molecule. In practice, with trace level analysis, the difference in the hydration of ions in the membrane vs. free solution, and the high water permeability of all ion exchange membranes will make it impossible to differentiate processes 2.2 and
2.3. If however the same process as that in 2.3 occurs on the ion exchange screens, the outcome will be different:
M^ereen+H% ^ H%creen + MV (2.4)
34 The screen ion exchange sites are regenerated on a much slower scale and process 2.4 will therefore lead to the production of MX in addition to the introduction of MOH. For poorly ionized analytes, only process 2.2 can occur. But for ionized analytes, processes
2.2/2.3 and 2.4 can occur in competition. If the latter dominates, the resuh will be a positive MX peak atop a MOH background. (The screen sites will be regenerated more slowly, basically resulting in an eventual change in baseline.) The results of using a suppressor for base introduction purposes result in the chromatograms shown in Figure
2.11. This behavior obviously results in an interesting and immediate differentiation
between strong and weak acid analytes and may be useful in some situations. The
possibility of co-eluting peaks in opposite directions may, however, complicate
interpretation of the data in real samples.
Illustrative Applications
Figure 2.12 shows a 2-D chromatogram with the two detector signals being
shown for several strong and weak acid anions. Weak acid analytes such as arsenite,
silicate, borate, and cyanide are invisible in the first detector and produce easily
measurable responses in the second detector.
Previous work has elaborated on how such 2-D data can be exploited for the
diagnosis of co-elution, estimation of analyte pKa values calculation of analyte
equivalent conductance (and thereby provide a means of identification) values, and
perform universal calibration.^'^ The advent of commercial electrodialytic eluent
generators has made possible nearly pure water backgrounds which, in conjunction with
35 passive base introduction devices make the practice of 2-D IC detection simpler, more sensitive and attractive than ever. User-friendly software that can fully utilize the 2-D data is needed for the complete exploitation of the technique. Recent advances in the understanding of ion exchange devices in ion chromatography may even make possible
3-D detection schemes (HX, MX, MOH).^^ However, even the present state of development provides a very useful tool to the interested user as detailed below.
Filter samples of airborne particulate matter have been collected and analyzed by ion chromatography, for example, during the supersite campaigns in Houston and
Philadelphia.^^ While major components such as sulfate, nitrate, chloride, etc., are readily identifiable and quantifiable, there are numerous other analytes also present in these samples that are often hidden by the major analyte peaks. Even with IC-MS, co- elution makes identifying the occtirrence and identification of trace constituents a very challenging task. (Contrary to popular belief, IC-MS provides considerably poorer
detection limits than either of the detectors in 2D IC, when a total ion scan must be
conducted for a totally unknown analyte.) Figure 2.13 shows a 2D chromatogram of an
air filter sample extract collected in Houston during the summer of 2000. Note that the
data immediately reveals that the asterisked peak is clearly an acid weaker than a
common aliphatic carboxylic acid (see response to acetate in Figure 2.12). This
information would have been impossible to discem by any other means. Of the
numerous other nuances that are present in this chromatogram but are too difficult to see
without further magnification, I focus only on the 18-21 min region. The peak at -19
min is completely invisible in the suppressed chromatogram and must be due to a very
36 weak acid. The peak at -20 min is seen as a perfectly clean Gaussian response in the suppressed chromatogram while the second dimension immediately reveals that it is actually a mixture of two partially co-eluting analytes, probably in an approximate ratio of-l:3.
In summary, 2DIC in its presently developed form is simple to implement and practice and asides from improving the detectability and response linearity characteristics of weak to very weak acids, it provides a wealth of information that is otherwise difficult or impossible to obtain.
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23. Dasgupta, P. K. US Patent 4,500,430, 1985.
24. Stillian, J. R. LC»GC Mag 1985, 3, 802-812.
25. Srinivasan, K.; Saini, S.; Avdalovic, N. Recent Advances in Continuously Regenerated Suppressor Devices. Abstract 136, 2001 Pittsburgh Conference, New Orleans, LA, March, 2001.
26. http://www.utexas.edu/researchyceer/texaqs/index.html http ://'www.cgeny. com/Narsto/
27. Samanta, G.; Boring, C. B.; Dasgupta, P. K. Anal. Chem. 2001,13, 2034-40.
39 LLO£p ^sajxlsaj>^^ tUDysn'esuodssu >iestl
40 strong acid, H2S04 background
0.40 Strong acid pure H20 bgnd
.> 'Z5u- 0) 0.20 E
-pK10
- pK9 pK8 Strong acid
O.OE+0 2.0E-5 4.0E-5 6.0E-5 8.0E-5 1.0E-4 Peak Concentration, eq/L
Figure 2.2. Cassidy plot of response sensitivity in linear axes. An ideally linear response produces a flat curve of zero slope. The top trace asstunes a 1 ^M H2SO4 background; all others assume a 10 |j,M CO2 background.
41 EEG Oven r^QU Enclosure 1—1 p
Water
Gas Pressure
KOH
Figure 2.3. Experimental system. Key: P, chromatographic ptimp (1 mL/min); EEG, electrodialytic eluent generator; V, injection valve(25 i^L); GC, AGl IHC (4 mm) guard; SC, AS 1 IHC separator; EDS, electrodialytic suppressor; Dl, first detector; BID, base introduction device; D2, second detector; R, exit restrictor. KOH flow into BID is 0.5 mL/min by nitrogen pressure.
42 flow out
flow In metal wire (A) connected to current source
screw hole
plexiglass slab
metal win
flow channel
•mA^ n Eluite out
Device B KOh Out KOMIn
Eluite out
Device C
Figure 2.4. Base introduction device designs: (a) planar sheet membrane design that can be operated electrodialytically or by Donnan leakage, (b) straight tube in shell design, and (c) filament-filledannula r helical design.
43 30.00
Fit 20.00 E All other Membranes
(U O c Thin PTFE, RAI CD •c V Nafion 417 •D C Dionex o n O 10.00 A Nafion 117 o Asahi Glass, Selemion 0 Sybron, MC 3470 o Asahi Glass, CMV o Asahi Glass, Flemion 0.00 1 \ 1 1 0.00 40.00 80.00 120.00 160.00 200.00 Current, uA
Figure 2.5. Ctirrent efficiencies observed with electrodialytic devices with different membranes.
44 Thin Radiation Grafted PTFE (RAI), 0.07 mm V Nafion 417, 0.43 mm 0.12 -^ • Dionex radiation grafted memrane, 0.10 mm (a) A Nafion 117, 0.18 mm O Asaiii Glass, Selemion, 0.15 O ^^ o Asahi Glass, Flemion, 0.15 mm, -COOH
Si
1 ' r 0.00 40.00 80.00 120.00 160.00 200.00 Current, uA
Figure 2.6. Backgrotmd noise in electrodialytic devices with different membranes as a function of (a) the observed conductance (0.1 mM KOH) 27.2 |iS/cm) and (b) the electrodialytic drive current. Internal flow, 1 mL/min in this and subsequent figures.
45 40 -n Dow PFSI, 0.15 mm, r'^2 1.0000
Thin Teflon, 0.07 mm, r'^2 0.9947 + RAI, 0.10 mm, r*2 0.9996 30 Asahi Flemion, 0.15 mm, r'^2 0.995 E Nafion 117, 0.18 mm, r'^2 0.9996
Nafion 417, 0.43 mm, r'^2 0.9986 0 — 0.00 0.20 0.40 0.60 0.80 Feed KOH Concentration, M Figure 2.7. Passive Donnan leakage of KOH through various sheet membranes as a function of feed KOH concentration. 46 0.80 -n Eluent Flow 1 mL/min # LiOH A 0.60 — O NaOH c o A KOH (0 + CsOH c 0) o c A o 0.40 — o X O T3 O A 0 CD 0 C 0 0.20 O O A 4^A % 0.00 n ^ \ ^ ^ ' r 100 200 300 400 500 Feed MOH Concentration, mM Figure 2.8. Donnan leakage of different alkali hydroxides through the RAI PTFE membrane. 47 0,25 —1 # Device B, 0.525 x 0.35 mm od x id, 90 mm long O Device B, 0.40 x 0.30 mm od x id, 90 mm long 40 80 120 160 200 Feed KOH, mM Figure 2.9. Dependence of Donnan leakage on tubular membrane dimensions. Nafion membrane tubes are used. 48 0.20 —1 0.00 — E o o -0.20 — ca c o O -0.40 — -0.60 4.00 8.00 12.00 Time, min Figure 2.10. Detection of 0.6 j^M borate in a sample mixture on the second detector. This presentation used a moving average routine to reduce baseline noise. The S/N= 3 LOD will be 0.6 |4.M based on the baseline noise observed in the raw detector signal. 49 35.00 Sulfate 34.00 — Phosphate E .o w 33.00 iL (D O c as o •D c 32.00 — o O J n - \ Ca r o 31.00 — •S<) cr 3 \ o a ' 3 o fi> o 30.00 20 0 10 20 Time, min Figure 2.11. Second detector response to various analytes using a commercial membrane suppressor (containing an ion exchange screen) as the base introduction device. 50 6.25 nmol nitrate, borate acetate, sulfate, ^^ ]S" re 12.5 nmol all others _a> u re 8.00 — u w ff s it o 9> o M re M 4-* o 0) a ' O) o p^« 1 ^-^ < AS11HC Column, Ramp D) E 0-3.0 mM KOH 0-10 min •2 £ ^ Hold at 3.0 mM till 15 min re ' i -^ 4.00 — Ramp to 10 mM 15-20 min Z 0) ' •o j 3o g •a 3 1' c ;i o (0 O .._--"'' ' ^^ — 0.00 — ,. ^ J ^- T.---,-- -'^--^rrmi^rr: - / '1ft i/^^' il 'i,' i'' i' _ \ '< W i/ O » fi>rb o < }rat e nat e licat e enit e id e -4.00 — III I O.C 0 10.00 20.00 30.00 40.00 Figure 2.12. 2D ion chromatogram tmder standard conditions using gradient elution, 25- |iL injection volume. 51 AS11HC 1 mL/min E u 4.00 8 c 3 •o C 8 0.00 0.00 20.00 40.00 60.00 Time, min Figure 2.13. 2D ion chromatogram of an air filter sample extract (Houston, TX, July 2000). The inset shows the 18-21-min region magnified. 52 CHAPTER III FIELD MEASUREMENT OF ACID GASES SOLUBLE ANIONS IN ATMOSPHERIC PARTICULATE MATTER USING A PARALLEL PLATE WET DENUDER AND AN ALTERNATING FILTER-BASED AUTOMATED ANALYSIS SYSTEM Introduction Many instruments exist for the rapid automated determination of gaseous constituents of ambient air. This includes, for example, all the gaseous criteria pollutants. Diffusion based collecfion and analysis of atmospheric gases have been reviewed.' In regard to suspended particulate matter, physical parameters such as optical or aerodynamic size distribution and mass concentration can be relatively readily determined by a ntunber of available commercial instruments. This is not the case for the (near) real-time determination of chemical composition of the atmospheric aerosol. The quest for instrumentation that can accomplish this objective began some three decades ago and continues today. Crider^ first demonstrated real time determination of aerosol sulfur with a flame photometric detector (FPD) by switching a filter that removes SO2 in and out of line. In many early methods, potentially interfering gases were first removed and the aerosol stream was then thermally decomposed under controlled temperature conditions to characteristic gases that were collected by a diffusion denuder and then measured 53 periodically. Much of the effort was directed to the specific measurement of sulfuric acid and the various ammonium sulfates.^ Similar methods were also developed for ammonium nitrate. One ingenious method for measuring aerosol acidity involved gas phase titration of the aerosol with ammonia.^ The flash volafilization (FV) technique of rapid thermal decomposition of a collected analyte^ became widely used for the measurement of aerosol sulfate in conjunction with a FPD.^ Although determinafion of nitrates by thermal decomposition was originally considered questionable,^ FV- NOx detection based meastirement of nitrate has been shown not only to be viable,^ recent innovations and adaptations by Stolzenbug and Hering'" have made it routine. This technique is also promising for the simultaneous measurement of aerosol S by an FPD and aerosol C by a CO monitor. Thermally speciated elemental vs. organic carbon measurements have been demonstrated." Direct introduction of an air sample into an air plasma has been shown to be viable for the direct measurement of metallic constituents.'^ More recently, Duan et al.'^ have described a field-portable low-power argon plasma that tolerates up to 20% air. Coupled to an inertial particle concentrator,'"* such an approach may be practical although the limits of detection (LCDs) are not as yet good enough for use in ambient air. For a given analyte, uniquely simple and sensitive solutions may exist; Clark et al.'^ reported that a single 100 nm diameter NaCl particle can be detected, free from matrix interferences, with an FPD. The application of mass spectrometry (MS) to aerosol analysis has had a long and illustrious history.'^ Electron and optical microscopic techniques were once believed to 54 be the best route to the analysis of individual particles.'^ Single particle MS can do this today and do so in real time'^. MS can provide information on not just specific components such as sulfates and nitrates, but on all material present in the particle. While MS may hold the key to the future, the cost, bulk, operator sophistication and the extensions needed to produce reliable quantitative data presently leave room for other, more affordable techniques. Since much of the aerosol constituents of interest are ionic, typical present day practice of aerosol analysis involves gas removal with a denuder, filter collection with subsequent extraction of the filter by an aqueous extractant and analysis by ion chromatography (IC). In this chapter, a fully automated IC-based approach to near real time aerosol analysis is described. Continuous impaction is one of the most straightforward approaches to accomplish aerosol collection but it is difficult to collect very small particles by impaction. This problem was solved by introducing steam into the aerosol flow and allowing the aerosol to grow.'' This general theme has been adapted and refined by others,^° as well as by this research group ^' and introduced in parallel by a Dutch group.^^ Although other approaches to collecting atmospheric aerosols into a liquid receiver coupled to IC analysis have been investigated," generally these could not exceed the efficiency of the vapor condensation aerosol collection approach across a large particle size range. The steam introduction approach is, however, not without its shortcomings. A small but measurable artifact is caused by the hydrolytic reaction of NO2, which is not appreciably removed by most denuder systems now in use. The resulting product is 55 measured erroneously as particulate nitrite (and to a much smaller extent, nitrate). Steam introduction requires a condensation chamber that increases the size of the instrument. Filter collection also potentially permits differential analysis via sequential extraction with different solvents not possible with direct collection in a liquid.This chapter describes a new instrument that is a fully automated analog of manual filter collection, extraction and analysis. Experimental The instrtunent was constructed using a full tower size personal computer (PC) case as the housing. Various components were anchored or attached directly to the PC chassis. Fully assembled, the particle collection and extraction instrument had dimensions of 55 cm x 76 cm x 76 cm (L x W x H; including instrument components placed outside the computer case). Gas Removal and Analysis Soluble gas collection is accomplished with a parallel plate wet denuder (PPWD). The current PPWD differs from previous designs as follows. The denuder is composed of Plexiglas plates with Teflon spacers. Non-glass construction eUminates fragility problems. The desired area of each Plexiglas plate is microstructured to render it wettable. The denuder is bolted to a stand consisting of a support base to which threaded pipe flangesar e secured by screws. The threaded ends of''/g in. i.d. steel piping, used as the support stands, are secured thereto. 56 For the measurement of gases and aerosols with the highest temporal resolution possible, it is necessary to dedicate individual IC units to the gas system and the aerosol system. There are two potential arrangements : (a) a PPWD supplying its liquid effluent to an IC dedicated to gas analysis and a second independent PPWD the gas phase effluent of which is directed to the particle collection system (PCS), which is coupled to its own IC, and (b) a single PPWD connected to the PCS, the liquid effluent from the PPWD and the PCS each going to separate IC units. Even though the latter arrangement may at firstsee m to be the simpler, in all field experiments,^'* the first option has been chosen. Among others, HNO3 and HCI are two gases that are of interest and both are known to be "sticky"; the very minimum of an inlet line must be used. On the other hand, it is generally desired to measure the aerosol composition in the < 2.5 Ijm size fraction,necessitatin g both a cyclone and a gas removal denuder prior to the aerosol collector. The cyclone cannot be placed after a wet denuder because of the growth in size of hygroscopic aerosols during passage through the denuder. Placing the cyclone before the denuder would entail loss and/or undesirable integration of the "sticky" gases. The general suggested arrangement thus involves the deployment of the gas analysis denuder in open air (typically immediately on the roof of the shelter where the analytical instruments are located) without a cyclone and with a very short inlet (< 5 cm of a perfluoroalkoxy (PFA) Teflon tubing). The air sample enters the denuder at the bottom. A peristaltic pump located in the instrument shelter pumps the liquid to and from the denuder. The transit timei n typical deployment is about 2 min and temporal gas analysis data are corrected for this transit delay. The denuder stand is sufificientiy tall to allow the inlet to be -60 cm off the support base. To minimize interaction of the inlet air sample with the stand components, 57 especially in still air, the iron support stand fromth e base to the bottom of the denuder is wrapped with Teflon tape. The denuder is shown schematically in Figure 3.1. Each denuder plate is 10.0 x 55 cm (Vg" thick) with the active wettable area of 6.5 x 42 cm, starting 7.5 cm from the top and 1.75 cm from each edge. The denuder liquid is forced through a fritted PVDF barrier to allow even flow down the plate and is aspirated from the apex of the V-groove, 4.5 cm from the bottom edge. The two plates are spaced by a 3 mm thick PTFE spacer. The air inlet/outlet holes, circular at the termini, are machined with a contour that becomes elliptical as they approach the interior of the denuder to allow for a smooth entrance/exit of the airflow. PFA Teflon tubing (I ga., 8.3 mm o.d., 7.5 mm i.d.) fit tightly into these apertures. The overall airflow arrangement and gas system liquid flow arrangement is shown in Figure 3.2a. Typically the air sampling rate is 5 Standard Liters per Minute (SLPM), controlled by a mass flow controller (MFC-D, Aalborg instruments AFC 2600D, Orangeburg, NJ). A diaphragm pump (PI, Gast DOA-PI20-FB) provides the sample flow, the same pump is used for flow aspiration on a filter FC (vide infra). Hydrogen peroxide (0.5 mM) is used as the denuder liquid at -0.5 mL/min on each plate, each stream pumped through disposable mixed bed ion exchange resin columns MB (0.67 cm i.d. X 15 cm, PTFE column filled with Dowex MR-3 resin) located immediately before the PPWD liquid entrance ports. The effluent streams are aspirated at -1 mL/min from each plate (using same peristaltic pump but larger tubing, 0.89 mm vs. 1.29 mm i.d. Pharmed® tubes are used for input vs. aspiration, peristaltic pump speed fixed at 6 rpm) 58 to ensure all liquid is aspirated from the bottom of the PPWD. The aspirated flow streams are combined and sent to the IC analysis system consisting of alternating TAC- LPl anion preconcentrator columns, AGl IHC guard and AS 1 IHC separation columns, and an electiodialytically regenerated suppressor (ASRS, operated at 50 mA). The chromatographic system itself consisted of a DX-100 pump and detector with 22.5 mM NaOH eluent flowing at 1 mL/min. In more recent work, an IS-25 chromatographic pump, coupled to an EG-40 electrodialytic eluent generator (15.5 mM KOH, 1.5 mL/min, LC-30 oven at 29°C) and an ED40 detector used as a conductivity detector (CD) have been used. Chromatography is conducted either on a 10-min or a I5-min cycle. A 4- chaimel peristaltic pump (Rainin Dynamax) is used for all liquid pumping. All chromatographic equipment and columns above and in the following were from Dionex Corp. Particle Collection Svstem A Teflon-coated aluminum cyclone (10 L/min; University Research Glassware, URG, Chapel Hill, NC) is used as the first element of the inlet system to remove particles larger than 2.5 i^m. The cyclone exhibits the desired size cut point only at the design flow rate. Referring to the overall airflow arrangement in Figure 3.2a, the air sample passes through the cyclone @ 10 SLPM and is divided by an Y-connector into two flow streams of 5 SLPM each. One is drawn through a 47 mm glass fiber filter Fl (Whatman type GF/B, filters were changed either at 12 h intervals or corresponding to daylight and nighttime hours and were used for archival purposes and IC-CD-UV-MS analysis of the 59 filter extract in home laboratory) via mass flow controller MFC-C (Aalborg AFC2600D). The cyclone and the filter holder are mounted on a modified camera tripod. The feet of tiie tiipod are bolted to the roof of the instrument shelter; the air inlet is maintained -2m above the roofline. The second flow stream from the cyclone exit proceeds through a copper conduit or aluminized PFA Teflon tube to a PPWD located within the instrument shelter. The metal is electrically grounded to minimize aerosol loss. The PPWD is fed with -1 mL/min streams of 10 mM Na2HP04 (adjusted to pH 7) containing 0.5 mM H2O2, on each plate that serves to remove both acidic and basic gases; the denuder effluent (aspirated at~l .5 mL/min) is sent to waste. The gaseous effluent from the denuder bearing the aerosol proceeds to the PCS. The first element of the PCS is a specially constructed rotary valve VI that directs the ambient air stream to either filter A or filter B. This valve must provide a straight passageway for the sample stream to one of the two sample filters without aerosol loss. The valve is shown in functional detail in Figure 3.2b. The stator plate has three holes, the central port is connected to the sample air stream (from the PPWD) while the two other ports are connected in common through a Y-connector to a sequential trap containing a particle filter (F2), acid-washed silica gel (Tl, 6-8 mesh, which removes NH3) followed by a soda-lime trap (T2, 4-8 mesh, that removes acid gases) and a heater (H) that thus provides a hot dry clean air source (Figure 3.2a). The rotor plate has two holes, connected to filter A (FA) and filter B (FB), respectively, and is rotated by a spring-return rotary solenoid (TRW/Ledex, Vandalia, OH, 30° rotation angle). The air transmission tubes to the valve are 7.5 mm i.d., 8.75 mm o.d. PFA tubing, push fit into 60 the stator and rotor plates of the valve. With the solenoid unenergized, ambient air is sampled on filter A and with the solenoid energized, ambient air is sampled on filter B; flow is thus switched without aerosol loss. Other air valves V2-V4 are '/2-NPT large- orifice, low power, on-off type solenoid valves (Skinner A10, Parker/Hannifin, 12 VDC) that govern airflow in the PCS. Plexiglas filter holders were machined to hold 25 mm diameter filters. Atop a stainless steel screen are placed a paper filter (Whatman grade 5) and a glass fiber filter (Whatman GF/B). Two 10-32 threaded ports on opposite sides of the top half of the filter holder provide entiy of wash liquids. The bottom half of the filter holder is designed as a shallow cone with the air outlet at the center. The liquid exit port is a 10-32 threaded aperture, located equidistant from the inlet apertures such that the inlet/outiet apertures constitute an equilateral triangle in top view. Air/liquid separators, constructed using 3-inch transparent polyvinyl chloride (PVC) pipe with PVC caps cemented to each end, constituting 500mL capacity reservoirs, were incorporated below each filter holder in the air exit path. These contained air in and exit ports, as well as a port to remove accumulated water (periodically, e.g., every 24 h) using a syringe. These separators serve to keep any wash liquid from entering the respective mass flow controllers (MFC-A, B; O-IO LPM; UFC- 1500A, Unit Instruments, Inc., Chaska, MN). The diaphragm pump (P2, same as PI) used for sampling is capable of aspirating at >8 L/min through each filter holder simultaneously. 61 Standard wall PFA Teflon tubes (ISW, Zeus Industrial Products) were used for connecting PCS components upstream of the filter holders. This tubing was externally wrapped with electiically grounded Al tape and then with bare Cu wire. This served the dual purpose of improving its structural strength and reducing electrostatically induced aerosol loss. Instrument components were machined to provide a leak-free push-fit with this size tubing. Flexible PVC tubing (Vg in. i.d.) was used for component connections downstieam of the filter holders. Filter Extraction System A 6-channel peristaltic pump (Dynamax RP-1, Rainin) provides liquid pumping. Valves V5-V8 are low power miniature liquid solenoid valves. Valves V5 and V6 are subminiature all-PTFE wetted part valves (161T031, Neptune Research, W. Caldwell, NJ) that direct the flow of deionized water to the filter holders. Prior to the filter holders, the pumped water (I mL/min total flow) is split into two flow streams. A 2 cm length of PEEK tubing (0.010 inch i.d., Upchtirch Scientific, Oak Harbor, WA) was placed immediately prior to the filter holder at each water entrance to provide flow resistance. This served to evenly distribute the flow from both inlets evenly on to the filters. Valves V7 and V8 (161P091, Neptune Research) handle filter extract in which stray glass fibers may be present. Therefore, these valves are pinch type valves that can tolerate such fibers without valve malfunction. A low volume fiber-trap-filter (FTF, Acrodisc CR, 5 ^m, 25 mm) placed prior to the injection valve prevents glass fiber intrusion to the preconcentration columns. Such intrusion can result in high-pressure drops, resulting in 62 decreased sample loading on the columns. Injection valve IV is a 10 port electrically actuated valve (Rheodyne) that contains two low-pressure drop anion preconcentration columns (TAC-LPI). PEEK peristaltic pump tubing adapters (PF-S, VICI) terminating in ^4-28 fittings were used. Male nuts (1/4-28 threaded) and ferrules were used to connect tubing to the pump adapters. Pharmed tubing (1.29 mm and 1.52 mm i.d., respectively) was used for pumping water to and from the filter holders (-1 and 1.5 mL/min); larger aspiration flow is used to prevent water backup at the filters. Similarly 1.29 and 1.52 mm i.d. Pharmed® ptimp tubes were used for pumping and aspirating liquid to and from each wall of the PPWD. All liquid transfer lines were 20 gauge standard wall PTFE tubing (20 SW, Zeus Industrial Products, Orangeburg, SC). For connections, PTFE tubes were butt-joined with Pharmed® pump tubing as sleeves. The chromatographic columns and suppressor were identical to that for the gas analysis system. The chromatographic system itself used either a DX-120 Ion Chromatograph and detector with a 22.5 mM NaOH eluent at 1.0 mL/min or a DX-600 system with an electrodialytically generated (EG 40) 14.75 mM KOH eluent flowing at 1.5 mL/min with columns thermostated at 31 °C and a CD 20 conductivity detector. Under either operating conditions, chloride, nifrite, nitrate, sulfate and oxalate were analyzed in less than 15 min. Occasionally the system was operated with 30min sample collection and 30min gradient elution rtms. 63 Instrtiment Operation Table 3.1 shows the air and liquid valves and their respective on/off status. Figures 3.3a and 3.3b illustrate the four states of the instrument cycle. The first state depicted in Figure 3.3a is 8.5 min in duration. In the particle collection system, the soluble gas denuded aerosol flow stream is directed to filter A by valve VI. Air passes through filter A though mass flow controller A (MFC-A), which regulates the airflow to 5 SLPM, and finally through valve V4, which is on during state 1. Valves V2 and V3 are off, and filter holder B (FB) is under airlock. In the liquid extraction portion of the instrument, deionized water is contained in a 2 L bottle (WB). The air entrance to the water bottle is equipped with a soda-lime trap to minimize acid gas intrusion into the bottle. Water from WB is aspirated and then pumped at 1 mL/min by the peristaltic pump (PP) through a mixed bed ion exchange column (MBl; packed with Dowex MR-3 resin, Sigma) to remove any trace impurities present in the deionized water. Valve V5 directs flow to valve V6, which in turn directs the water to filter FB. The water enters FB through the two ports in the top of the holder and is simuhaneously aspirated from the bottom of FB through valves V7 and V8 by the peristaltic pump. Since FB is under airlock, water does not enter the air outiet tubing at the bottom of the filter holder. The extracted material from the filter is pumped through the fiber trap filter (FTF) to remove glass fibers from the fiow stream before passing to the appropriate preconcentration column. Valve IV is configured such that while one preconcentiation column is chromatographed, the other preconcentration column is 64 loaded with sample or washed with water. In the present case, preconcentiation column PCI is loaded with sample. Following 8.5 minutes, state 2 begins (Figure 3.3b). During state 2 in the PCS, ambient air continues to be sampled on FA, just as in state 1. Valves V2 and V3 are activated in state 2, allowing clean hot air to pass through filter FB for the duration of this state. Clean (ammonia/acid gas and particle free) air, produced by passing ambient air through F, Tl and T2 is heated to -75°C by passing it over a siliconized resistance heater (Watlow, St. Louis, MO) contained in a PVC cylinder housing that is powered by 110 VAC power (-20 W) via a DC relay that is switched in parallel with valve V2. This clean, hot air is aspirated through the previously extracted filter FB, to dry it prior to state 3. Within the PVC cylinder housing the heater, a thermal cutout device is located in close proximity to the heater and is connected in series with the heater such that the heater shuts off in the event of overheating (t > I43°C). Note that at the time the instrument enters state 2 from state I, although all the analyte has been extracted from filter FB and preconcentrated, the last portion of the wash water is still contained in the filter housing. This water is aspirated into the trap bottle ahead of MFC-B. Water that enters into the trap bottle is generally of the order of ImL/cycle. This volume may be used to monitor the filter extraction process; excessive water accumulation in the water trap bottle indicates fiow problems through the filter or through the relevant preconcentration column. In the liquid extraction system, valves V5 and V8 are activated. Valve V5 now directs water used to wash filter FB in state 1, back into the water bottle. This recycling procedure helps maintain the purity of the water in WB. As a resuh of liquid being 65 aspirated faster from the filter housing than it is pumped in, air bubbles inevitably enter into the preconcentration column. To remove the air bubbles before the sample is injected, valve V8 is activated and water is aspirated by the pump through a mixed bed ion exchange coltimn (MB2) through V8 and piunped through the preconcentration column PCI. The dtiration of state 2 is 6.5 minutes. After state 2 ends, state 3 (8.5 min) and state 4 (6.5 min) follows. States 3 and 4 are identical to states 1 and 2, respectively, except that the roles of filters A and B are interchanged relative to those in states 1 and 2. States 1-4 constitute an instrument cycle; state I starts at the end of state 4 and this continues until deliberately shut down. The chromatographic system is calibrated by a valve-loop combination in which each side of the valve is separately calibrated volumetrically by filling the loop with an alkaline solution of bromothymol blue of known absorbance, injecting, collecting all the effluent into a 5 mL volumetric flask, making up to volume and measuring the absorbance. Such a calibration takes into account the internal volumes of the valve ports, etc. Standards containing chloride, nitiite, nitiate, sulfate and oxalate are then injected using the loop, keeping the concentrator column ahead of the guard column to match actual experimental dispersion. Multipoint calibration curves are constructed in terms of absolute amount injected in ng versus peak area. Electrical The main ac power to the instrument goes to a PC-style power supply (that comes with the PC chassis) providing +5 and +/-12 V power, of which only the +12 V supply is 66 used (rated at 8A, <2A used at any time). A separate power supply board (+/- 15 and +5 V) is used for the mass flow controllers. Even the lowest rung IC (DX-120) used with the PCS provides 2 TTL outputs from the ion chromatograph. These can be temporally programmed in the DX-120 operating method. Table 3.1 shows the temporal state of these outputs. The schematic shown in Figure 3.4a is then used to control the instrument. The two TTL outputs are fed into a demultiplexer chip. Normally, the output from this demultiplexer is high; low output signals are generated at distinct pin numbers based on the DX 120 TTL signals input to it. Outputs from the demultiplexer chip are inverted and then used to address the logic level N-Channel MOSFET switches (RFM8N18L, Harris) to control the valves. The power supply grotmd is connected in common to all the source pins of the MOSFET switches, while the valves are connected between the positive supply and individual drain pins of the MOSFET switches with an intervening diode (rated @3A) to provide diode logic control. All valves operate from the 12 V power supply except VI, for which a separate power supply (18VDC, 2.5 A) was constructed. Figure 3.4b shows the electronics associated with the mass flow controllers. The schematic governing MFC-A is shown (that for MFC-B is identical). The MFCs can be manually controlled by 3-position, center-off toggle switch SWIA. Grounding terminal D or terminal J results in fully opening or fially shutting dovra the control valve, respectively. In the center-off position (normal), a 0-5 V contiol signal provided to terminal A of the controller governs the flow rate. This signal is provided by the 10 K 10-tum potentiometer RIA (numeric dial readout) and is normally set to provide 2.5 V so 67 that airflow is controlled at 5 SLPM on these 10 SLPM flow controllers. The output signal from the MFC (5 VFS) is divided 50:1 using a simple voltage divider network (R2A, R3A) and displayed on a 200 mV FS 3'/2-digit panel meter (DPM-A) that displays the air flow rate in SLPM. Two DPDT relays (R4 and R5) are used for controls that affect the filter drying airflow. The two relay coils are in parallel with valves V2 and VI, respectively. One half of relay R4 is used to apply AC power to the air heater during the filter drying cycle (only V2 is on at this time). The common pin of the other half of R4 is grotmded and the corresponding NO pin is connected to one of the common pins in relay R5. The corresponding NO and NC pins are connected to D-pins of MFC-A and MFC-B, respectively. Referring to Table 3.1, the net resuh is that when V2 is on and VI is off, MFC-A is opened fully to allow maximtim flow through filter A to dry it; conversely when V2 and VI are both on, MFC-B is opened fiilly to allow maximum flow through filter B. When V2 is off, both MFCs remain under front panel control. Total power consumed by the instrument, not including the IC, was measured to be 0.9-1.1 A @ 117VAC, under 150 W total. IC-CD-UV-MS Analysis of Filter Extracts. Filter extraction and analysis were done at Kodak Research Laboratories (Rochester, New York). Sampled 47 mm filters were individually folded and placed in Centricon centrifiigal filter devices (YM-IO, 10,000 MWCO, Millipore). Filters were handled with Nitrile gloves and plastic forceps. To each Centiicon was added 2.0 mL of water as extractant. Two centrifugations were done on the same day with the filtrate 68 passed back through the device for re-extraction. After the second pass, the filtrate was again tiansferred to the upper chamber and the devices were capped and placed in a refrigerator for 28 h. Finally, it was centriftiged for the third and final time (this was done to soak the filters to provide better analyte recovery). Two blanks were extracted in the same fashion and the average was subtiacted from the sample data (this correction was insignificant for most analytes). Chromatography was conducted on a GP-40 gradient pump, an ATC-2 cleanup column to clean the NaOH eluent, a 2 mm AS-15 column, an ASRS-Ultia suppressor in the extemal water mode (2.0 mL/min), an ED-40 conductivity detector, a PD-40 photodiode array UV detector (all from Dionex; the UV detector was scanned from 195-350 nm, essentially only the 205 nm response was used). Chromatography was conducted with a 5-85 mM linear gradient in hydroxide concentration over 25 min and a final hold of 5 min with a constant concentration of 5% methanol in the eluent and with a total flow rate of 0.25 mL/min. The injected sample volume was 100 |aL. Ion exclusion was also used to help differentiate between malic and succinic acids (the latter was not eventually detected) which co-elute in anion exchange with hydroxide gradients. An ICE-AS6 column with an AMMS-ICE suppressor was used for this work. The mass spectrometer was a SCIEX API 365 in electrospray mode with negative ion detection. 69 Chemicals All chemicals were analytical reagent grade. Nanopure water, >18 MQ«cm was used throughout. Hydrogen peroxide (30%), Na2HP04 and 50% NaOH were obtained from J.T. Baker. Aerosol and Gas Generation A vibrating orifice aerosol generator (Model 3450, TSI Inc., St. Paul, MN) was used to generate monodisperse aerosols containing (NH4)2S04 and put through a Kr-85 neutralizer (TSI 3054). A Venturi-type nebulizer was used to generate polydisperse aerosols. A laser-based optical particle counter (Model A2212-01-115-1, Met-One, Grant's Pass, OR) was used for size characterization. Other details of the aerosol generation and characterization system have been published. Clean air was supplied by a zero air generator (model 737-14, AADCO, Clearwater, FL, 100 SLPM). Gas standards were generated as previously described. Field Deployability The instrtiment is designed to be used in the field and is readily transportable (32 Kg). Air/liquid separators and fiUer holders were placed outside the instrument for ease of maintenance. PVC air/liquid separator holders are mounted with thumbscrews on each side of the instrument console and readily disassembled. A Plexiglas plate, held on the front panel of the instrument by similar thumbscrews, accommodates filter holders A and 70 B in recessed housing. All user settable items including mass flow controller readout and controls, are easily accessed from the front panel. The peristaltic pump body was affixed within tiie top of the computer case with the case cut out in the front and the top such that the pump head exits through the top (tubes are readily changed) and the pump panel is accessible through the front. Resuhs and Discussion Instrument Performance Filter Collection Efficiency Recovery and Carryover Glass fiber filters are known to display essentially zero breakthrough for particles over a large size range. In the present work breakthrough through these filters was studied using a polydisperse KBr aerosol (Mass median aerodynamic diameter 0.57 |xm, Gg 1.47) at concentrations of 21 and 25 |J.g/m^. Breakthrough was determined by allowing the system to sample through FA and FB for 4 hours each and installing a separate pre-washed 47 mm quartz fiber filter downstream from each of these. The latter were manually extracted and analyzed. Bromide was chosen as the test aerosol because tiie filter blank for this analyte was below the limit of detection (LOD). Bromide remained below LOD after 4h sampling (n=6). The capture of the aerosol by the filters is thus deemed to be quantitative. Recovery of the bromide collected on FA and FB, following the standard wash and preconcentiation period of the instrument was 97.1 ± 3.4% (n=6) compared to parallel sampling on a 47 mm filter, manual extraction and analysis. System carryover was determined by spiking the sampling filter with 100 ^ig 71 aliquots of bromide, continuously washing the filter thereafter and preconcentrating every successive wash for 8.5 min and analyzing the same. The first wash recovered 98.6 ±0.3% and every successive wash contained exponentially decreasing amounts such that following four wash cycles, the signal was below the LOD. Limits of Detection: Filter Blanks and Filter Pretreatment Instiiimental LODs (S/N=3 ) for chloride, nitiite, nitrate, sulfate and oxalate with electiodialytically generated, electrodialytically suppressed eluents are very low, under current experimental elution condhions these are typically in the 5-25 pg range for a properly operating system using current state-of-the-art commercial hardware. (It would be even lower for the fast eluting fiuoride, formate, methanesulfonate, etc. but citing these LODs may not be relevant because under the current standard elution conditions these are not resolved). For a 75 L air sample, these would translate into LODs that are of the order of 0.1 ng/m^ for the above anions were it not for the filter blanks. Glass fiber (GF) filters contain high levels of some ions, most notably chloride and sulfate. If used as such, they must go through cycled instrument operation for several hours before the chloride and sulfate values still leaching from the filter become insignificant in comparison to typical urban background levels. All of the following strategies can be successfully used: (a) use high purity prewashed quartz fiber fitters, (b) pre wash several GF filters on a Biichner funnel with copious amounts of DI water, store refrigerated singly in pre washed plastic containers. (NOTE: Do not ultrasonicate or apply any other similarly energetic measures to wash GF filters: they will disintegrate.) (c) soak 10-12 72 filters at a time in a beaker of deionized water. Decant and replace with fresh water at least four times at 15 min intervals. After the last disposal, cover tightiy with Parafilm® and store refrigerated. Strategy a is convenient but expensive, strategy c involves least labor and is what has generally been used, discarding the first three cycles of data when the filter is first replaced. Under these conditions, typically filter blanks (or more accurately, variations in filter blanks) are sufficiently reduced such that LODs for all of the above ions equate to <10 ng/m^ and after a few hours of operation, approach I ng/m^ Blank issues do not constitute a significant consideration for the gas analysis system (except for analytes eluting very close to the carbonate (CO2) peak). LODs in the 0.1 -1 ng/m are routinely obtained for the target gases. Choice of Filter, Filter Replacement Frequency Glass fiber (GF) filters have the drawback that during the washing cycle, fibers are shed. Fouling of the preconcentration column by the fibers is prevented by the paper filter underneath the GF filter and by the fiber trap filter (FTF, see Figure 3.3). Current manufacturer's specifications on the preconcentrator columns used are such that the pressure drops at the desired preconcentration fiow rate are at the limits of performance for many peristaltic pumps. When fouled, the pressure drop increases and in the worst case, liquid can back up on the filter housing. In the first field deployment in Atlanta in 1999, The system was operated without the paper backup filter for several days, and one preconcentration column was marginally fouled, decreasing die flow rate and consistently producing lower results on that channel. The work of Buhr et al." has already 73 demonstrated that fritted glass filters may not result in efficient capture of small particles. No filter media other than glass/quartz fiber has been found that offer the combined advantages of (a) high flow rates with minimal pressure drop (b) quantitative retention of particles across the size range, (c) efficient extractability with minimum volume of a purely aqueous extractant and (d) high flow rate in wet condition to permit rapid drying. The frequency with which the filter needs to be replaced seems to depend on particle loading. Note that water-insoluble substances remain on the filter and gradually accumulate, increasing the pressure drop. In at least one location the filter surface was accumulating substances that were rendering it hydrophobic. Once this happens to a significant extent, washing ceases to be uniform and the filter must be replaced regardless of pressure drop issues. In various field sampling locations, it has been found that the necessary filter replacement frequency vary between 1 to 3 days. In this context, it is interesting to note that carbonaceous (soot-like) compounds are not water soluble and accumulate on the filter. In urban sampling, much as k happens on hi-volume samplers, the filter surface becomes dark as it is used. It would be relatively simple to accommodate LED(s) and detector photodiodes within the filter housing to measure this discoloration and thus obtain a crude "soot index". Denuder Liquid Considerations for IC Coupling A Dedicated Denuder for the Particle System With an IC as the analyzer of focus, water-soluble ionogenic gases are the analytes of interest. Acid gases include SO2, HCI, HF, HONO, HNO3, CH3SO3H and various 74 organic acids, primarily CH3COOH, HCOOH, and (C00H)2. Ammonia is the only basic gas of importance under most condhions. If water is used as a collector, sulfur dioxide is collected as sulfurous acid. Henry's law solubility of SO2 is limited and quantitative collection may not occur under these conditions. Additionally, some of the bisulfite formed undergoes oxidation to sulfate either in the denuder and/or the IC system, leading to both sulfite and sulfate peaks. This unnecessarily complicates quantitation. Recent evidence^^ indicates that when a denuder is cooled, very little oxidation to sulfate occurs - this suggests that the oxidation within the IC system may be limited. However, this is likely a function of the degree of trace metal fouling of the chromatographic system/column. Addition of a small amoimt of an oxidant like H2O2 to the denuder liquid eliminates this problem and results in virtually instantaneous oxidation of the collected SO2 to sulfate. For the gas analysis denuder, the recommended denuder liquid is thus 0.5 mM H2O2. All other collected analytes, including nitrite (originating from HONO) is completely unaffected by the H2O2. Dilute H2O2 is also easily cleansed of ionic impurities by passing it through a mixed bed ion exchanger. Recently Zellweger et al.'^" pointed out a potential problem with collection of the weaker acids in high SO2 environments. It is easily computed that in an atmosphere containing 100 ppbv SO2, quantitative collection at an air flow rate of 5 LPM and a total liquid effluent flow rate of 1 mL/min will lead to 20 [iM H2SO4 (pH -4.4) in the liquid effluent. Many weak acid gases may have solubility limitations in such a solution. Particular concern was expressed about HONO (pKa 3.1-3.2), although the sitiiation is 75 obviously worse with gases like acetic acid (pKa 4.75). Zellweger et al. proposed a dilute solution of their chromatographic eluent, ~ 50 i^M NaHC03, as the PPWD feed. Unfortunately, this may not provide a generally applicable solution. In the presence of large amounts of SO2, the low concentration of influent NaHC03 used solution may be overwhelmed. The following arguments can be made in favor of not adding any alkaline modifier: (a) weak acids dissolve in aqueous solution both by their ionization and through their Henry's law partition (intrinsic solubility). If the latter is high, (HCN, a very weak acid, has a very high intrinsic solubility, for example^^) then good collection is maintained; (b) levels of SO2 -> 100 ppbv are found sporadically as a plume impacts a sampling location but such levels on a sustained hdisxs are not common, at least in the US; the suggested approach may be meritorious in an exceptional case but generates problems for other, more common situations; (c) a large amount of carbonate in the sample is incompatible with hydroxide eluent based anion chromatography, presently the preferred practice. Use of a carbonate containing PPWD liquid generates a substantial amount of carbonate in the effluent, a broad tailing carbonate peak can obscure smaller analyte peaks in that region; (d) an alkaline denuder liquid will inhibit uptake of ammonia, if ammonia is to be analyzed in the same sample. Although it has not been explicitiy so stated, the different composhions tried for the denuder liquid by the ECN group^' makes it clear that they too have grappled with this problem. A complete solution is not yet available. Note that gases that are not collected by a denuder preceding the PCS, will generally be collected by a PCS (especially a steam condensation based PCS), causing positive error. While 76 subquantitative collection of gases by the gas analysis denuder cannot be easily corrected for. errors in the particle composition measurement can be prevented by simply using a separate gas removal denuder for the PCS. This denuder uses a denuder liquid buffered at pH -7 with sufficient buffer capacity and at enhanced liquid flow rate that allows complete removal of both acid gases and ammonia. In principle, a similar approach can be practiced with the gas analysis denuder, if the buffer material used is removed completely by suppression or is invisible to a conductivity detector. Ito et al.^^ used a zwitterionic buffer to remove high levels of acidic gases (as may be present in indoor environments when a kerosene-fiieled heater is operated) or high levels of ammonia (which have been encountered in homes with live-in pets) before aerosol analysis. While these approaches have not been demonstrated when the denuder effluent is to be preconcentrated and analyzed, zwitterionic buffering may still be useful. Glycine, for example, has an appropriate pKa to be useful as a buffer and is suppressible. Morpholinoethanesulfonic acid and Bis-tris should be among other potentially useful suppressible zwitterionic buffers, which will provide a low conductivity background.'"' Initial experiments with such materials appear promising and future investigation of an optimum choice is required. Meanwhile, the conflicting needs of incorporating a cyclone of an appropriate cut point before the PCS and of having no inlet system for analyzing sticky gases in a gas analysis system still suggests that the PCS has its own gas removal denuder, regardless of denuder liquid considerations. 77 Illustrative Field Data The instiument has been deployed in several summertime field studies, each with 4-6 week duration: Atlanta Supersite (1999, during which an imtial version of the instrument was used), Houston Supersite (2000, during which the presently described version of the instrument was used) and Philadelphia (2001, during which the gas phase portion of tiie instrument was used). Figure 3.5 shows the concentrations of nitric acid/particulate nitrate, nitrous acid/particulate nitrite (the latter is nearly zero - establishing that this type of filter based measurement do eliminate artifact nitrite formation) and sulftir dioxide/particulate sulfate for a few days from the Atlanta site. Figure 3.6 shows the concentrations of hydrochloric acid/particulate chloride, oxalic acid/particulate oxalate for a few days from the Houston site. Typical chromatograms for the gas and particle analysis systems are shown in Figure 3.7. When carefully examined for minor components, the chromatograms, especially those for the aerosol samples, reveal a far greater degree of complexity. A gradient chromatogram of a 30 min sample collected in Atianta is Shown in Figure 3.8, with overlays representing lOx and lOOx magnifications of the base chromatogram. Considering that the baseline is essentially completely flat for a blank run even at the lOOx magnification, the number of real components present in such a sample becomes readily apparent. Not surprisingly, a majority of these peaks are organic acids. While MS is uhimately the only completely unambiguous means of identification when confirmed by a matching standard, in many cases the charge on the analyte ion can be estimated by determining void voltime corrected retention times (^R) under isocratic 78 elution conditions at 3 or more different eluent concentrations. Under these conditions, it is well known that the slope of a log ^R VS. log [eluent] plot is equal to the ratio of the charge on the analyte ion to that on the eluent ion (unity for hydroxide).^' This is shown in Figure 3.9. With this information and the nature of UV response of the analyte, h is often possible to determine the identity of the analyte. At the very least, it provides clues for selecting confirmation standards for MS. Table 3.2. lists average daytime and nighttime aerosol composition for a relatively polluted period during the Atlanta measurement campaign. The analysis was conducted by IC-CD-UV-MS by Drs Martin and Smith at Kodak, with identification confirmed by MS and conductivity providing quantitation. Several peaks remain imidentified; numbers in parentheses provided for these are calculated from the conductivity peak areas based on the average response. These should be taken as lower limits because the average response per imit weight is dominated by strong acid anions and these unidentified species are almost certainly organic acids for which response per unh weight is likely to be smaller. I have also performed qualitative IC-MS analysis of fiher extracts. The filters were collected in two field studies in Philadelphia and Houston and archived for lab analysis. The resuhs are shown in Table 3.3. Oxalate, Succinate, Methylmalonate, Malonate, Malate, Maleate, and Oxalate were present in almost every sample. Lactate, Phthalate, and Butyrate have been identified in some samples; however, in others, they were either below the LOD of the instrument or unpresent. To the author's knowledge, this is the first attempt to decipher the total anionic composition of ambient urban aerosol. In a global context, it is most remarkable that the list of the organic acids 79 identified here overlaps in a major fashion with the list of aliphatic organic acids that are used as metabolic pathway markers in the human physiological system.^^ Conclusion An automated particle collection and extraction system has been presented. When coupled to an IC for analysis, the system mimics the standard procedure for the determination of the anion composition of atmospheric aerosols. The instrument provides high sensitivity and allows analysis of anions in aerosol in only a fraction of the time and cost of conventional techniques. A wide range of aerosol constituents can be determined by simply changing the analytical technique used to analyze the filter extract. The instrument is field worthy. In the Houston field experiment, of a total of continuous deployment over 872 hours, the particle (gas) analyzer instruments respectively produced meaningfiil data 85 (90)%) of the time, was being calibrated 5 (5)% of the time and was being equilibrated (fitter wash), in maintenance, or down 10 (5)% of the time. Acknowledgments I would like to thank Charles Bradley Boring who gave his time and effort to put this instrument together and Zhang Genfa who operated the instrument in Atlanta in 1999 before I was able to use it in Houston in 2000.1 also would like to thank Michael W. Martin and William F. 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Environ. 1995, 29, 2609-2624; Liu, S.; Dasgupta, P. K. Talanta 1996, 43, I68I-1688; ibid. Anal. Chem. 1996, 68, 3638- 3644; Karlsson, A.; Irgum, K.; Hansson, H. J. Aerosol Sci. 1997, 28, 1539-1551; Liu, S.; Dasgupta, P. K.; Microchem. J. 1999, 62, 50-57. 24. Atlanta 1999: http://wrvyw-wlc.eas.gatech.edu/supersite/ Houston 2000: http://vyww.utexas.edu/research/ceer/texaqs/ Philadelphia 2001: http://www.cgenv.com/Narsto/ 83 25. Appel, B. R. ACS Adv. Chem. Ser. 1993, 232, 1-40; Koch, T. G.; Fenter, F. F.; Rossi, M. J. Chem. Phys. Lett 1997, 275, 253-260; Neumann, J. A.; Huey, L. G.; Ryerson, T. B.; Fahey. D. W. Environ. Sci. Technol. 1999, 33, 1133-1136; Komazaki, Y.; Hashimoto, S.; Inoue, T.; Tanaka, S. Atmos. Environ. 2002 (in press). 26. Samanta, G.; Boring, B.; Dasgupta, P. K. Anal Chem. 2001, 73, 2034-2040. 27. Chang, I. H.; Choi, N. H.; Lee, B. K.; Lee, D. S. Bull. Kor Chem. Soc 1999, 20, 329- 332; Chang, I. H. Ph.D. Dissertation, Yonsei University, Korea, August 2001. 28. Kuban, V.; Dasgupta, P. K. Anal. Chem. 1992, 64, 1106-1112. 29. Keuken, M.; Schoonebeek, C. A. M.; Wensveen-Louter, A.; Slanina, J. Atmos. Environ. 1988, 22, 2541-2548; Wyers, G. P.; Otjes, R. P.; Slanina, J. Atmos. Environ. 1993. 27A, 2085- 2090; Slanina, J.; Wyers, G. P. Fres. J. Anal. Chem. 1994, 350, 467-473; 0ms, M. T.; Jongejan, P. A. C; Veltkamp, A. C; Wyers, G. P.; Slanina, J. Int. J. Environ. Anal. Chem. 1996, <52,207-2I8; Jongejan, P. A. C; Bai, Y.; Veltkamp, A. C; Wyers, G. P.; Slanina, J. Int. J. Environ. Anal. Chem. 1997, 66, 241-251. 30. Ivey, J. P. J. Chromatogr. 1984, 257,128-132. 31. Small, H. Ion Chromatography, New York: Plenum, 1989, 68-69. 32. http://oxmedinfo.ir2.ox.ac.uk/Pathwav/Miscell/24028.htm 84 X O o o o I o o o O b b 00 O z > o O o b o b > o o o b b b b O b b > O o O b b b b z > O § O o b z b z > o o o o b b Z 2 > O o O § b b b z b > O o O § b b b b z O O z o o 00 £5 (N (N 3 U u bu bu u b b < OX) b .,.H an d 00 c 03 2 pa b .S tftC8 b 03 &> ^ J3 Q O OX) b t: hin g o g ^rt CO p 4) iS ^-OX») T3 'o o o n F A C r Purg ( gon F spirat e onto P o fa Si u 00 o c cd Wa t > CQ ade d mpl i .g gsti l igl o pli n "H. "& dr y b gin ' gins a .5 td E E P E s D CQ Hi o CQ 85 Table 3.2. Average anion composition of day and night time aerosol in midtown Atlanta, August, 1999' Retention time , min Concentration Micrograms per Cubic Meter Conductivity UV Analyte Day Night Detector Detector Samples Samples 8.34 Fluoride 1.1 0.58 8.95 Glycolate 0.28 0.19 9.37 Acetate 0.58 0.25 9.56 Lactate 0.81 0.32 9.83 Formate 0.91 0.71 10.96 a-Hydroxyisobutyrate 0.02 0.03 11.23 Unknown [0.015] [0.02] 11.87 Methanesulfonate 0.05 0.04 13.04 Chloride 9.8 5.5 13.27 Pyruvate tr tr 14.93 Unknown [0.004] [0.01] 15.52 Nitrite 0.11 0.15 15.60 Carbonate nd nd 16.23 Malate 0.30 0.24 16.57 Malonate 0.36 0.26 17.23 Sulfate 16 11 18.13 18.34 Oxalate 0.34 0.27 20.46 Unknown [0.01] [0.02] 21.58 Phosphate 0.03 0.03 23.28 23.52 Nitrate 1.9 1.7 24.33 24.66 Unknown [0.02] [0.03] 24.87 Unknown [0.03] [0.03] 25.87 26.06 Unknown [0.004] nd 26.72 Unknown [0.003] [0.007] 28.50 28.83 o-Phthalate tr tr 29.10 Unknown [0.004] [0.072] ' Retention times are as per the chromatographic protocol described in text. Numbers in parentheses provided for unknown peaks are calculated from the conductivity peak areas based on the average response. These likely the lower limits. 86 Table 3.3. Organic anion composition of aerosol filter samples collected in Houston, TX, 2000 and Philadelphia, PA, 2001 and identified by IC-MS Study Period of Analyte collection Boston, TX Aug 22, 8:30 pm- Succinate, Malate, Lactate, Maleate, Oxalate, Phthalate, August 12 - Aug 23, 8:40 am September 25, 2000 Aug 23, 8:40 am - Succinate, Methylmalonate, Malonate, Malate, Lactate, Aug 23, 7:50 pm Maleate, Oxalate, Phthalate, Butyrate Aug 28, 8:30 am- Succinate, Methylmalonate, Malonate, Malate, Lactate, Aug 28, 9:00 pm Maleate, Oxalate, Phthalate Sep 7, 8:30 pm - Succinate, Methylmalonate, Malonate, Malate, Maleate, Sep 8, 9:30 am Oxalate Sep 10,8:30 am- Succinate, Methylmalonate, Malonate, Malate, Maleate, Sep 10,8:30 pm Oxalate, Phthalate Sep 12,8:30 am- Succinate, Methylmalonate, Malonate, Malate, lactate, Sep 12,8:00 pm Maleate, Oxalate, Phthalate Sep 16,8:30 pm- Succinate, Malonate, Lactate, Maleate, Oxalate, Phthalate, Sep 17, 8:45 am Philadelphia, PA July 6, 7:40 am - Succinate, Methylmalonate, Malonate, Malate, Lactate, July 1-July30, July 6, 8:00 pm Maleate, Oxalate, Phthalate 2001 July 10,8:30 am- Succinate, Methylmalonate, Malonate, Malate, Lactate, July 10,8:40 pm Oxalate, Phthalate July 16, 10:00 pm- Succinate, Methylmalonate, Malonate, Malate, Oxalate July 17,8:30 am July 16,8:30 am- Succinate, Methylmalonate, Malonate, Malate, Lactate, July 16, 10:00 pm Maleate, Oxalate July 21, 9:00 am- Succinate, Methylmalonate, Malonate, Malate, Lactate, July 21, 9:00 pm Maleate, Oxalate, Phthalate July 21, 9:00 pm- Succinate, Methylmalonate, Malonate, Oxalate July 22, 8:40 am 87 AO o o o < LI o o LR WA Figure 3.1. Wetted denuder shovra schematically. AI/AO: Air in/out aperttires, LI/LO: Liquid in/out apertures, LR: Porous polyvinylidene fluoride element acting as a liquid flow restrictor, WA: wetted area, S: PTFE spacer, SH: Screw holes for affixing two denuder plates together. * MFC-C MB PPWD MB IC1 PP H202 [if i P2 FC MFC-D Ambient Air Ir ^ n ;=Ki:ir^ Ambient FA (a) F P H Rl Air In T MFC-B MFC-A P1 Q- (b) V1 ON OFF Figure 3.2. Particle collection system (a) Total system airflow and gas analyzer liquid flow schematic. PPWD: Gas system wet denuder, MB: mixed bed resin deionizer columns, IC: Gas analysis system ion chromatograph (uses 10-port dual concentrator column injector as in PCS IC in Figure 3; FA,FB: Glass fiber filters, T: Trap bottles, MFC-A,B,C,D: Mass flow controllers, C; Cyclone, FC: 47 mm filter for MS analysis, PI,2: Air sampling pumps, PP: Peristaltic pump, F: Filter, P: Purifer, H: Heater. The dotted section including the denuder is on the roof while the air pumps are either below the instrument shelter or in a modified doghouse with forced air ventilation. VI: aerosol switching valve, shown in detail in (b). 89 (U l> X) uT T3 3 C /N c« > I 0) .2 tn o D (U ^ t: r c b A ^-S E< 3 90 »5 Vdc - -•6 Vdc QND- -ONO U2.2 TTL2ln . -QND D.13 — U1 -nL1 In OND — U2.4 —ETH U2.8 — D.7/10/12 —ID I 04/8/11 — D—IsM MF3A 14 U24—(a 2 13 D.I/9 9 12 QND 4 U2 " 5 10 MF9- C U2< — 0 (a) QNO- 7 •"«"*'= D.9,4- D I OND—S RIA10K *5V 10-Turn (b) r Fuse. 0.25 A •—^t^!r®~^ LB LA - SAMPLING ON AT FILTER A LB - SAMPLING ON AT FILTER B Figure 3.4. Schematic ofelectronics governing instrument operation, (a) Ul (ECG74155AN) demultiplexer takes chromatograph TTL signals and produces demultiplexed outputs at pins4-7, these are inverted by hex inverter U2 (ECG 7404) and addresses gates of logic level N-Channel MOSFET switches (RFM8N18L) to turn on/off various valves via diode logic, (b) Air heater and hot air flow control. 91 Nitric Acid Nitrate o E u !Q 3 U 0) a (0 E S O) S u E c 0) u c oo i Sulfur Dioxide < A Sulfate c fl> (0 o 8/16/99 8/18/99 8/20/99 Figure 3.5. HNOs/Nitrate, HONO/Nitrite and S02/Sulfate patterns at a Midtown location in Atlanta, GA. Note nocturnal maxima in the middle panel and opposite behavior in others. 92 o o 13 (0 S ^ 3 CO o a> 2P (u re .S ^ >< X SH :: O O o ^.a. sen 9/4/ 0 O ° ^.J M O o -s emi s 2/0 0 it e c l "v " D 0) V. 15 .a O 2 XI o •fiC/3 u.*-) o l-^l CO oo e larg il y ind i ^^ o a> ^ o Si o tn a c CM X CO C/5 CO 13 S > o Ji -^ o o la £3 ^ X § O S 8/2 3 acid / isk n U (U *-R^j 'V"*"3 * 00 / r> c/i Thi : e, 0 H -o 8/ 2 hlor i ,TX . ^ S su ®g o o 0) H 1 ' T V 00 3 CO (Vj 00 ^ CiO 9\BKu ojqno jacTsujejSojDjUJ 'uouBJjuaDuoo |OSOjav pue seg b 93 o in H H X fui/SntrO'O 'i»>Bl«xo o c £uijSn sff 'a;»Jins E ?5 c C o o V3 U9l O Sis in jiu^nj-s'apuona u K g = st{is pi}< nntSiio ini!i[3 -|s laipo S (U +-» '8}iiiuoj'8iB)83y'apuaij ai O !§! s >-. o M o Ji ^—f— d 13 CO to CM q cvi qddfi^T'ZOS 13^ qtW|79io'eoMH O.E 00 E zoo oT o o E o •s 00 f? c qddrro'osDH o I qddao'cw o fo DC > 00 a o 5 (3l|/ 94 •a c 0 u o> c 0 fO o ** .0 4-u> re re re u u !C readil y re (6 ) sulfa t nif i gn i £ O) re re re E Onl y 5. 0 c E X itrate . '•B X 0 (M 0 0 0 T- ^ xperiment . onate , (5 ) n 0. 0 B J3 c>« a 0 c 5(U ?R E ^•s 0) ^C 0 t- *rH /'-s 10 1- B '^ ^ c T3 „ 0 (11 Tj Elu t 1 collec t 2 ) chlor i 0 0 ^^ 0 «3 (U m o f anae r nate , aceta t 5. 0 o n chromatogra i (I ) fluoride , fo n 0 0 ak s ien t (ij -0 0 1 ox a - 0 luo/sn 'aouepnpuoQ [^ tul S 00 b 95 1.50 -1 Unk4 Unk7 slope-1.8 slope -2.061 1.00- E 0) Unk3 slope -2.6 ^ Unk6 slope-1.1 Nitrate Slope-1.17 0.50 Sulfate Slope-1.80 arbonate slope-1.62 Nitrite slope-1.10 Cd Chloride slope .90 Unk9 slope-1.1 S 0.00 UnkS slope-1.01 o o -0.50 - Unki slope -1 •1.00 1.10 1.20 1.30 1.40 1.50 log [Hydroxide Eluent Concentration, mlVl] Figure 3.9. Log tRversus log [eluent] plots reveal charge on analytes, aiding search for a confirmatory standard. 96 CHAPTER IV CONTINUOUS ANALYZER FOR SOLUBLE ANIONIC CONSTITUENTS AND AMMONIUM IN ATMOSPHERIC PARTICULATE MATTER Introduction The health effects of particulate matter (PM) has been a subject of intense and growing discussion. ' For the most part, the available evidence is epidemiological, rather than direct and hence creates a controversy.^ PM is an umbrella term that includes different species that vary widely in chemical composition, size and toxicity.'* It is particularly important to have high temporal resolution PM monitors that provide chemical composition information, along with simultaneous information on gaseous species and meteorological data, to better understand the chemistry of aerosol formation and transport, thermodynamic equilibrium or lack thereof.^ Such information is also invaluable in performing source apportionment. Several approaches are available towards automated near continuous measurement of chemical composition of particulate matter. Mass spectrometry (MS) 7 0 has been effectively used for online real time analysis of particulate matter. " Presently MS is capable of single particle analysis down to nm size particles and provide information about particle size, morphology and composition.'° However, response is strongly matrix dependent and the results tend to be qualitative and limited by cost and the complexity. 97 More conventional chemical analysis must automate and reasonably integrate the steps of collection and analysis. Very small particles are hard to collect by impaction. The concept of growing particles with steam prior to impaction followed by ion chromatography (IC) analysis was introduced by Dasgupta et al.,'"''^'^ and almost simultaneously by Khlystov et al.'^'^ Kalberer et al.'^ and especially Loflund et al.'"' have described sophisticated systems that are largely modeled after the first design. Weber et al. ' presented a particle-into-Iiquid system that is based on the particle size magnifier design of Okuyama et al.'^'^ that also uses steam. The sample is analyzed by a dual IC system with a reported LOD of 10-50 ng/m'^ and time resolution of 3.5-4 min. Steam introduction has proven to be one of the most efficient means to grow and collect particles. Yet, available denuders do not remove NO and NO2 effectively. The reaction of steam with these gases produces nitrite, and to a lesser extent, nitrate. On a continuously wetted glass frit, Buhr et al. found higher levels of nitrate than observed on a conventional filter based instrument. The steam introduction technique involves generation, injection, and condensation; this also adds to instrument complexity and size. Attempts to obviate the use of steam have recently been underway. Boring et al. recently described a filter based automated system^^ coupled with IC for measurement of anions in PM.^'* The system uses a parallel plate wetted denuder (PPWD) and two glass-fiber filters that alternate between sampling, and washing/drying. The filter wash is preconcentrated for analysis. The filter based system has its own merits but leaching of fibers from presently used fibrous fdters leads to fouling of dovmstream components and presents problems. In addition, the filter system intrinsically operates on a batch mode. To 98 accommodate the needs of future continuous analysis systems, a truly continuous analysis system is desirable. Of PM constituents, sulfate and nitrate are of the greatest interest. Monitors that specifically monitor particulate sulfate and nitrate have been introduced. Hering and Stolzenburg^^-^^ described a system that samples air at 1 standard L/min (SLPM) through a 2.5 pm cut cyclone inlet, followed by a carbon impregnated denuder to remove the gases. The particles then pass through a Nafion humidifier and are collected by impaction on a metal sfa-ip. For analysis, the strip is directly heated electrically and the liberated gases (SO2 from sulfate, NOx from nitrate) are measured by gaseous SOa/NOx monitors.^^ A nitrate analyzer that removes NOx, collects nitrate on a quartz fiber filter, thermally decomposes the nib-ate and measures the NOx has been described by Allen et al. These researchers have also tested a system in which a sulfur gas free sulfate aerosol stream is thermally decomposed to SO2 prior to measurement by a modified gaseous SO2 analyzer. ^^ The above instruments operate on cylinder gases as the only consumable and are therefore attractive. IC analysis is attractive for a different reason: it can provide simultaneous analysis of multiple constituents. Present day IC's can also operate on pure water as the only consumable. In this vein, a simple, robust device for semi-continuous collection of soluble ions in particulate matter is developed. The collector is inspired by the designs of Cofer and Edahl^^'^' who developed a device to collect and concentrate trace soluble atmospheric gases from large volumes of air into small volumes of liquid with high efficiency by a nebulization-reflux techniques. Janak and Vecera used the 99 same principle of nebulization/reflux shortly thereafter, again for gas collecfion. A similar principle to collect particles, after prior removal of soluble gases is used here. The present device can be designed with an optional inlet that can provide a particular size cut. This PC has been extensively characterized in the laboratory and deployed in a number of major field studies. Experimental Section Particle Collector Extractor Figure 4.1a and 4.1b show the two designs of the PC investigated in this work. The PC is essentially a sealed cylindrical chamber (3 in. o.d., 2.5 in. i.d., 3.75 in. tall) made of Plexiglas to which the sample airflow is introduced through a constricted nozzle. The simpler version, shovm in Figure 4.1a, does not provide any size cut. In this design, the soluble gas denuded air stream flows straight into the PC through a Plexiglas orifice. The nozzle bearing the orifice is machined to have a smooth inner surface and a gradual taper (-75 °) without an abrupt edge. It fits snugly over a perfluoroalkoxy (PFA) Teflon inlet tube (8.75 mm o.d., 7.5 mm i.d., 1 SW, Zeus Industrial Products), that serves as the exit tube of the PPWD and connects it to the PC. The PPWD is identical to that used in chapter III.^'* DI Water is pumped peristaltically (PP5) at 1 mL/min into the PC chamber through a stainless steel capillary (0.56 mm o.d., 0.30 mm i.d., type 304 stainless steel, B- HTX-24, Small parts Inc., Miami Lakes, FL) that delivers the water to the air stream just exiting the nozzle. The water is aerosolized by the high velocity air, creating a fine mist. The mist attaches to the particulate matter in the sampled air. 100 A hydrophobic microporous PTFE membrane filter (Fluoropore FHLP, 0.5 pm pores, 47 mm dia., Millipore) constitutes the top exh of the PC. The filter rests between the cylindrical PC body and the inverted funnel shaped air suction outlet affixed together by six 4-40 threaded '/z" long stainless steel screws evenly positioned around the perimeter. To assure an airtight seal around the filter, an 0-ring, put in an appropriately machined groove on the top perimeter of the cylindrical section of the PC, provides sealing. A mesh machined in a Plexiglas disk provides back support for the filter. The water mist coalesces on the hydrophobic filter surface as large droplets. These eventually fall to the bottom of the particle collector chamber. The pressure drop needed to aspirate liquid water through the highly hydrophobic filter is large. As such, liquid water is not aspirated through the filter. The system thus behaves as a reflux condenser where the liquid refluxes from the filter. The bottom of the PC is not flat but slopes to a slightly off-center low point, much like a shower drain, such that water runs to this point. An aspiration aperture is provided at this point. Two stainless steel rods (0.064 mm dia.), placed radially across the aperture serve as a conductivity sensors. Using the conductivity probes as a simple logic sensor, the presence of water across the electrodes (high conductivity) causes appropriate electronics to turn on a dedicated one channel peristaltic pump P2 (FIA 8410, BIFOK, Sweden) to aspirate the liquid for analysis. As shown in Figure 4.1b, in lieu of using a separate cyclone, the air inlet of the PC can be designed similar to a cyclone to provide a particular size cut. The gas-denuded air sample enters the interior cylindrical chamber of the PC through a tangential inlet with 101 the interior cylinder serving as the cyclone. The cylinder ends in a 1 mm orifice at the top of a cone. A 360 ^im o.d., 250 ^m i.d. capillary tube serving as the DI water inlet comes through the bottom of the PC (affixed at the bottom plate with a compression fitting) and just protrudes through the nozzle orifice. Tvpical Field Installation The entire instrument was located inside an air-conditioned trailer. The general layout is shown in Figure 4.2. The preferred sampling arrangement involved a 6 in. PVC pipe vertically traversing the shelter, extending I m above the rooftop with a U-joint on top to prevent precipitation ingress. Underneath the shelter, a blower fan BF was attached to the PVC pipe to aspirate air @ 100-150 L/min, below turbulent conditions but with a sufficiently fast flow rate to minimize wall losses. If a wet denuder is installed before the PC, it can change the original particle size distribution due to aerosol hydration. For this reason, the PC with a built-in cyclone was not used in the field studies with the PPWD units. A stainless steel tube SI (lO.O mm i.d., 12.4 mm o.d., 26 cm long), fashioned into an approximately semicircular/U shape, breaches the PVC tube at a convenient height within the shelter such that one end of the steel tube is located at the precise center of the PVC tube, pointing upward in the direction of the incoming airflow. In experiments where total particle composition was measured, no cyclone was used and the stainless steel tube directly terminated in the bottom air inlet of the PPWD which in turn had the PC connected in top. The PPWD was strapped to the PVC conduit as shown in Figure 4.3. In experiments using this arrangement, the gas composition was 102 also measured and tube SI was lined inside with a tightly fitting PFA tube. In other experiments, where PM2 5 composition was measured, a Teflon-coated Aluminum cyclone (URG-2000-30EN, University Research Glassware, Chapel Hill, NC) C was interposed between the stainless tube inlet and the PPWD. (The principal flow stream of interest through the PP WD/PC is 5 L/min, the cyclone is designed for 10 L/min. For simplicity, the Y-joint between C and the PPWD and the auxiliary exhaust system that aspirates the balance 5 L/min has not been shown in Figure 4.3.) In this configuration, gas sampling was conducted with a different train altogether using a second denuder. This is because the loss of certain gases, notably HNO3, in the cyclone was deemed inevitable. A water trap T and a minicapsule filter MF were placed after the PC. This prevents any water condensation downstream of the PC entering the mass flow controller (MFC, model AFC 2600, Aalborg, Orangeburg, NY, O-IO SLPM). Aspiration is provided by an air pump (model DOA-P120-FB, Gast Manufacturing Corp, Benton Harbor, MI). All air ptrnips were typically located below the shelter to reduce noise in the work environment. Liquid Phase Analytical Svstem Referring to Figure 4.3, aside from pump P2, the dedicated liquid aspiration pump for the particle system, liquid was pumped using a variable speed 8-channel peristahic pump (Dynamax RP-I, Rainin, PPI-7) at a fixed pump speed of 4.5 RPM. Some of the operational details of the denuder and chromatographic systems are similar to those reported by Boring et al.^' Pharmed® pump tubing was used throughout '74-28 threaded 103 PEEK tubing adapters (PF-S, VICI). Pump lines 1-2 (1.29 mm i.d., P/N 95709-32, Cole- Parmer) feed the denuder with liquid, one on each side, @ ~1 mL/min. In most of our work, we used 0.5 mM H2O2. This nonionic liquid is compatible with the effluent being subjected to analysis by IC for determining gas composition.^"* Questions have been raised, however, about the ability of such a liquid to remove weak acid gases, notably HONO and HO Ac, particularly in the presence of large SO2 concentrations.^^ However, as shown in Figtire 4.3, the PPWD effluent in the particle sampling train is simply discarded, whenever separate dedicated denuders are used in the gas and particle sampling trains. Any liquid can therefore be used in the particle system denuder. A 0.05 M phosphate buffer in the pH 6-7 range is applicable as the scrubber liquid and is particularly effective in removing soluble basic/acidic gases ranging from NH3 through HONO to SO2 to strong acids. Pump channels 3-4 (1.52 mm pump tubing, P/N 95709- 36, Cole-Parmer, to ensure that the input liquid is completely removed) takes the denuder effluent to waste. For cases where the PPWD effluent is used for gas analysis, the considerations have been outlined in chapter III. In essence, the liquid flow rate into the denuder must be large enough under all operating conditions to keep the denuder wet at all times; however, any flow in excess of this should be avoided because of the need to pump the effluent through preconcentration columns and the upper pressure limitation of peristaltic pumping. Channel PP5 pumps house-deionized water through a mixed bed deionization column (6.7 mm i.d, 20 cm long, filled with Dowex MR-3) MB into the particle collector 104 at 1 mL/min (1. 29 mm tubing). Pump P2, actuated by the conductivity sensor, aspirates the water containing the dissolved aerosol and any undissolved solid and pumps h through a filter F (0.2 fxm, 25 mm dia membrane filter, P/N 6809-4022 Whatman) and through cation preconcentrator columns CC1/CC2 (contained in valve VI) and anion preconcentrator colunms ACI/AC2 (contained in V2), in sequence. P2 aspiration rate must be equal to or higher than that of PP5 (1 mL/min), and is typically between 1.2 - 1.8 mL/min; a significantly larger flow rate is avoided because of backpressure caused by the preconcentrator columns. CCl and CC2 are 5 x 35 mm columns (Dionex) filled with a 1:1 mixture of Dowex-50Wx8 H'^-form 200^00 mesh strong acid resin with a diluent (chloromethylated polystyrene-divinylbenzene, Bio-Beads S-Xl, 200^00 mesh, Bio- Rad Inc.). ACl and AC2 are Dionex anion preconcentrator columns that were originally custom-made for this instrument but are now commercially available (P/N TAC-ULP, 5 x 23 mm, Dionex Corp.). VI and V2 are both 10-port electrically actuated valves, respectively of the low- and high-pressure types (C22Z-3180EH, VICI; EV750-I02, Rheodyne). Pump channel PP6 (1.29 mm i.d. tube, 1 mL/min) pumps either water or 10 mM NaOH as selected by 12-V all-PTFE solenoid valve V3 (161T031, NResearch, Caldwell, NJ) through CCI/CC2 through one side of the membrane device PMD to waste. The final pump channel PP7 (0.51 mm i.d., 0.3 mL/min, Cole-Parmer 95709-18) pumps water, freshly deionized through mixed bed resin column MB (identical to that before the PC), through the other side of the membrane device PMD in a countercurrent fashion to a standalone conductivity detector CD25, a restrictor tubing R (0.125 x 60 mm), to waste. 105 Except as stated, all liquid transfer lines are 20 gauge standard wall PTFE tubing (0.86 mm i.d., 20 SW, Zeus Industrial products). Operation and Analysis Protocol Valve V4 is a 6-port low-pressure manually operated loop injector (C22Z-31EH, VICI) that is used for calibrating the system. The injection volume of the loop in this valve was carefully determined (by filling with a dye solution, injection, making up the injected material to volume, measuring absorbance and comparing with the absorbance obtained for the same solution after a known dilution) to be 35 pL. An equimolar mixttire of (NH4)2S04 and NH4NO3 at different concentrations was used to calibrate the system. During this calibration, air sampling is shut off When V4 is filled with the calibrant and switched to the inject position, P2 pumps the injected sample downstream where the ammonium is captured by CCI/CC2 (CCl is in position in Figure 4.3 as drawn). The anions pass through the cation exchanger and are captured by AC1/AC2. Placing the cation exchange preconcentrator ahead of the anion preconcentrator is important because these anion preconcentrators contain agglomerated anion exchange latex on cation exchange beads and cation exchange sites are still accessible. If the sequence is reversed, ammonium will be captured by the anion exchange column. NaN02 and Na2C204 solutions were similarly used to calibrate for nitrite and oxalate. VI, V3, PP6-7, PMD, CD25 and associated components constitute the ammonia analysis system. In principle, a second IC can provide complete soluble cation analysis in lieu of the arrangement chosen here (although it may be necessary to have respective 106 preconcentrators in parallel, rather than series, to avoid eluent counterion contamination between systems). However, ammonium is often the dominant cation of interest in atmospheric fine particles and can be determined in a simpler fashion, as in this work. The measurement of ammonitun in a sample by basification and diffusion of the resulting gaseous ammonia into a receptor stream across a membrane was originally introduced by Carlson^'*'^^ and subsequently used in many arenas, including the measurement of aerosol ammonium.'" The present work differs from extant reports in cation exchanger preconcentration and elution by a strong base. The latter elution technique is uniquely practiced for a weak base cation and is vital for preventing anion contamination in a serially connected anion chromatography system. The typical operational sequence involves two 15-min halves of a 30 min cycle. As an example, dtiring t = 0-15 min, the PC effluent is preconcentrated sequentially on CCl and ACl. At 15 min, VI-V3 all switch. CC2 and AC2 now take the positions of CCl and ACl to perform preconcentration. 10 mM NaOH pumped by PP6 elutes NH4'' from CCl as NH3 which flows through the donor side of porous membrane device PMD. The PMD is made of two Plexiglas blocks, each containing a flow channel (600 pm deep, 5 mm wide, 98 mm long) accessed with 10-32 threaded ports that serve as liquid inlet and outlet. A porous membrane (Metricel polypropylene, 0.1pm pores. Pall Corp. P/N XE20163), separates the two flow channels; a number of screws hold the blocks together. (Note that this membrane is asymmetiic and the transfer extent does differ on which side of the membrane is made the donor.) The difftised ammonia is received by the DI water flowing countercurrent on the receiver side and is carried to the 107 conductivity detector CD25. Restrictor tubing R prevents any bubbles in the detector. All indicated components as well as connecting tubing are placed inside the chromatography oven maintained at 29-30 °C. V3 switches back to water at t = 23 min to wash CCl with water such that residual NaOH is removed from it before VI and V2 are switched back at t = 30 min for CCl/ACl to begin preconcentration again. At t = 15 min, as V2 switches, chromatography begins on ACl with a 14.75 mM KOH eluent generated by an electrodialytic eluent generator EG40, the chromatographic unh (Dionex DX 600) consisting of an GS50 pump, an AGl 1-HC guard (4 x 50 mm) and ASl I-HC (4 X 250 mm) separation columns. A thermally stabilized conductivity cell (DS-3) is used in conjimction with a CD25 detector. The DS-3 conductivity cell, like the identical cell used for the ammonia system, is maintained inside an LC 30 oven. Both conductivity detector signals are acquired on an IBM laptop computer interfaced with the system through a LAN card (Linksys, Etherfast 10/100 integrated PC card) via aNetGear EN308 network hub with Dionex PeakNet 6.2 software. The cycle repeats every 30 min until deliberately shut off or until a preprogrammed number of cycles have run. System automation and valve control is achieved via PeakNet software via the TTL and Relay outputs in the chromatographic hardware. 108 Chemicals All chemicals were analytical reagent grade. Nanopure water (Barnstead, 18 MQ cm) was used to prepare all standards and eluent. H2O2 (30%) and NaOH (50%) (NH4)2S04, NaN03, NaN02, and Na2C204 were obtained from standard sources. Particle Generation Fluorescein-doped particles of different sizes were generated using a vibrating orifice aerosol generator (VOAG, model 3450, TSI Inc. St. Paul, MN). The VOAG generates nearly monodisperse aerosols. The charge on the generated particles were brought to Boltzmann charge by a Kr-85 discharger and characterized by a laser-based optical particle counter (model A22I2-0I-115-1, Met-One, Grant's Pass, OR). The general experimental arrangement and details of VOAG operation have been previously described.^^ The aerosol generator feed solution was (NH4)2S04 doped with fluorescein; all related measurements were made using a spectrofluorometer (model RF 540, Shimadzu) using excitation and emission settings appropriate for fluorescein. The fluorescein content was negligible relative to the (NH4)2S04, except for the smallest size particles generated in this manner. After inttial design experiments were completed, particle size-cutoff characterization of the final version of the PC of Figure 4.1b was conducted with standard polystyrene microspheres (Bangs Laboratories, Fisher, IN). These spheres (density 1.05) were dyed (where the dye was not extractable by water but acetone- extiactable) by equilibrating a stirred suspension of the polystyrene beads with a 109 Rhodamine-B solution. The beads were centriftiged, resuspended in water, recovered by filtration through a membrane filter and washed several times with water. To generate aerosols containing these beads, a diluted suspension of the dyed beads were used in the VOAG. The 20 pm orifice disk was replaced with a larger orifice and the liquid filter in the VOAG was removed. Particle Characterization In a VOAG, the eventual equivalent spherical diameter of the dry particle is equal to the cube root of the feed solution concentration multiplied by the primary droplet volume and divided by the dry particle density.^^ Under otherwise fixed experimental conditions, the particle size can be varied by varying the (NH4)2S04 feed solution concentration. The size of the particles computed from the VOAG operating conditions was cross checked by the laser-based particle counter data, consisting of number counts of particles in discrete size ranges of 0.1-0.2 pm, 0.2-0.3 pm, 0.3-0.5pm, 0.5-1.0pm, 1.0- 3.0pm and >3.0 pm. The geometric mean diameter was taken to be equal to the count median diameter (CMD). The mass median diameter (MMD) and mass median aerodynamic diameter (MMAD) were then calculated from the geometric standard deviation of the log normal size distribution of the aerosol, the density of anhydrous (NH4)2S04 (1.77) and including slip correction. The relevant data are reported in Table 4.1. 110 Results and Discussion PC Cyclone Inlet Design The horizontal and vertical position of the air inlet, relative to the cylindrical cyclone body, as well as its angle of entrance, affects the removal efficiency and the sharpness of the size cut. All experiments were conducted at a flow rate of 6 standard liters per minute. Predictably, the sharpness of the size cut and the coarse particle removal efficiency were better with a tangential entry than straight entry of the sampled air; all further work was carried out with the tangential entry design. With the cylindrical portion of the cyclone having a height of-35 mm and an inner bore of 18.5 mm; the tangential inlet of 4 mm bore was placed at a height of 4, 18 and 31 mm from the bottom (bottom, middle and top positions). Placing the entry at the top of the cyclone body allows more room for cyclone action and the 50% cut point observed changed from 7.8, to 6.1 to 4.9 pm from the bottom to the middle to the top position. An increase in the sharpness of the cut-off behavior was also observed in moving the entry to the top. To obtain a 50% size cutpoint (D50) in the desired 2.0 to 2.5 pm range, further changes were, however, clearly needed. Reducing the inner diameter of the cyclone cylinder and reducing the air entry ttibe diameter are both effective in reducing Dso- The chosen values for these two parameters in the final design were 12 and 2.5 mm, respectively. The penefration of size standard polystyrene particles in this device is shown in Figure 4.4. At 6 L/min, D50 for this device was 2.15. The sharpness of the cyclone, defined as (D^efD^f^ where D16 111 and D84 are the aerodynamic diameter of the particles at 16 percent and 84 percent penetration efficiency, respectively,^^ is estimated from Figure 4.4 to be 1.60. The PC with a size cut inlet eliminates the need for a separate device to provide the desired cut. This is attractive in systems where particles are of primary interest and dry denuders can be used to remove potentially interfering gases. Particle Losses in the Inlet Svstem With a wet denuder and the PC of Figure 4.1a following h, minimal particle losses prior to the PC are desired. Losses for fluorescein-doped (NH4)2S04 aerosol within the nozzle inlet of the PC alone (without the PPWD ahead of it) was found to be 0.21, 0.96, 1.29, 1.62, 2.62, and 5.25% for particles of MMAD values 0.21, 0.55, 0.99, 2.6, 4.8, and 7.8 pm, respectively (mean of two experiments). The PC hself thus exhibits very little loss of particles up to 2.5 pm size. This and the following experiment were conducted at a flow rate of 5 SLPM; this was also the sampling rate used in all field experiments. With the PPWD ahead of the PC, the particle size specification pertains merely to that entering the PPWD; the aerosol size doubtless grows upon passage through the PPWD. Indeed, as Table 4.2 shows, substantially higher losses were observed when the aerosol was first passed through the PPWD(two separate experimental runs were made). At 2.5 pm, 11-12% total loss was observed, the large bulk of the loss occurring in the PC nozzle. The nozzle was redesigned using a much more gradual, 75° taper instead of the original 45° taper, and the nozzle diameter was increased from 0.397 mm to 0.500 mm. The loss in the PC nozzle decreased to 3.6+0.2%, with a total loss in the system in 112 the 5-6% range. The growth of less hygroscopic particles will be less and total losses are likely to be lower than that observed with the (NH4)2S04 test aerosol. Testing for breakthrough of a fluorescein-doped (NH4)2S04 aerosol in the size ranges stated through the PC, was accomplished by putting a quartz fiber filter after the PC at sampling rates up to 6 SLPM. In the worst case, <0.5% of the total fluorescein was present in the backup filter extract. The PC would thus appear to be a neariy quantitative collector. Response Time and Carryover The PC operates under continuous air and liquid flow. The liquid sample coalescing on the inner walls of the PC or the filter is continuously collected and sent on for analysis. At a liquid input rate of 1 mL/min, each sampling cycle involves 15 mL of the liquid sample in and out of the PC. To evaluate the response time, generated fluorescein particles were sampled and the liquid sample was directly sent into a fluorescence detector for continuous detection. The system was allowed to sample clean air for 7 min, then the fluorescein aerosol sample was sampled for 15 min, followed by clean air again. The fluorescence signal rose to half the plateau value in 3 min and the 10-90% rise time was 5.5 min. The 90-10% fall time was slightiy longer at 6.8 min. Both were adequate for a 15 min sampling cycle. 113 Performance and Detection Limits Using electrodialytic generation and suppression of the eluent, current state of the art in IC technology, the LOD (S/N = 3) for chloride, nitrite, nitrate, sulfate and oxalate were each < O.I ng/m^ for a 75-L total sample volume (15 min at 5 L/min). This is adequate to make measurements of not just polluted urban air but of a pristine background environment. Ammonium is measured as ammonium hydroxide; the latter is a weak base and a quadratic (or higher polynomial) based calibration equation must be used for quantitation. The S/N =3 LOD for ammonium in our system was 8 ng/m^ Typical instrument outputs are shovm in Figure 4.5 for (a) ammonium and (b) anions in particulate matter using data from Tampa, FL. Note that very low levels of particulate nitrite are being measured, even though it is a relatively high NOx envirorunent. While some of the nitrite being measured may still be an artifact from the reaction between water and NOx (not removed by the PPWD), the level of artifact nitrite produced from a comparable instrument using steam is significantly higher. System Maintenance For continuous prolonged operation, periodic attention to the following items is necessary. Adsorption of organics causes the filter eventually to lose its hydrophobic character, causing water leakage through the pores. Insoluble particles slowly block the filter pores, increasing the pressure drop to an unacceptable level. In urban sampling, the first generally precedes the latter, requiring replacement in 2-3 weeks. While the system has been operated as long as 5 weeks without problems, the current practice is to replace 114 the filters as a routine procedure every two weeks. Replacement requires less than 5 min and the data from the next two cycles are discarded because of potential contamination. Peristaltic pump tubes are replaced after three weeks of continuous operation. The anion preconcentrator column (5x 23 mm) provides for low pressure and cannot be replaced witii the more common 4 x 35 mm type; this results in more frequent pump tube replacements and can cause other problems due to higher pressure drop. The membrane filter after the PC (F, Figure 3) is replaced every 4 weeks. Despite the presence of F, the inlet frh of columns CCI/CC2 can get clogged with very fine insoluble PM that passes through F, generating backpressure. These are inspected for soiling every two weeks and replaced as needed. Illustrative Field Data The system has been deployed in a number of field studies. Although comparison between conventional integrated filter measurement techniques and high time resolution meastirements such as that provided by the present instrument have the intrinsic flaw that the high temporal resolution data will have to be averaged back over a much longer period, one is always interested in these comparisons with established methods. In that vein, Figure 4.6 shows a comparison of integrated sulfate concentrations (3-, 6- or 9-h samples) measured independently by Brigham Young University researchers by their PC- BOSS system^^ with data from the present instrument, during a study in Lindon, UT in the summer of 2002. Considering that the sulfate data are all <2 pg/m^ and the problems 115 of getting good filter based measurements at low levels, the observed agreement is very good. Figure 4.7 shows two-week segments of data for nitrate and sulfate collected in Tampa, FL and Philadelphia, PA. In Philadelphia, sulfate levels are generally much higher than the nitrate levels. It will be further noted that the experimental site is probably impacted by at least two sources, one in which the sulfate and nitrate peaks are coincident in time and another in which they are not correlated. In both Tampa and Philadelphia, the levels are predictably much lower during the weekend. In Tampa, nitrate levels are substantially higher than in Philadelphia and peaks in nitrate and sulfate are much better correlated. Gas concentrations were also measured in most of the field studies. In Tampa, the average HCI concentration (0.71 ppb) was found to be nearly twice that measured in Houston, TX and four times that measured in Philadelphia. Both Houston and Tampa have elevated particulate chloride concentrations relative to more inland sites like Philadelphia or Lindon, UT. In Tampa, the pattern of HCI and particulate nitrate concentrations (Figure 4.8) strongly suggests that at least in part, HCI formation is related to nitrate formation. The particle collector data shovm in this case was from an instrument without any cyclone inlets. (The nitrate levels were very much lower when a 2.5 pm cut point cyclone was put in the line, suggesting that nitiate was in a coarse particle fraction.) These observations can be reconciled if at least in part the genesis of particulate NO3" involves the reaction of NO2 or HNO3 on moist sea-salt. 116 The acidity of the particles, in particular, the ammonium to sulfate ratio on an equivalents basis, is often of interest. Figure 4.9 shows the sulfate and ammonium concentrations for a two-week-segment of the Tampa measurements. The sulfate/ammonium ratio in equivalents is almost always greater than unity (corresponding to (NH4)2S04) and frequently greater than 2 (more acidic than NH4HSO4). The latter events are mainly associated with day time. Note that the relative high acidity events are short-lived and will not be detected by integrated measurements. In Tampa, ammonium and sulfate are all in the fine particle phase where as nitrate is predominantly found in a size greater than 2.5 pm. Thus no major errors are made in assessing relative acidity when looking at the ammonium to sulfate ratio rather than ammonium to total anions. It is also interesting to note that dtu:ing the May 11-12 weekend, except for a few hours on Sunday morning (perhaps due to religious reasons), the ratio persists at tmity, characteristic of an aged aerosol. In this context, it is also worthwhile noting that we have encotmtered situations in other campaigns where the aerosol is distinctiy alkaline, i.e., the total measured ammonium equivalents exceeds the total measured anion equivalents. In agriculturally intensive areas there are significant concentrations office ammonia measured in the gas phase. At high humidity, the aerosol has significant amounts of liquid water and ammonia is taken up therein. The present systems (or comparable steam-based collection systems) see this excess ammonia but in integrated filter samples, most of this excess ammonia evaporates. 117 References 1. Pope, C. A.; Thun, M. J.; Namboodiri, M. M.; Dockery, D. W.; Evans, J. S.; Speizer, F.E.; Heatii, C. W.; Am. J. Resp. Crit. Care, 1995, 151, 669 - 674. 2. Schwartz, J.; Environ. Res., 1994, 64 68-85. 3. Schlesinger, R.B.; Inhal. Toxicol, 1995, 7, 99 - 110. 4. Wang, H.; Shooter, D.; Atmos. Environ., 2002, 36, 3519 - 3529 5. Kitto, A. M. N.; Harrison, R. M.; Atmos. Environ., 1992, 26A, 235 - 241. 6. Air quality criteria for particulate matter. National Center for Environmental Assessment, Office of Research and Development, US. EPA. Research Triangle Park. NC; EPA/600-AP-95-I00IA, 1996. 7. Suess, D. T.; Prather, K. A.; Chem. Rev., 1999, 99, 3007 - 3035 8. Johnston, M. V.; J. Mass Spectrom., 2000, 35, 585 - 595. 9. Noble, C. A.; Prather, K. A.; Mass Spectrom. Rev., 2000, 19, 248 - 274 10. Maynard, A. D.; Philos. Trans. Roy Soc A, 2000, 358, 2593 - 2609 11. Blatter, A.; Neftel, A.; Dasgupta, P. K.; Simon, P. K.; in: Angletti and G. Restelli (Eds.), Physico-Chemical Behavior of Atmospheric Pollutants, Proc.6' European Symposium, Report EURI5609/2 EN, Luxembourg, 1994, pp. 161-111. 12. Simon, P. K.; Dasgupta, P. K.; Anal. Chem., 1995, 67, 71 -78. 13. Simon, P. K.; Dasgupta, P. K.; Environ. Sci. Technol, 1995, 29, 1534 - 1541. 14. Khlystov, A.; Wyers, G. P.; Slanina, J.; Atmos. Environ., 1995, 29, 2229 - 2234. 15. Slanina, J.; ten Brink, H. M.; Otjes, R. P.; Even, A.; Jongejan, P.; Khlystov, A.; Waijers-Ypellan, A.; Hu, M.; Lu, Y.; Atmos Environ., 2001, 35, 2319 - 2330. 16. Kalberer, M. Ammann, M., Gaggeler, H. W., Baltensperger, U. Atmos. Environ., 1999,33,2815-2822. 17. Loflund, M.; Kasper-Giebl, A; Tscherwenka, W.; Schmid, M.; GeibI, H.; Hitzenberger, R.; Reischl, G.; Puxbaum, H.; Atmos. Environ., 2001, 35, 2861 - 2869. 118 18. Weber, R. J.; Orsini, D.; Daun, Y.; Lee, Y. N.; Klotz, P. J.; Brechtel, F.; Aerosol Sci. Technol, 2001, 35, 718-727. 19. Orsini, D. A., Ma, Y., Sullivan, A., Sierau, B., BaumannK., Weber, R. J. Atmos. Environ. 2003, 37, 1243-1259. 20. Okuyama, K.; Kousaka, Y.; Motouchi, T.; Aerosol Sci. Technol, 1984, 3, 353 -366. 21. Dasgupta, P. K., Poruthoor, S. K. Pawliszyn, J. Ed., Wilson and Wilson's Comprehensive Analytical Chemistry Series, Vol. XXXVII, Elsevier, 2002, 161-276. 22. Buhr, S. M., Buhr, M. P. Fehsenfeld, F. C, Holloway, J. S. Karst, U., Norton, R. B., Parrish, D. P. Sievers, R. E. Atmos. Environ. 1995, 26, 2609-2624. 23. Samanta, G., Boring, C. B., Dasgupta, P. K. Anal Chem. 2001, 73, 2034-2040. 24. Boring, C. B.; AI-Horr, R.; Genfa, Z.; Dasgupta, P. K.; M. W. Martin and W. F. Smith. Anal Chem., 2002, 74, 1256-1268. 25. Stolzenburg, M. R.; Hering, S. V.; Environ. Sci. Technol, 2000, 34, 907 - 914. 26. S. Hering, M.R. Stolzenburg, Integrated collection and vaporization particle chemistry monitoring, US Patent, 5,983,732, November, 1999. 27. http://vyww.rpco.com/products/ambprod/brochures/brochtu'e8400n pages.pdf http://www.rpco.com/products/ambprod/brochures/brochure8400s pages.pdf 28. Allen, G. A.; Koutrakis, P.; Ding, Y. US Patent 6,503,758, January 7, 2003 29. Allen, G. A. Personal Communication, April, 2003. 30. Cofer, W. R.; Collins, V. G.; Talbot, R. W.; Environ. Sci. Technol, 1985, 19, 557. 31. Cofer,W. R.; Edahl, R. A.; Environ. ScL Technol, 1986, 20, 979. 32. Janak,L; Vecera, Z.; Anal Chem., 1987, 59, 1494 - 1498. 33. Zellweger, C; Ammann, M.; Hofer, P.; Baltensperger, U. Atmos. Environ. 1999, 33, II3I-II40. 34. Carlson, R. M.;Anal Chem.,l9n, 50, 1528-1531 35. Carlson, R. M.; US Patent 4,206,299 June 24, 1980 119 36. Hinds, W. C. Aerosol Technology; New York, Wiley: 1982, p 381. 37. Kenny, L. C; Gussman, R.; Meyer, M. Aerosol Sci. Technol, 2000, 32, 338 - 358. 38. Eatough, D.J.; Obeidi, F.; Pang, Y.; Ding, Y.; Eatough, N.L.; Wilson, W.E. Atmos. Environ. 1999, 33, 2835-2844. 120 Table 4.1. Cotmt median diameter, mass median diameter and mass median aerodynamic diameter of particle generated by VOAG with different feed (NH4)2S04 solution doped with fluorescein. (NH4)2S04 + Fluorescein Count Median Mass Median Mass Median Diameter Diameter MMD, Aerodynamic CMD, nm nm Diameter MMAD, ^m lX10"'mM+500ng/L 0.20 0.411 0.547 0.1mM + 500|ig/L 0.93 0.869 1.155 1.0mM+500ng/L 1.99 2.695 3.584 4.0 mM +800 ^g/L 3.16 4.168 5.544 8.0 mM+1000 ng/L 3.98 5.241 6.969 121 Table 4.2. Loss of aerosols in the PPWD and the air-inlet nozzle of the PC^ % Loss Mass Median Aerodynamic Diameter (pm) MMAD, pm 0.21 0.55 0.99 2.55 4.79 7.78 Dry Denuder Inlet and Outlet 0.9 1.2 0.26 1.04 2.1 2.6 1.4 2.6 0.6 1.1 4.3 1.4 Wet Denuder Plates 0 1.26 1.52 2.29 3.7 9.09 0 2.05 0.8 2.17 4.75 9.46 PC Nozzle Inlet 0.5 1.1 4.36 8.85 9.75 9.91 0 3.2 5.01 7.82 9.69 10.05 ^Two separate experimental runs are shovm. 122 Air Suction 0.25 in Air Suction (b) Water Out Air Inlet Water Inlet Air Inlet Water Inlet Figure 4.1. Particle collector with (a) straight Air Inlet (b) with cyclone-like size cut Inlet 123 Ambient Air In PVC Trailer Roof C 0MF SI Ambient Air In MFC Trailer Floor Ambient Air Out Figure 4.2. Field sampling and airflow schematic: PC, particle collector; PPWD parallel plate wet denuder; C, cyclone; SI, stainless steel ttibeinlet ; PVC, 6 PVC pipe; 1, water trap; MF, minicapsule filter; MFC, mass flowcontroller ; P, air sampling pump; BF, blower fan. 124 /\ PC .w p P5 -^M'^-^^-D^ I ] H2C Figure 4.3. Total particle collection/analysis system, air and liquid flow schematic. C, cyclone; PPWD, parallel plate wet denuder; PC, particle collector; T, liquid trap; MF, minicapsule filter; MFC, mass flow controller; P, air pump; PPl-7 peristaltic pump lines; P2, one channel peristaltic pump; MB, mixed bed resin deionizer; F, filter; CCl and CC2, cation preconcentration columns; ACl and AC2, anion preconcenfrator columns; GS50, chromatography pump; EG40, eluent generator; SRS, self regenerating suppressor; GC, guard column; SC, separation column; VI, low presstire 10 port injection valve; V2, high pressure 10 port injection valve; V3, 3way solenoid valve; V4, 6 port injection valve; S, Injection Syringe; PMD, porous membrane device; CD25, conductivity detector; R, restrictor; W, waste. 125 100—1 80 — o c .2 60 o It: HI c "I 40 0) 0) 20 — n ^ 1 ' r 2 4 6 8 Aerodynamic diameter, jum Figure 4.4. Penetration curve of standard size polystyrene beads in the particle collector with a cyclone-style inlet. 126 0.80 Tampa, FL BRACE Study May 6, 2002 1:15 PM E u 0.60 - Ammonium Ammonium (A Preconcentrator 1 Preconcentrator 2 C 0.89 Mg/m3 0.92 Mg/m3 0.40 1 8 0.20 3 •o C 8 0.00 -0.20 0.00 10.00 20.00 30.00 Time, min 1:00 to 1:15 PM, 5/6/02 Tampa, FL 8.00 (VJ a. 1 E R 3. u \ 6.00 d iT (S 1 (/) (I? c d 1 :s E I •o 3 I 5 (vi tn SI 0) •IS ^"2 4.00 o I o d Q I I a. 1 o SI 1 a. 2.00 c d S (0 3 "D C 0.00 u u 6 Y Preconcentrator 1 Preconcentrator 2 Cycle A Cycle B -2.00 0.00 10.00 20.00 30.00 Time, min Figure 4.5. Representative system output (a) ammonium response (b) anion chromatogram, over two cycles, Tampa, FL. 127 3 —I 1:1 Correspondence Line^ # # # 9-h sample CO E D D D 6-h sample "o) 2 — O O O 3-h sample IS. o 3 (0 (fi (A O QQ I 1 - O Q. 1 ' r 1 2 Present Instrument Sulfate, |ag/m^ Figure 4.6. Integrated sulfate measurements versus sulfate measured by the present instrument. The line shown is the 1:1 correspondence line, not the best-fit line. 128 # Sulfate • Nitrate 30 - 20 - CO 1 10 - 7/a/01 7/10/01 7/12/01 7/14/01 7/16/01 7/18/01 7/20/01 7/22/01 7/24/01 7/26/01 Date 20 -I • Sulfate ^ Nitrate 16 - 12 - oi 5/2/02 5/4/02 5/6/02 5/8/02 5/10/02 5/12/02 5/14/02 5/16/02 5/18/02 5/20/02 Date Figure 4 7 Sulfate and nitrate concentrations in (a) Philadelphia, PA, July 2001, and (b)Tampa, FL, May 2002. The enclosed areas are the mghttime hours (stmset to sunrise). 129 6 -1 HCI, ppbv NOj" ng/m* 4 — .C2 '•S 2 <-> c a> u c o o 2 - T I ' I I \ I \ I \ I \ I \ ' I ' \ I I 4/30/02 5/2/02 5/4/02 5/6/02 5/8/02 5/10/02 5/12/02 5/14/02 5/16/02 5/18/02 5/20/02 Date Figure 4.8. HCI and particulate nitrate patterns in Tampa, FL. May 1, 2002-May 18, 2002 130 (aeq/m^ sulfate neq/m^ ammonium sulfate/ammonium ratio r- 0.3 — 0.2 E a> 0.1 - 0 5/4/02 5/6/02 5/8/02 5/10/02 5/12/02 5/14/02 5/16/02 5/18/02 Date Figure 4.9. Sulfate/Ammonium equivalent ratio with sulfate and ammonium equivalent concentration patterns, Tampa, FL. 131 CHAPTER V SEMI-CONTINUOUS MEASUREMENT OF MAJOR SOLUBLE GASEOUS AND PARTICULATE CONSTITUENTS IN SEVERAL MAJOR US CITIES Introduction Exposure to high levels of fine particles is believed to be responsible for tens of thousands of deaths each year in the US.' Fine particles have been associated with hospital admissions from cardiopulmonary diseases and mortality.^ While fine particles come fi-ommyria d sources and contain hundreds of inorganic and thousands of organic components, fossil fiiel combustion is typically the single most important source. Secondary aerosols are formed via atmospheric reactions. In terms of mass, fine particles are composed of primarily sulfate, nitrate and ammonium ions; organics and mineral dust make up most of the rest.'*'^ The complex interaction of gases, namely that of sulfur dioxide, nitrogen oxides, nitric acid, nitrous acid, and ammonia with each other, wdth other oxidants and with photochemically generated intermediates underlies the genesis of ionic inorganic constituents in Particulate Matter (PM). Formation and transport are both subject to meteorological variables. Sulftir dioxide is predominantly oxidized through homogeneous oxidation by OH radical^ and heterogeneous oxidation by H2O2, and O3 ^ to form sulfate as an end product. The hydroxyl radical is the only significant gas phase oxidant. It reacts with SO2 to form an adduct free radical (HOSO2) which reacts with O2 to form SO3. Sulftir trioxide then 132 reacts readily v^th water forming sulfuric acid. Aqueous phase oxidation proceeds by dissolution of SO2 in water followed by oxidation with H2O2.* The overall reaction rate depends on relative humidity, sunlight intensity and concentrations of oxidants. Sulfate generated as H2SO4 reacts with gaseous ammonia to form ammonium sulfate and ammonium bisulfate.^ These secondary sulfate aerosols exist almost exclusively in the fine aerosol fraction (< 2.5 pm), and are also associated with reduced visibility problems due to their hygroscopic nature."^ Nitric acid, HNO3, is formed primarily through the homogeneous reaction of NO2 with OH radical, hydrogen abstraction by NO3 from aldehydes, or reactive hydrocarbons, or hydrolysis of N2O5. The NO2-OH radical reaction is the major source of HNO3; this takes place during daytime, whereas hydrolysis of N2O5 is the dominant nighttime source." Gaseous HNO3 reacts with gaseous NH3 to form solid NH4NO3 in an equilibrium, however, the precise value of the equilibrium constant is greatly affected by temperature and relative humidity.'^' '•^ While sulfate and ammonium exist mainly in the fine mode, nitrate exhibits a bimodal size distribution. The nitrate size distribution depends on location and meteorology. In coastal areas, coarse nitrate is typically present as NaNOs, formed by the reaction of HNO3 and NOx with NaCl sea salt aerosol. This also resuhs in significant amoimts of gaseous HCI. Nitrous acid is formed by the heterogeneous reaction of gaseous NO2 with water adsorbed on surfaces '^' '^ this reaction may also be mediated by black carbon. ' In daylight, HONO photolyzes to NO and the OH radical.'^ Nitrite in the aerosol phase can be oxidized to nitrate by oxidants,'''^° including the hydroxyl radical. 133 Several measurements of soluble ionogenic gases and their corresponding aerosol phase components have been conducted in order to establish a comprehensive database to enhance the understanding of tropospheric chemistry and gas-particle chemical and physical interactions^' in different environments.'^^''^^ High temporal resolution gas composition measurement and meteorological data acquisition has long been possible; aerosol composition meastirement with good time resolution has been difficult. Simultaneous, coordinated particle and gas composition and meteorological data with good time resolution can provide an altogether different dimension of understanding of atmospheric processes. In this chapter, data collected in field measurement campaigns latmched at or in the vicinity of fotu- major urban US cities and one suburban area are presented. All of the measurements were conducted in the summertime. This chapter focuses on data collected during TexAQS 2000 (Texas Air Quality Study, Houston, TX); NEOPS 2001 (North East Oxidant and Particle Study, Philadelphia, PA); BRACE 2002 Study (Bay Region Atmospheric Chemistry Experiment, Tampa, FL); and a measurement campaign in Lindon, UT, a suburban location, in 2002. The focus is on incidents that highlight the importance of continuous analysis in better understanding gas-particle partitioning, heterogeneous chemistry of PM formation, relations between PM growth and the precursor gases. An overview of the observed chemistry at the different sites is also presented. 134 Sampling Sites The Texas Air Oualitv Study (TEXAOS 20001 The Texas Air Quality study ^"^ took place during July and August, 2000. Houston has been cited as having numerous air quality problems; it is presently in violation of some of the national ambient air quality standards.'^^ The study was conducted to better plan for how the Houston-Galveston regional area and the state can better meet the air quality objectives. The 2000 population of greater Houston (Houston -Galveston- Brazoria) was 4.7 million, ranking lO"* in the US. The combination of heavy emissions with the coastal weather patterns adds to the complexity of Houston's air quality problems. Southeast Texas has the largest petrochemical manufacturing industry in the US. It is estimated that around 2.5 million people in Houston area are exposed to PM concentrations that exceed 15 pg/m^ (annual average).^^ Many different groups participated in TexAQS 2000. Experimenters were distributed among a significant ntimber of experimental sites. The data discussed here was obtained at Houston Regional Monitoring Site 3 (HRM3, EPA site number 48-201-0803) located dovrawind from the heavy industrial area of the Houston ship channel. The site itself is located next to a petrochemical and a chemical manufacturing complex where contributions from primary emissions can be occasionally significant. The land-sea and land-bay breezes are Oft responsible for diurnal flow reversal and alternating periods of clean and polluted air. As in most other southern cities, the most severe pollution episodes occur during the summer when generation of secondary PM peaks. 135 The Philadelphia Study The study she in Philadelphia, PA was one among a network of sites in the North East Ozone and Particle Study NEOPS.^^ The study was conducted thorough the month of July, 2001. The site was located 13 km northeast the city center of Philadelphia at the Baxter Water Treatment Facility on the banks of the Delaware River. Philadelphia lies along the northeast corridor between New York and Baltimore (-120 km Southwest of New York;-180 km Northeast of Baltimore) yet more inland (- 200 km offshore) than both, land-sea breeze patterns here has much less effect than Houston. Philadelphia- Wilmington—Atlantic City metropolitan area has a 2000 population of 6.2 million, ranking 6* in the US. The BRACE sftidv BRACE^^ was held in Tampa, Florida in April and May, 2002. There were a ntimber of experimental sites; the principal site where our instilment was located was located in Hillsborough County, near the Valrico Waste Water Treatment Plant (Valrico WWTP, Valrico, FL), 20 km West of Tampa city center and 16 km northeast of the bay. The site was in an open agricultiiral area along the predominant northeasterly wind trajectory; h is subject to local traffic emissions and occasionally to plumes from tiie Tampa Electric Company coal-fired power plants (Gannon and Big Bend plants). The Tampa-St. Petersburg-Clearwater metropolitan area has a 2000 population of 2.4 million. 136 The Lindon Study In Lindon, UT, the sampling site was located at the Lindon Elementary School, where a State of Utah air quality sampling site is also located. Lindon is 13 km west nortitwest of Provo, UT and 53 km south southeast of Salt Lake City, UT. The Provo- Orem area has a 2000 metropolitan population of 0.37 million (rank no. Ill) and the Salt Lake City - Ogden area has a 2000 metropolitan population of 1.3 million (rank no. 35). The sampling site is expected to be impacted predominately by emissions from mobile sotirces. There were no significant point sources that were expected to impact the site during the study dates in August 2002. Experimental Table 5.1 shows the different sampling locations, associated sampling periods, measured species and the techniques by which they were measured. All the listed gases (HCI, HONO, HNO3, SO2, H2C2O4, and NH3) were collected using a high efficiency parallel plate difftision denuder with 0.5 mM H2O2 as denuder liquid described in chapter III. Air sampling rate was 5 standard L/min (SLPM) throughout. The denuder liquid effluent is preconcentrated on sequential cation and anion preconcentrators. Using a 10 or 15 min cycle time, the collected ions were eluted and analyzed. Ammonium captured by the cation preconcentrator is eluted with NaOH and is passed across an asymmetric porous membrane device, which allows the ammonia from the alkaline donor stream to difftise into a deionized water receiver stieam flowing countercurrently. The conductivity of the receiver effluent was measured and provides a measure of the 137 collected ammonium. The anions were measured by a ftilly automated ion chromatography system. With tiie exception of the measurements made at Tampa, the gas and aerosol sampling trains were separate. In principle, it is possible to take the wet denuder effluent and send it to one analysis system for the measurement of the collected gases and send tiie effluent from tiie particle collector following it. This is precisely the configuration tiiat was used in Tampa, where prior available evidence indicated that nitrate may have significant presence in a coarse size fraction and no size cut inlet was implemented. Implementing a size cut, e.g., to measure PM2.5 is difficult in a single train where both gases and particles are to be measured. Implementing a device like a cyclone upstream of the denuder can lead to large losses of reactive gases, especially HN03.^^ On the other hand, incorporating the cyclone after the wet denuder does not impose a size cut on the aerosol that is relevant to the original aerosol population as the aerosol grows significantly in size dtiring passage through the wet denuder. As such, two independent trains (PPWD for gas, Cyclone-PPWD-Particle collector for PM2.5) were used whenever both gas and PM2.5 compositions were of interest. For the particle collector, in Houston, the automated alternating filter-based system^"'^' described in Chapter III was used. This system uses two glass-fiber filters that alternate between sampling, and washing and drying. The frequent washing and drying does, however, cause leaching of fibers from these filters that can lead to fouling of downstream components and thus requires significant maintenance. In all subsequent studies, a more robust and compact mist reflux system^^ that is described in Chapter IV 138 was used. Briefly, the denuder effluent airflow enters a compact Plexiglas chamber through an inlet nozzle. DI water is delivered through a capillary into the center of the airflow. The generated water mist attaches to the aerosol, which impacts on a hydrophobic PTFE membrane filter that constitutes the top of the PC and the airflow exit. Water drops coalesce on the filter and fall into a cavity equipped with a liquid sensor. The solution containing the dissolved constituents is aspirated by a pump and pumped onto serial cation and anion preconcentrator columns. With a 15 min analytical cycle and a sampling rate of 5 L/min, the limit of detection (LOD) for ammonium is 8 ng/m^ and for sulfate, nifrate and oxalate is O.I ng/m^. Results and Discussions Overview The average concentrations of PM components and gases are shown plotted in Figures 5.1 and Figure 5.2. The minimum (usually zero) and maximtim excursions are numerically shown on each bar. The median, rather than average particulate Cl" values in Houston is shown because even after washing, filter blanks in newly put in filters may contribute significantly to the measured chloride content and maximum chloride content information may also not be meaningful. Not surprisingly, sulfate, nitiate and ammonium constitute the majority of the soluble inorganic mass of the PM. The sum of the average concentiations of all soluble anions in PM was the highest in Houston followed by Philadelphia and Tampa. Conversely, total soluble anions was the lowest in Lindon; this follows closely tiie extent 139 of urbanization. The fraction of sulfate that constitutes the total measured anions (on an equivalents basis) was the lower in Houston (0.36) than at the other sites. Particulate chloride content was by far tiie highest in Houston (median 3.8 pg/m^), followed by Tampa, which averaged about a third of that in Houston and all other chloride concentrations were lower still by factors of 2-4. On the average, the aerosol was most acidic in Tampa and Lindon; in Houston and Philadelphia, the measured ammonium equivalents exceeded tiie measured anion equivalents. The Houston aerosol contained the largest amotmt of NRt"^ compared to any other sites. Some caveats may be in order regarding the data in Houston. There were other adjacent industrial sources on other sides. It is possible that because of the very close proximity of the sampling location to industrial sources, the resuhs for some of the species are not representative of the typical regional air quality. However, at the same time it is also true that many other parameters measured at this location have been indicative of highly polluted air in the region. For example, concentrations of HCHO, a secondary product formed through photochemical reactions, exceeded 25 ppbv on numerous afternoons and the maximum measured concentration exceeded 47 ppbv, 2-3 times the maximtim concentration measured in urban Los Angeles in the late '80s. ' Particulate Chloride and HCI Concentrations The high chloride concentration in Houston, substantially higher than that observed in Tampa, is all the more remarkable because not only is Houston a more inland location, PM2.5 measurements were made in Houston and TSP measurements were made 140 in Tampa (actual sampling inlet geometiy probably resulted in a size cut of-20 pm). The size cut in the particulate sampling protocol imposed in Houston would have excluded tiie majority of the sea-salt aerosol that typically will be at a larger size fraction tiian PM2,5, especially at relative humidity typical of summertime Houston. Despite the particulate chloride concentration being much higher in Houston than in Tampa, the gaseous HCI concentrations were significantly higher in Tampa than in Houston. At both sites, there is no correlation between particulate chloride and HCI (r^ values were both well below 0.01). This is to be expected because even if the genesis of HCI is connected to particulate chloride, e.g., by reactions with NO2, HNO3 or H2SO4, it is the availability of these reactants rather than the availability of particulate chloride that is likely to be the limiting factor. The close correspondence of Na* with Cl" as a fimction of particle size in the Tampa aerosol ^ leaves little doubt about the sea-salt origin of the chloride in this sample. Sodium was not directly meastu-ed in the Houston aerosol. However, the cation-anion equivalent balance in this case does not indicate that an amotmt of Na"^ corresponding to the large amount of chloride fotmd is likely. Rather, h appears likely that local sources in the immediate neighborhood of the sampling site are responsible; h is knovm tiiat one of the nearby plants is among the largest emission sources of chlorine-containing- compounds in the region and another deals with polyvinyl chloride. Some appreciation of the potential impact of local sources impacting the HRM-3 site can be gleaned from the photograph of the site in Figure 5.3. While industrial operations on the back of the 141 site are visible, not visible are indusfrial operations to the left of the photograph and on the back of the camera location. Sulfur Dioxide and Sulfate The rate of conversion of SO2 to S04^' is a function of multiple factors, most importantly the concentration of oxidants, sunlight intensity and relative humidity. The relative ratio of sulfate aerosol to SO2 in a pitune is indicative of the age of the plume. Air masses that impact a sampling site come from different sources, have had different processing histories and are of different age. For most of the data in the present chapter, meteorological data are available. It is in principle possible to calculate back trajectories of the air masses and discuss each significant case individually. This is, however, beyond the scope of the present chapter. Nevertheless any significant degree of correlation between SO2 and sulfate shows the genesis relationship between the species; this correlation will increase as the air mass arrives with a mean transport time close to the mean half-life for the conversion of SO2 to sulfate. A positive correlation (p) between the gas and particle phase exists in all sites (pTampa= 0.21, pHouston = 0.28, pphiiadeiphia = 0.46). Tampa has distinct episodes where the air mass originates from the open ocean or elsewhere, e.g., from further south in the State. Philadelphia had tiie highest average mass of sulfate among the four cities. The average sulfate concentration in Philadelphia is 1.57 and 1.39 times that in Houston and Tampa, respectively. This is not directiy associated with the precursor SO2 levels measured in these locations. In fact, the SO2 level is slightly higher in Houston and only intermediate in Philadelphia. This lack of direct 142 association between SO2 and S04^" levels in different locations, in addition to the their significant correlation tiiat exists in Philadelphia may be due to the location of Philadelphia in tiie Nortiieast corridor and being subject to a photochemically more developed air mass. Figures 5.4, 5.5 and 5.6 show a representative one-week plot of SO2 and S04^" concentiations in each tirban location. It can be clearly seen from the figures that the best correlation between SO2 and S04^" exists in Philadelphia. Figure 5.4 shows a clear diurnal pattern for both SO2 and S04^' in Philadelphia with the daily sulfate maxima lagging that of sulfur dioxide. SO2 levels start increasing between 6:00 and 8:00 am reaching their maximum levels at around 9:30 am, while sulfate levels reach maximtim at around 3:00 pm. The observed sharp increase and decrease in SO2 concentration seems associated with the rush in traffic expected each morning. In accordance with either gas phase or aqueous phase SO2 oxidation by OH radical or H2O2, respectively, smoother and more gradual increase and decrease is observed for sulfate levels than for SO2. Gaseous SO2 supplied to the atmosphere is removed principally by three processes: direct scavenging in precipitation, oxidation to aerosol sulfate with subsequent deposition by vertical and horizontal precipitation, and 'dry' deposition. The rates of these removal processes, which vary with environmental conditions, along with the transport velocity must be known in order to understand the fate of SO2. In a typical summer day tiie -5/: estimated lifetime for SO2 in the atmosphere is about 1.5 days. In Houston, however, the maximum SO2 concentration occurs at night while the sulfate maximum precedes it by few hours (Figure 5.5). This seems in accordance with 143 tiie argument presented before that the site is located in an industrial area with heavy local nighttime SO2 emissions from nearby sources (flaring in petrochemical industries is notoriously carried out late at night and nocturnal inversion may also help trap the plvune). In Tampa, sulfate and SO2 exhibit patterns with muhiple spikes observed during the day (Figtire 5.6). The site is predominantly affected by local traffic, however, occasionally plumes from coal power plants passed directly over the site and were detected by the instrument as can be observed by the fact that the maximum measured concentiation of SO2, SO4 ' and HNO3 were measured in Tampa (Figure 5.2 and Figure 5.1). The pattern of sulfate in Lindon is similar to that of sulfate in Philadelphia (Figure 5.7). Despite the much lower concentration, a relatively clear diurnal pattern is observed. Nitious Acid. Nitrite. Nitiic Acid, and Nitrate Table 5.2 shows the day and night correlation values among N03', N02', HONO, and HNO3. The mean NO2' and HONO concentrations are higher tiian the respective mean NO3" and HNO3 concentrations in Philadelphia. The ratio of the average N02' to NO3" concentrations and HONO to HNO3 concentrations are 1.27 and 1.32, respectively. This close ratio in the particle and gas phase associated with the relatively high concentiations of both HONO and N02" is not observed in the other tiiree locations. Also a far more significant positive correlation exits between N03' and HONO in Philadelphia than in Houston or Tampa. Due to the expected nighttime abundance and rapid daytime photolysis of HONO, such a correlation with HONO suggests tiiat the concentration of nitiate is higher during nighttime than daytime. Indeed the ratio night/day concentration 144 of nitiate in Philadelphia is 2.57 while that of nitric acid is 0.33. At nighttime the formation of NO3" has been reported to occur due to hydrolysis of gaseous N2O5 on wet surfaces and aerosol particles to form aqueous HNO3. ^^ N2O5 is formed at night by the reaction of nitiate radical NO3 with NO2. In turn, NO3 radical is formed by the oxidation of NO2 with ozone. Thus the formation of nitrate aerosols in Philadelphia is dominated by nighttime formation.''^ While in Tampa, Houston, and Lindon the nitrate seems to be dominantly formed dtiring daylight via OH radical. Figure 5.8 and Figure 5.9 show the pattern for gaseous HONO and HNO3, and particulate NO3' and NO2" in Philadelphia, respectively. Nitrate does exhibit a nocttimal maximum associated with that of HONO in Philadelphia. This can be seen very clearly dtiring the night of July 16/17 when the concentrations are higher than those of previous days. Furthermore, the diurnal variation of both gases and particles are well resolved, but unlike NO3", NO2', and HONO; HNO3 shows a daytime maximtim typically occurring between 1:00 and 3:00 PM. The pattern of NO2", NO3", and HONO are broadly similar but HONO shows the most variation. The significant nighttime correlation between HONO, N02', NO3" may suggest that gaseous NO2 is high and more liquid water is available due to condensation. Indeed the heterogeneous reaction of NO2 with H2O adsorbed on surfaces or aerosols produces HONO(g) and aqueous HN03.^^ Also, both HONO and NO2" can be oxidized in aqueous particles to form NO3". However, it is more likely that the nighttime formation of N03' is due to the hydrolysis of N2O5. Unlike in Philadelphia, NO3" has an insignificant nighttime correlation and daytime correlation with HONO in Houston. The diurnal pattern appears more clearly for 145 tiie gases than tiie particles; however, an increase in daytime nitrate can still be clearly seen in Houston. The lowest measured average concentration of HNO3 is in Tampa. The average concentiation of nitiic acid in Tampa is less than half that measured in Philadelphia or (Figure 5.2) Houston; however, the average concentration of nitrate is more than double that in Houston and three times higher than that in Philadelphia or Lindon (Figure 5.1). In Tampa, a significant correlation exists between overall (day and night) HNO3 and total NO3" (p=0.44). Since overall NOx concentrations are not that disparate, this strongly suggests that HNO3 is being converted to particulate nitrate in Tampa. Indeed, the high average concentiation of total NOs" is due to the formation of lutrate on coarse sea salt particles by the reaction of HNO3 (and possibly NO2) with NaCl. This is discussed in greater detail in a later section. The coordinated variation between nitrate and nitric acid is obvious in their pattern. The close diurnal pattern can be clearly seen in Figure 5.12 between May 7 and May 11,2002, as well as on the afternoon of May 13, 2002. Notice also the simultaneously low levels of nitiate and nitric acid on the days between May 7 and May 13. Figure 5.13 shows nitrite and nitrous acid levels in Tampa. Both nitrite and nitious acid levels are relatively low, but HONO shows strong interesting variations between day and night. Notice the gradual increase in nitrous acid concentration as the night progresses and the relatively sharp drop in the morning. Nitrate and Nitrite levels like otiier PM levels are low in Lindon, however a stronger variation and clearer diurnal pattern is seen for nitrate than for nitrite (Figure 5.14). 146 Observation of High PM p,nH Tr^ce Gases F.pi.nHes in Philadelphia During tiie NEOPS study, three major events of high PM and trace gases were observed. The first and second episodes occurred on July lO'Vd July I7'^ respectively and were relatively brief, lasting for only one day. However, the third episode started on July 22"", and lasted till tiie 26"^. During this episode strong diurnal pattern for both PM and gases were observed and the highest levels were measured on the 25"'. Figure 5.15, Figure 5.16 and Figure 5.17 show tiie variations of N03', S04^", SO2 and HONO3 during tiie first second and tiiird episode, respectively. The wind direction and solar radiation for tiiese episodes are shown in Figure 5.18. All those episodes were strongly correlated with a south, southwest wind, which brings the air mass from the city center to the study site. The second episode, which took place between July 17 and July 18, serves as a good representation of the other two episodes. July 17 started with a northern wind associated with low levels of pollution. Just after midiught the wind became southeast blowing a different air mass over the site. A sharp increase in SO2, S04^" and NO3' levels was observed that lasted until early morning hotirs. The close similarity in the concentration profiles of SO2, S04^", and NO3' in the early part of the night suggests that these species have originated from the same sotirces and/or has been simultaneously photochemically processed during the previous day. By morning hours, the wind direction became from the southwest. The correlation between gas and particle concentrations, specifically between SO2 and SO4 ' immediately deteriorated. While sulfate maintained its high nighttime level of-15 pg/m^ SO2 levels increased sharply exceeding 30 ppb at 9:00 am before dropping sharply at noon. This is 147 probably associated witii tiie local morning emissions of SO2, especially since the wind was blowing from tiie city center to the site. S04^" and HNO3 are associated with photochemical activity, thus increased rapidly during daytime and reaching their maximum levels in the afternoon. The next day was dominated by a northeriy wind associated with substantially lower levels of gases and particles. This relation between wind direction and elevated levels of PM and gases can be seen on an extended scale in the last episode. The episode was longer, lasting 4 days, and associated with a rectirring ditimal pattern with incremental levels. Nitrate/Chloride Replacement on Sea Salt Particles in Tampa. FL Recent studies of size resolved particle analysis in Tampa Bay has revealed the predominant existence of nitrate in the coarse PM size fraction and sulfate in fine PM size fraction.''^ The average PM2.5 nitrate composhion measured in Tampa from May I to May 9, 2002 is 0.29 pg/m^ while the average TSP nitrate composition is 2.09 pg/m^ for the same period. However the average fine and total sulfate for the same period are 5.18 pg/m^ and 5.58 pg/m^ respectively. The PM2.5 were measured by different instrument tiiat has been developed by URG Corp. The instioiment uses steam to grow and collect particles. The large difference between the average total and fine nitrate fraction is attributed to the reaction of gaseous HNO3 or other NOx/NOy species with particle surfaces and compounds thereon. The most significant of these reactions is tiiat between HNO3 and NaCI(s, aq) in sea salt particles which resuhs in the production of HCI(g). Indeed the highest average HCI concentration was measured in Tampa. In addition, the 148 correlation between HNO3 and HCI is significant (p- 0.734) reflecting the direct relationship between reaction of HNO3 and liberation of HCI gas. The correlation between NO3" and HCI is 0.35. Despite being significant it is smaller than that between HCI and HNO3. This may be atfributed to formation of coarse nitrate through other documented reaction patiiways such as the reaction of NO2 with NaCl.'*^ Figure 5.19 shows representative one -week patterns of HCI, HNO3 and N03' in Tampa. The close correlation in the pattern of HCI and HNO3 can be cleariy noted in the figure. The relative concentration of fine and coarse nitrate and the scarcity of fine nitrate in Tampa are related to the different nature of nitrate in the fine and coarse PM fraction. Fine NO3' is predominantly NH4NO3, formed by the reaction of NH3 and HNO3 and requires a certain partial presstire product of NH3 and HNO3 to exist. The reaction is reversible, thus relating the existence of fine nitrate to sufficient abundance of ammonia, which in turn is related to the acidity of fine particles and the level of sulfate neutralization. In Tampa the ratio of sulfate equivalents to those of ammonium is more than unity, i.e., the aerosol is acidic at the level between NH4HSO4 and (NH4)2S04. Under these conditions if nitrate were present as NH4NO3, HNO3 would form and be driven into the gas phase and in turn will react with sea salt aerosol to form coarse NaNOs. Thus the lack of sufficient ammonia for complete neutralization of sulfate, in addhion to the abundance of sea salt NaCI may be behind the almost exclusive presence of nitrate in the coarse PM fraction. Figure 5.20 shows the patterns of HCI, Cf and % relative humidity (RH) in Tampa. An inverse variation between HCI and relative humidity is clearly observed in the 149 figure witii HCI maximum occurring at RH minimum. The degassing of formed HCI from sea salt particles depends on relative humidity. Thermodynamic calculations predicted that 90% of the initial HCI concentiation is lost from droplets at relative humidity less than 97%, but under extremely humid conditions HCI will not be depleted from large droplets.'*^ The abundance of HCI gas suggests that relative humidity was not sufficiently high to prevent the degassing of HCI from the particle phase. Ammonia. Ammonium, and PM Neutralization Semi-continuous measurement of NH3 and NH4 has a particular advantage in eliminating significant errors associated with long term collection. Underestimation of NH3 and overestimation of NILt"^ can be caused by absorption of NH3 to the collection medium itself or the already collected particulate matter. Absorption of NH3 to acidic aerosols has been reported in the determination of H2S04,'^'' The opposite can happen as well. A presstire drop over the collection medium as well as changes in humidity, temperature and pressure during sampling, might change equilibrium condhions for NH4NO3 aerosols and cause evaporation of NH3.'*^ Such errors are significantly reduced by reducing the residence time of particles and gases on the collection medium. The ratios of the total measured anion equivalents to ammonitim equivalent are 0.77 and 0.61 in Houston and Philadelphia, respectively. Figure 5.21 and Figure 5.22 show a plot of the meastu-ed ammonium equivalent, total measured anion equivalents, and measured NH3 levels in Philadelphia and Houston, respectively. In Philadelphia, the ratio of the total measured anion equivalents to ammonium equivalent is biased by tiie 150 values of tiie last few days of the study, specifically, from July 18 till July 30. During tiiis period the measured equivalent ammonium is significantiy higher than that of total measured anion equivalents and this can be observed in Figure 5.21 as well. In fact, the ratio of the total measured anion equivalents to ammonium equivalent is 1.23 and 0.37 for tiie periods from Julyl to July 18 and from July 18 to July 30, respectively. In the latter period the excess ammonium may be due to the uptake of anmionia by aerosols having significant amounts of liquid water in a high humidity environment. The present system can see tiiis excess ammonia but in integrated filter samples, most of this excess ammonia evaporates. Or it may be due to association of ammonium with organic anions in particulate matter, which may be significant during that period. In Houston, ammonia from petiochemical sources may be significant and it is very likely that it is being taken by water containing aerosols. Figure 5.21 and Figure 5.22 reveal the close association between the equivalent concentrations of ammonium and total meastired anions. The correlation between the total anion equivalents and that of NIL,"^ are 0.49 and 0.30 in Philadelphia and Houston, respectively. Furthermore, consistent with previous indications that the air mass meastired in Philadelphia is relatively more aged than that in Houston, the correlation between gaseous NH3 and UlU*' is higher in Philadelphia than in Houston (pHouston= 0.144, pPhiladelphia= 0.34). In Tampa both nitrate and chloride are associated with sea salt particles rather than being neutralized by ammonium. Thus, sulfate remains the only predominant anion to be neutralized by ammonia. The equivalent ratio of sulfate to ammonitim in Tampa is 1.09. Though, total sulfate was measured, sulfate is almost entirely present in fine 151 particles and seems to be associated mainly with NH4^ rather than Na* or Mg* present iin coarse sea salt particles. Figure 5.23 shows the equivalent sulfate and ammonium and ammonia levels measured in Tampa. Notice the coordinated variation in the levels of ammonium and sulfate. A ftirther indication of the strong association between sulfate and ammonium is their high correlation (p= 0.82). Figure 5.24 shows a plot of equivalent ammonium versus equivalent sulfate in Tampa. The majority of the points lie in the region between NH4HSO4 and (NH4)2S04 suggesting that sulfate is only partially neutialized by ammonium. In Lindon, the correlation between equivalent ammonitim and total anion equivalents is (p == 0.62), but when only equivalent sulfate and nitrate are correlated with eqtuvalent ammonium the correlation increases (p = 0.71). The equivalent ratio of the total measured anions to ammonium is 1.79, suggesting that among all locations the most acidic particles are measured in Lindon. However, the equivalent ratio of only nitrate and sulfate to ammonitim is 1.19. The difference is largely due to the significant equivalent contribution of chloride relative to sulfate, nitrate and ammonium. Chloride constitutes 11% of the equivalent anionic composition of PM in Lindon and may be associated with other cations rather than ammonitim. Figure 5.25 shows the equivalents of sulfate + nitrate vs. the equivalents of ammonitim in Lindon. The close time-coordinated variation of anions and ammonium can be clearly observed, especially at the higher concentrations. 152 Conclusion Fifteen minute measurements of inorganic soluble gaseous and particulate constituents in 3 urban and 1 suburban locations in the United States are presented. The data among different locations and among gases and PM constituents were compared and correlated. . Among all locations the concentration of PM was highest in Philadelphia and lowest in Lindon. S04^" levels were compared to precursor SO2 levels in each location and the correlation between the two was measured in each site. In Houston, localized pltunes with significant concentrations of SO2 observed during nighttime impacted the site location. The predominant formation of coarse nitrate on sea-salt NaCl particles in Tampa was specifically investigated and the levels of HNO3 were correlated with the production of HCI gas. The acidity of particles and extent of neutralization by ammonium was also studied. In Houston and Philadelphia the ammonium equivalents exceed those of sulfate nitrate chloride and oxalate. Particles are slightly acidic in Tampa and Lindon. 153 References 1. Kaiser, J.; Science, 2000, 289, 22-23. 2. Pope, C. A.; Thun, M. J.; Namboodiri, M. M.; Dockery, D. W.; Evans, J. S.; Speizer, F.E.; Heath, C. W.; Am. J. Resp. Crit. Care, 1995, 151, 669 - 674. 3. Wang, H.; Shooter, D.; Atmos. Environ., 2002, 36, 3519 - 3529. 4. Saxena, P., Hildemann, L. M., J. Atmos. Chem., 1996, 24, 57 - 109. 5. John, W.; Wall, S. M.; Ondo, J. L.; Winklmayr, W.; Atmos. Environ., 1990, 24A, 2349 -2359. 6. 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Ammann, M.; Kalberer, M.; Jost, D.T.; Tobler, L.; Rossler, E.; Piguet, D.; Gaggeler, H.W.; Baltensperger, U. Nature, 1998, 395 , 157-160. 154 18. Zellweger C; Ammann, M.; Hofer, P.; Baltensperger, U. Atmos. Environ., 1999, 33, 19. Koutrakis, P.; Wolfson, J. M.; Bunyaviroch, A.; Froehlich, S.E.; Hirano, K., Mulik, J. D. Anal Chem., 1993, 65, 209-214. 20. Geyh, A.S.; Wolfson J.M.; Koutrakis, P.; Mulik, J.D.; Avol, E.L.; Environ. Sci. Technol, 1997, 31,2326-2330. 21. Chow, J. C; Watson, J. G.; Lowenthal, D. H.; Egami, R. T.; Solomon, P. A.; Thuillier, R. H.; Magiliano, K.; Ranzeiri, A.; Atmos. Environ.. 1998, 32, 2835 - 2844. 22. Tanner, R. L.; Parkhurst W. J.; J. Air & Waste Manage. Assoc, 2000, 50, 1299 - 1307. 23. Brook, J. R.; Dann, T. F.; Burnett, R. T.; J. Air & Waste Manage Assoc, 1997, 47, 2-19. 24. http://vvfv^fw.utexas.edu/research/ceer/texaqs/ 25. Cooke, G. A. Federal Register, 67 (148) (2002) 49895-49897. August I, 2002 26. http://uts.cc.utexas.edu/-gcarch/HoustonSuperSite/ 27. http://www.cgenv.com/Narsto/ 28. http://www.hsc.usf edu/publichealth/EOH/BRACE/Bracelink.html 29. Li-Jones, X.; Savoie, D.L.; Prospero, J.M..; Atmos. Environ., 2001, 35, 985-993. 30. Boring, C. B.; Al-Horr, R.; Genfa, Z.; Dasgupta, P. K.; M. W. Martin and W. F. Smith. Anal Chem., 2002, 74, 1256-1268. 31. Samanta, G.; Boring, C. B.; Dasgupta, P. K.; Anal Chem. 2001, 73, 2034-2040. 32. A Continuous Analyzer for Soluble Anionic Constituents and Ammonium in Atmospheric Particulate Matter, R. Al-Horr; G. Samanta; P. K. Dasgupta. 33. P. K., Dasgupta; S., Dong; and H., Hwang; Aerosol Sci. Technol, 1990, 12, 98-104. 34. Lawson, D. R.; Biermann, H. W.; Tuazon, E. C; Winer, A. M.; G. I., Mackay; Schiff, H. I.; Kok, G. L.; Dasgupta, P. K.; Fung, K.; Aerosol Sci. Technol, 1990, 12, 64-76. 155 35. Campbell, S. W.; Evans, M. C; Poor, N. D.; Atmos. Environ., 2002 36, 4299^307 36. Finlayson-Pitts, B. J.; Pitts Jr., J. N.; Chemistry of The Upper and Lower Atmosphere: Theory Experiments and Applications; San Deigo: Academic Press, 2000, Ch. 8, 296 -297. 37. Detener, N. M.; Crutzen, P. J.; J. Jeophys. Res., 1993, 98, 7149 - 7163. 38. Wayne, R. P.; Barnes, I.; Biggs, J. P.; Burrows, C. E.; Canosa-Mas, C. E.; Hjorth, J.; Le Bras, G.; Moortgat, G. K.; Pemer, D.; Poulet, G., Restelli, G.; Sidebottom, H.; Atmos. Environ., 1991, 25A, 1-203. 39. Lammel, G.; Cape, J. N.; Chem. Soc Rev., 1996, 25, 361 -369. 40. De Bock, L. A.; Van Malderen, H.; Van Grieken, R. E.; Environ. Sci. Technol, 1994, 28,1513-1520. 41. Ro, C; Oh, K.; Kim, H.; Kim, Y. P.; Lee, C. B.; Kim, K.; Kang, C. H.; Osan, J.; Hoog, J. D.; Worobiec, A.; Grieken, R. V.; Environ. Sci. Technol., 2001, 35,4487- 4494. 42. Weis, D. D.; Ewing, G.E.; J. of Phys. Chem. A, 1999, 25, 103, 4865-4873 43. Clegg, S. L.; Brimblecombe, P. Atmos. Environ., 1985, 19, 46, 5-470. 44. Koutrakis, P.; Thompson, K. M.; Wolfson, J. M.; Spengler, J. D.; Keeler, G. J.; Salter, J. L.; Atmos. Environ., 1992, 26 A, 987-995. 45. Forrest, J.; Tanner, R. L,; Spandau D. J.; D' Ottavio T.; Newman, L.; Atmos. Environ., 1980, 14, 137- 144. 156 Table 5.1. Sampling locations and available measurements Location Sampling Gases^ PM System Period Houston, TX August 12 - HCI, HONO, PM2 5 (N03", PPWD + TexAQS 2000 September 25, HNO3, SO2, N02-, S04^", PPWD- 2000 H2C2O4, NH3 €204^", NH4^) altemating filter/automated IC Philadelphia, PA July 1-30,2001 HCI, HONO, PM2.5 (NO3-, PPWD + NEOPS HNO3, SO2, N02', S04^", PPWD-Mist H2C2O4, NH3 €204^", NH4*) Reflux Automated-IC Tampa, FL April 26-May HNO3, HONO, TSP (NO3", PPWD-Mist BRACE 2002 30,2002 SO2, HCI, NH3, NO2 , S04^-, Reflux C2O4H2 €204^", NH4*) Automated-IC Lindon, UT August 1-30, PM2.5 (N03-, PPWD-Mist 2002 N02', S04^", Reflux C204^", NH4*, Automated-IC Cl) 157 Table 5.2. Day and night correlation of NO3", NO2", HONO, and HNO3 measured in fotir cities Houston, TX Philadelphia, PA Tampa, FL Lindon, UT Day Night Day Night Day Night Day Night Correlation 0.16 0.21 0.18 0.25 0.11 0.21 HNO3, NO3 Correlation 0.41 0.044 0.32 0.041 -0.040 0.084 HONO, NO2 Correlation -0.061 -0.095 0.33 0.29 0.057 0.19 HONO, HNO3 Correlation 0.042 0.14 0.17 -0.044 -0.12 0.09 0.012 -0.05 NO2, NO3 Correlation -0.19 -0.14 0.56 0.38 0.14 -0.39 NO:", HNO3 Correlation 0.045 -0.012 0.63 0.44 0.35 0.026 NO3, HONO 158 •'3 96U-9Z0 J 3 h c ZIP 900 I see - ZOO Q. 0 .2 E X E 901^- ZOO < c -I 0 980-frOO L CO 0) h '•p 999- 0 C (0 B Q. 0 C8C-300 I 0 X X 893-0 -I s 0 o c frZ-9-ZOO -J c h 0 9-8C-990 (_ Q. a, oe-frzoQ a X (A .S 0) 0 C 0) i/i 9S€ - l£ k c 4-) u _o 0 u 3 o 0) J c I ZZ 8 - UOO o 0 h 39Z-C1.0 I "^ (0 o 5 Q. a 90Z - UOO o 1.9-9-ZWO E X Q. (0 c _0 o u t^o- uoo 0) C9"6- 900 z !E 0) 3 a Cl-C- 300 0 c (0 0 "3 tZP - ZOO T5 a 3 (0 0 I a •a a u jUiyen 'uoBBJiuaouoo 00 159 6S^-0 [ h d) „ 303-^1.0 [ 9^ X 6C6 - S€i0 [ X z O e^o - 0 [ h o (0 600-o[ Q. c a 0 (53 0 SSO-0 [ z 'S n O o u CM 0 X 889- 300 [ 0) c etc-zoo [ •o cd c 00 a0 tn 0) I I.66 - kOO o 0 C9Z - 300 I o o Cl 0 si-e-coo I o •c0 z VPL - WO n I X •0 u cu 0) o 3frC- 0 3 u (A SO-fr -900 [ (0 CO 0 0 £ 3C - ZOO L (0 z 0) 0) 0 n X 0 a I.ZI.-300 [ h •a X D. I- 9k-0 Q. 'a I O X Q. I- X X u > < 1 ' I >ri (L> Aqdd 'uoji&i^uaouoo ap 160 •> a o o 00 c o XI u o >, 13 ii C/3 -a 1 T3 ca c CJ o _o (L> Q S 00 161 H O (0 (M o in o CM o CO < 0) PL, 13 M CM OH H o a, CM •T3 ur e ^ u H O au ^ T3 o CM ox i :3 o 00 H 3 1 .UJ/3ri 00 162 o o X I- c X o H (/) d 3 o o X O (U (0 i N 3 O U ca (L> o 3 I 3 iri in u o o o to o 00 00 PI ,uj/§n 163 CM O 00 u> o CM m in CM o a> S^ o cf 1-1 (0 *>. ir> H c CM o e *>> a% >«. m a, u CM o a >*. 00 u m O CM 3 o I C+H 3 in C/3 lU/Srl op 164 CM O 00 00 CM o 00 CM o 00 o •o CM o in 00 O (0 CM o O 00 CM "5 O ^ H rH t^ ^ c 00 •To3 C CM KJ o ,c ^ CM C/1 iH ^ u 00 iOH CM o ^ su r iH ca iH (U ^ 00 ua o o o o o +-• o q q q q 3 od to d C/D t-j iu/3ri in Iui 3 00 b 165 C/3 o < u Q. a I op Q. "S •o (0 ca i u 0) V3 c u m 0 0 z Z 0 1 I •.a CL, o § o o q 00 (M in H (U gUi/Sn >-, op 166 -^ Nitrate Philadelphia, PA 8.0 -1 -^ Nitrite 4.0 0.0 7/12/01 7/13/01 7/14/01 7/15/01 7/16/01 7/17/01 7/18/01 7/19/01 Date Figure 5.9. Pattern of NO2" and NO3" in Philadelphia, PA. The enclosed areas are the nighttime hours (sunset to sunrise) 167 o a I 00 u ca i/i ca 0) T3 ii Vi 13 O s=i ii a H >< H o c/3 O O g o e i PL, op 168 o o i^ Oi c« V. 7^rr^<.'t C/3 o o o CO 00 0) O o a o a —**CnV9 ^ ^ 00 ii •^-» o o o to 0) CO 13 o o u o u ® 0) • • ^V» m H >< Oi H C o o ^-» o CO 3 O *_» • Z A» > 3 • ^ o o o o z Ct- o o o CM PL, 0> q o m (M a u CO ;-( O 3 UJ/SrI 00 169 CM O H N. in CVJ o •s. ri o in XI CM ii o a CM 00 rH in ii •3 CM a CO o ca a T3 in 0) ii (U Vi a u3 E V. ii (0 O H 1- in a> PL, CO ca" CM o ra t z o X z in O CM o 00 in o CM o ii rs i in PL, O o o o o CM 00 (NJ * in a u, o 3 00 ui/Sri 170 o XI ii a I 00 •a -3 a ca cCOa ii ii Vi O (0 1 a ii X E H I- J Z -^ H z z ' rs o o z o C*-( o E u i a, ,UJ/Srl in i 00 PH 171 (M O OO CO eh o (M a o tti X H^ 00 X 3 00 1 N u O 10 u H ^ s 00 dar e N a so ViO in H en d 00 Q) a 4^ (0 XH N O 0 H N. D ^ 3 O 00 •a CoM 3 s m H 3 oo o o (M H N o OO z o O ii PL, 00 op .UJ/3rl 172 giu/Sri o CN 3 3 •-> cd X 13 1 X a. e OH o z o o qdd in in i op 173 elU/Srt O o o o CN oo" o 12 o 3 0) H "s rs >—>3 2 X o _&, o 13 X oV OH 3 7/1 8 at e Q patt e o z o -o o o IS H •s fs O V3 o Vi in qdd Si 3 op 174 gUi/Srl 175 • Wind Direction 360.00 —1 Solar Radiation 1000.00 I — 500.00 IS — 0.00 1000.00 ^ 500.00 O (0 & j5 0.00 1 1 1 1 1 1 1 1 1 7/17/01 7/18/01 7/19/01 ^ Wind Direction « 360.00 —I Solar Radiation V 270.00 — c c o S 180.00 —Y /\>^iW'4Wrr'Ty Q 90.00 — •o c 0.00 I ^« f^" (B S 0..0 0 -| 1»—1 1 1 1 1 \ 1 1 1 1 1 1 I I I I I I I I T" 7/21/01 7/22/01 7/23/01 7/24/01 7/25/01 7/26/01 7/27/01 Date Figure 5.18. Wind direction and solar radiation in Philadelphia during high PM and tiace gases episodes. 176 CM O CO in CM o CM in CM o in CM o a o E H (tj in 1- 3 d)V A (0 0) PL, m t; ca" o SJ CM o Xo z .t: X z OO H .3 in Vi CM iiB ): o rs i «s I in O CM o z CO in oCM Ss m o in o o o o OS cvi d >n ,uj/3n i op 177 A)!P!UjnH aAUcpd % PL, ca OH H 3 e1/3 3. 0) fl] 0) _ o > Z U £ GC Z O ,B 13 o o X o CN ;Ui/§n >n S op Pu 178 qdd '^HN < PL, d 13 B X PH < .3 Q. 3 O ta 0) u •o o fl] 3 o o X z z 3 > 3 u 3 3 > 3 O B o H CN in a 3 ui/baT^ 00 179 qdd '^HN X H d ^-o» o CO o 3 CO o X 0> 3 o o o CO s. o 0> 3 o o o X o z s, 0> 0) o (0 o z^-» CM 3 0> 3> 3 o cy o ii H 3 u 3 00 > 3 o a* o a 3 (3) O CM CO ca o o o H rs CN CN CM 180 qdd '^HN pL, ca" H .3 3 O •^ 0) CJ 3 O o X z z 3 > 3 3 (Q 3 Q. > fl) 3 ii 3 O 3 o H (^ CN iun i-3i .uj/barl op 181 Tampa, FL 0.80 —I Ammonium Bisulfate Ammonium Sulfate 0.60 — E O" 0) 0.40 — (0 <^ 3 V) 0.20 — 0.00 0.00 0.20 0.40 0.60 0.80 Ammonium, jxeq/m^ Figure 5.24. Equivalent anmionitmi versus equivalent stilfate in Tampa, FL 182 CM O o d esg o 00 3 '3. _S CM 3 o O oo ta H oo o 3 o M o O m 0) (0 H (0 z 00 Q CM o 3 H 3 CO > 3 CM a o 3 CM 3 H > CO (L> 3 O CM o o cd H ^-» o 00 H >n CN in u -ui/barl 3 op 183 CHAPTER VI SUMMARY AND CONCLUSIONS Environmental policies and regulations have always spurred hot debates for their enormous socioeconomic implications. When the Environmental Protection Agency (EPA) set standards for fine PM in 1997, the agency acknowledged that the uncertainties associated with setting standards for particles relative to other gaseous pollutants are significantly higher. Despite a major increase in PM related research over the past few years, these major uncertainties remain. Atmospheric modeling is helpful in explaining or predicting atmospheric events but often it does so with a wide range of uncertainty and large number of asstunptions. The context of this research was to provide tools that scientists, as well as practitioners of atmospheric analysis can use to measure species contributing to atmospheric pollution. There is no argtiment about the need for systems that can automatically measure chemical composition of PM and of the precursor gases with high temporal resolution. Beside providing a better understanding of the chemistry of gas and aerosol formation and transport, such measurement is also cost effective and does not suffer from problems associated with long term collection; such as particle evaporation, gas-particle interaction, and particle-particle interaction on the collection media. 184 Two Dimensional Detection in Ion Chromatographv The recent commercial availability of electrodialytic eluent generators capable of producing highly pure hydroxide eluents, which lead to nearly invariant backgrounds even with gradient elution, makes two-dimensional ion chromatography (2DIC) more attiactive tiian ever before. The work described in chapter II elaborates on previous studies that utilized base introduction after a conventional suppressed IC. It differs from other work in that passive, rather tiian electrodialytic base introduction is used, requiring no electronic control. After suppressed conductometric detection of an electrolytically generated hydroxide eluent and an electrolytic suppressor, the eluent is passed into a membrane device where potassium hydroxide (KOH) is passively introduced into the eluent stieam using Dorman forbidden leakage. The background conductance, measured by a second downstream detector, is typically maintained at a relatively low level of 20 - 30 pS/cm. Weak acids are converted to potassium salts that are fully ionized and are detected against a low KOH background as negative peaks. Further, different commercially available membranes have been studied in different physical designs and in different thickness with different bases to determine the optimtmi conditions so that resuhs as good as the best of the previous electrodialytic base introduction efforts can be realized in a simpler maimer. Device configurations investigated include a planar 2- channel device, a tubular device and a filament filled helical (FFH) device. The FFH device provides more effective mixing of the penetrated hydroxide with the eluent stream resuhing in a noise level < 7 nS/cm and a band dispersion value of less than 82 pL. 185 In conclusion, 2-D IC in hs presentiy developed form is simple to implement and practice. Aside from improving the detectability and response linearity characteristics of weak to very weak acids, it provides a wealth of information that is otherwise difficult or impossible to obtain. 2-D data can be exploited for diagnosis of co-elution and performing universal calibration. It can be used for the estimation of analyte pKa values and the calculation of analyte equivalent conductance , both as means of identification. However, user-friendly software that can fiilly utilize the 2-D data is needed for the complete exploitation of the technique. Recent advances in the understanding of ion exchange devices in ion chromatography may even make possible 3-D detection schemes (HX, MX, MOH). However, even the present state of development provides a very useful tool to the interested user. Measurement of Acid Gases and Soluble Anions in Atmospheric Particulate Matter Using a Parallel Plate Wet Denuder and an Alternating Filter-Based Automated Analysis Svstem Chapter III describes a fitlly automated instrument for the meastirement of acid gases and soluble anionic constituents of atmospheric particulate matter. Soluble gas collection is accomplished with a parallel plate wet denuder (PPWD). The denuder liquid effluent is then preconcentrated on anion exchange preconcentrator colunms, and then analyzed by IC. In a second independent chatmel, a new instrument collects particles in a fully automated procedure. The system mimics the standard procedure for the determination of anion composition of atmospheric aerosols. A cyclone removes large particles and the aerosol stream is then processed by a second wet denuder to remove potentially 186 interfering gases. The particles are then collected by one of two glass fiber filters which are alternately sampled, washed and dried. The washings are preconcentrated and analyzed by IC. The instrument provides high sensitivity and allows analysis of anions in aerosol in only a fraction of the time and cost of conventional techniques. A wide range of aerosol constituents can be determined by simply changing the analytical technique used to analyze the filter extract. Detection limits of low to subnanogram per cubic meter concentrations of most gaseous and particulate constituents can be readily attained. Ftuther, an attempt to decipher the total anionic composhion of urban particulate matter by IC with on-line confirmation by MS revealed the complexity of particle's compositions. Several organic anions were identified and quantitated, most commonly, formate, acetate, oxalate, lactate, glycolate, malate, and malonate. A Continuous Analvzer for Soluble Anionic Constituents and Ammonitmi in Atmospheric Particulate Matter The filter based instrument described in chapter III is field worthy and has been extensively field-tested. However, leaching of fibers from presently used fibrous filters has led to fouling of downstream components of the analytical system. In addition, the filter system intrinsically operates on a batch mode. To accommodate the needs of future continuous analysis systems, a truly continuous analysis system is desirable. Thus, A new continuous soluble particle collector (PC) has been developed. Described in Chapter IV; this device does not use steam, and avoids the problems associated with fibrous filter leaching. The PC is essentially a sealed cylindrical chamber (3 in. o.d., 2.5 in. i.d., 3.75 in. taII).This compact collector permits automated collection and continuous extraction of 187 soluble anions and ammonium in atmospheric particulate matter. The PC is mounted atop a parallel plate wetted denuder for removal of soluble gases. The soluble gas denuded air enters the PC through an inlet. One version of the PC contained an integral cyclone-like inlet. For this device, penetration of particles as a fimction of size was characterized. In the simpler design, the sampled air enters the PC through a nozzle and deionized water is pumped peristaltically at 1 mL/min into the PC chamber through a stainless steel capillary that delivers the water to the air stream just exiting the nozzle. The water is aerosolized by the high velocity air, creating a fine mist. The resulting water mist attaches to the aerosol, which impacts on a hydrophobic PTFE membrane filter that constitutes the top of the PC and the airflow exit. Water drops coalesce on the filter and fall below into a purpose-machined cavity equipped witii a liquid sensor. The water and the dissolved constituents are aspirated by a pump and pumped onto serial cation and anion preconcentrator columns. Ammonium captured by the cation preconcentrator is eluted with NaOH and is passed across an asymmetric membrane device which allows the ammonia from the alkaline donor stream to difftise into a deionized water receiver stream flowing countercurrently. The conductivity of the receiver effluent is measured and provides a measure of ammonium. The anions on the anion preconcentrator column are eluted and measured by a fiilly automated ion chromatography system. The total system thus provides automated semicontinuous measurement of soluble anions and ammonium. With a 15-min analytical cycle and a sampling rate of 5 L/min, the limit of detection (LOD) for ammonitim is 8 ng/m'^ and those for sulfate, nitrate and oxalate are <0.I ng/m^. The system has been extensively field tested. The system has been extensively operated in several field studies, averaging 94% data capttire (not including calibration or maintenance) which indicates instrument robustness and reliability. Although only the ammonium among soluble cations has been measured, the system can be configured with an additional ion chromatograph to measure other major soluble cations. In principle, a second IC can provide complete soluble cation analysis, however, it may be necessary to have respective preconcentrators in parallel, rather than in series, to avoid eluent counterion contamination between systems. Semi-Continuous Measurement Of Maior Soluble Gaseous And Particulate Constituents In Several Maior U.S. Cities The data collected in four field studies held in Houston, TX; Philadelphia, PA; Lindon, UT; and Tampa, FL; using the above described systems is presented in chapter V. Sulfate, nitrate and ammonium constitute the majority of the soluble inorganic mass of the PM. Among all locations the concentration of PM was highest in Philadelphia and lowest in Lindon. Concentrations of different gases and ionic constituents of PM were compared and correlated. The correlation between S04^" and SO2 levels was also highest in Philadelphia. In Houston the site location was impacted by a fresh air mass with significant concentrations of SO2 observed during nighttime. Particulate chloride concentrations were highest in Houston, but gaseous HCI concentrations were highest in Tampa. This in addition to the large difference between the average total and fine nitrate fraction measured in Tampa was attributed to the reaction of gaseous HNO3 or alternatively NO2, NO3 or N2O5 with coarse sea salt particles. A significant correlation between total measured equivalent anion PM composition and equivalent ammonium 189 exits in all location. However, The ratios of the total measured anion equivalents to ammonium equivalent varied significantly among locations. The data collected provide a wealth of information that is of tremendous value. For most of the data presented, meteorological data are also available from other participants in the studies. In principle, it is possible to calculate back tiajectories of the air masses and discuss each significant case individually. Conclusion The systems described in this research were fully automated and possessed a degree of robustness adequate for field deployment. The measurement was based on a 15- min cycle for collection and analysis. The current temporal resolution was mainly limited by the chromatographic separation. Future effort directly involved with these systems will be focused on developing significantly faster analysis allowing for even higher temporal resolution while maintaining adequate sensitivity and limits of detection. While the scope of this research constitutes an important contribution to atmospheric measurement of gases and particles, it was mainly limited to the measurement of soluble inorganic gases and inorganic ionic composition of particulate matter. Measurement of organic gases and organic species present in PM is another even more challenging and interesting dimension of atmospheric analysis. Organic compounds constitute a large fraction of the total chemical composhion of atmospheric particles. Present available methodologies and instrumentation are unqualified for such a task. In recent years, mass spectrometers that have the ability to provide real time measurement 190 of tiie chemical composition of a single particle has been developed. However, these instruments are fairly expensive and currently not suitable for reliable quantitative analysis. The development of less expensive alternative instrumentation that can provide more reliable quantitative real-time analysis of organic gases and organic composition of PM, will be among the future projects that I would like to research. There is significant interest in developing systems with a capacity to detect bio- agents for early detection of airborne bacterial and viral contamination. This year the U.S. government is proposing 6 billion dollars for a bioshield program. A significant portion of it will tmdoubtedly be spent on developing necessary early detection technology. Again, The cost and complexity of mass spectrometry provide an opportunity for developing less expensive and more specific technology. The tmcertainty of any ambient air analysis is largely affected by problems associated with the instrument inlet. Losses of gases and particles in the system prior to collection are among the most common problems. Uncertainties remain even if the instrument was carefiilly characterized and calibrated with the appropriate gases or particles. This is because inlet losses depend on factors like humidity, temperature; in addhion, to the relative concentration of gases and density and composhion of particles measured which are often variable and hard to predict. Therefore, my fiiture work will certainly involve developing gas and particle system inlets that will have a high degree of flexibility, but will eliminate or at least decrease the level of gas or particle loss within. 191 Finally, In the past few years miniaturization has been the trend of many chemical applications. It would be particularly interesting to develop miniattirized systems that can provide similar analysis. 192