Measurements of Halogen and Peroxy Radicals by Chemical Amplification

Cristian M. Mihele

A thesis submitted to the Faculty of Graduate Studies in partial fulfilment of the requirements for the degree of

Doctor of Philosophy

Graduate Programme in Chemistry York University Toronto, Ontario February 1999 National Library Bibliothèque nationale du Canada Acquisitions and Acquisitions et Bibliographie Services services bibliographiques 395 Wellington Street 395. rue Wellington Ottawa ON K1A ON4 Ottawa ON KIA ON4 Canada Canada

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Cristian M. Mihele

a dissertation submitted to the Faculty of Graduate Studies of York University in partial fulfillment of the requirements for the degree of

DOCTOR OF PHILOSOPHY

01999 Permission has been granted to the LlBRARY OF YORK UNIVERSITY to lend or seIl copies of this dissertation, to the NATIONAL LIBRARY OF CANADA to microfilm this dissertation and to lend or seIl copies of the film, and to UNIVERSITY MICROFILMS to publish an abstract of this dissertation. The author reserves other publication rights, and neither the dissertation nor extensive extracts from it may be printed or otherwise reproduced without the author's written permission. Abstract

In this work, two halogen radical detectors were developed by modiQing the

chernical amplifier for ROx radical measwements (ROx =HOz+ ROz + OH + RO). The halogen radical detectors are based on converting the halogen radicals into peroxy radicals, which are then measured with the ROx radical amplifier. The CIOx radical detector (CIOx = OC10 + Cl0 + CI) does not have the sensitivity required for ambient measurernents. Furthermore, the ROx radical amplifier is sensitive to CIO, radicals. The

BrOx radical detector (BrOx = Br0 + Br) is capable of measuring very low concentrations of bromine radicals.

Measurements of bromine radicals, in conjunction with measurements of photolysable halogens, were made during the Polar Sunrise Experiment 1997 at Alert,

Canada. These measurements were used to bring new insights about the spring Arctic ozone depletion phenornenon.

The performance of the ROx radical detector was fiirther improved and the water vapour effect on this instrument was studied. Ambient water vapour was found to have a major impact on the sensitivity of the ROx radical amplifier by decreasing the chah length. Since this instrument is usually calibrated in the presence of less than 1% of the ambient water vapour concentration, this study shows that the ROx radical ambient measurements made using this analytical technique have been severely underestimated.

The main cause is due to increased wdl losses for HO2 radicaIs with increasing hurnidity.

One possibility to minimise the humidity effect is to heat the walls of the reactor and to reduce the residence time in the reactor. Ambient ROx radical measurements, collected dunng an oxidant study in

Northern Michigan (F'ROPHET 97),were corrected to take into account the humidity effect on the chah length. A box model was developed and constrained accordhg to the arnbient measurements of CO, 03,NO, NO2, PAN and isoprene. The output of this model for ROx radicds was compared with the radical measurements. The agreement between the model and measurements suggests that our current understanding about rural tropospheric chemistry is quite advanced. 1 would like to thank the following people:

Dr. D.R. Hastie for his supervision, wonderfbl advice and his ability to find the time and

the patience to discuss ideas, many of them not presented in this dissertation for

obvious reasons.

Dr. M. Mozurkewich and Dr. L.A.Bante, members of my supervisory cornmittee, for

very useful discussions during my research evaluations.

Dr. M.C. Arias, my lab partner, for very productive discussions and getting me started

with the chernical amplifier.

Dr. G.A. Impey for his help during Our collaborative for PSE 97 and for being a good

fiiend and a challenging opponent on the tennis court.

Dr. S-M. Li for an advance copy of the CREAMS modelling package.

Dr. KG.Anlauf for the ozone data for PSE 97.

Al1 the "PROPHETers" for support during the Michigan study and for allowing me to use

their data.

Al1 the CAC members for helping me 'see the lights' of atmospheric chemistry, which

sometimes seemed covered by thick clouds.

My parents who had the courage to immigrate to Canada.

My wife Andreea for her warm love, help and support. Table of contents

1 Introduction 1.1 The importance of peroxy radicals measurements 1.2 The importance of tropospheric halogen radical measurements 1.2.1 Halogen radicals in the chemistry of marine boundary layer 1.2.2Haiogen radical during Arctic ozone depletion episode 1.3 Measurement techniques for tropospheric radical measurements 1.3.1 Chernical amplifier for ROx radical measurements 1.4 Objectives of this research

2 Description of the chernical amplifier for ROx radical measurements 2.1 NO2 detector 2.2 Reagents 2.3 HOz Calibrations 2.4 Instrument operation and data acquisition 2.5 Safety considerations

3 Analytical method development 3.1 Development of the chlorine radical detector 3.1.1 The principle of the chlorine radical detector 3.1.2Modelling the chemistry of the chlorine radical detector 3.1.3 Calibration source for chlorine radical detector 3.1.3.1 OC10 Preparation 3.1.3.2 Quantification of OClO in the gas phase 3.1.4 Instrument adaptation for chlorine radical measurements 3.1.5 Experimental evaluation of the chlorine radical detector 3.1.6New advances in understanding the chemistv related to the chlorine radical detector 3.1.7 OClO as a ROx radical source 3.1.8 Conclusions for the chlorine radical detector 3.2 Development of the bromine radical detector 3.2.1 The principle of the bromine radical detector 3.2.2Bromine radical source 3.2.3 Conversion agent for BrOx to HOx transformation 3.2.4 Linearity of the bromine amplifier 3.2.5 Calibrations of the bromine radical detector 3.2.6 Conclusions for the bromine radical detector

vii 4 New improvements for the ROr radical detector 57 4.1 Measurement of the amdo~~,ozratio for the radical source based on water photolysis 4.1.1 The importance of measuring G~~o~~,~~ratio 4. t .2 Theoretical aspects for a~2&~f,()2 detennination 4.1.3 Experimentai for a~z&EF,~2determination 4.1.4 Results for CTH~~CTEF,OZdetennination 4.2 The effect of arnbient water vapour on the radical amplifier 4.2.1 Methods for studying the humidity effect on the ROx radical amplifier 4.2.2 Results for the humidity effect on the chah length of the radicai amplifier 4.2.3 Measurement of the heterogeneous wall losses for peroxy radicals 4.2.3.1 Experirnental set-up for measuring the wall losses for HO2 radicals 4.2.3.2 Sources for CH302and CH3CH302radicals 4.2.3.3 Wall loss rate coefficients for HO2, CH302and CH3CH302 radicals 4.2.4 The effect of increased NO concentrations on the radical amplifier's sensitivity to relative humidity 4.2.5 Modelling of the radical amplifier sensitivity to water vapour 4.3 Optimisation for the operation of the radical amplifier 4.3.1 Enhancement of the Signal to Noise Ratio 4.3.2 Minimisation of the water vapour interference

5 Polar Sunrise Experiment 1997 5.1 Site location 5.2 Instrumentation deployed 5.3 Field operation for the brornine radical detector 5.4 Results and Discussion 5.5 Conclusions for the BrOx measurements during PSE 97

6 PROPHET 97 field campaign 6.1 Site location 6.2 Instrumentation deployed 6.3 Configuration of the radical amplifier for PROPHET 97 6.4 Data analysis for ROx radical measurements made during PROPHET 97 6.4.1 ROx radical data for PROPHET 97 6.4.2 Theoretical considerations for ROx correlation with the square root of [O3]*UVB Radiation

viii 6.4.3 Interpretation of the evening radical levels 6.5 Modelling the PROPHET 97 ROx data 6.5.1 Mode1 description 6.5.2Modehg the total radical concentration 6.5.3 Modelling studies for ROx sensitivity 6.5.3.1 Modelling studies for ROxsensitivity to isoprene 6.5.3.2Modelling studies for ROxsensitivity to ozone 6.5.3.3Modehg studies for ROxsensitivity to NOx 6.5.4 Modeliing studies on quantification the secondary radical sources 6.6 Radical levels estimated using Pseudo Steady State Approximation 6.7 Local ozone production for PROPHET 97 6.8. Conclusions for PROPHET 97

7,Conclusions and Future Directions 7.1 Conclusions 7.2 Future directions 7.2.1 Further testing of the brornine radical detector 7.2.2 Further development of the chlorine radical detector for measurernents in the Arctic troposphere 7.2.3 Further deveiopments of the radical detector for HO2 and ROz measurements 7.2.4Modelling studies for PROPHET 97 7.2.5Improvement of the ROx radical detector

Appendix 14

References 15 List of Tables

TabIe 1.1 Measurement techniques for tropospheric radical measurements 1 Table 3.1 6FJO2]~/6~02]~for different NO and CO concentrations 4 Table 3.2 Chain length determination for bromine radical detector 5

Table 4.1 Detemination of ~~2d~~~,~~ratio 6 Table 5.1 Unsaturated compounds present in the Arctic troposphere 9 Table 6.1 Instruments used during PROPHET Summer Intensive 1997 10 Table 6.2 Rate coefficients for the self- reactions of the peroxy radicals 11 Table 6.3 Cornparison between observed and calculated net ozone production for PROPHET 97 13 Table Al Rate coefficients for the reactions needed to mode1 the chemistry of 14 the ROx radical amplifier Table A2 Rate coefficients of the reactions added to the mode1 for the ROx 14 radical amplifier to study the chemistry of the CIO, radical detector List of Figures

Figure 2.1 Field configuration for the chemical amplifier Figure 2.2 The HOz radical source based on water photolysis at 184.9 nm, Figure 2.3 Data acquisition for the chemical amplifier Figure 3.1 Proposed configuration for the chiorine radical detector Figure 3.2 Mode1 results for the chain length of the chlorine radical detector; [CO]=i O%, [Cl0]inP~=4pptv Figure 3.3 The Cl0 source and the experimentai setup for measuring [OC101 by NO titration . Figure 3.4 The 4-way valve for ethane modulation used for the chlorine radical detector Figure 3.5 Chain length determinations for the OC10 source Figure 3.6 Effect of ClNO addition to the chemical amplifier operated in 3 different modes Figure 3.7 Proposed configuration for the bromine radical detector Figure 3.8 Bromine radical source based on Br2 photolysis at h2 330 nm Figure 3.9 Br to HOn transformation using propene as conversion agent Figure 3.10 Dilution method for bromine radicals Figure 3.1 1 Linearity of the bromine radical detector

Figure 4.1 ~02cross section and lamp emission spectra operated with Iess cooling (A) or more cooling and plasma voltage reduced by - 3 (B) in arbitrary unit s Figure 4.2 Detemination of oH20/cEF,02for [H20]=2100 ppmv using the 33 mm ID photolysis cell; the regression lines were forced through zero Figure 4.3 Experimental configuration for studying the ambient water vapour efTect on the ROx radical amplifier using the radical source based on water photolysis at 184.9 nm Figure 4.4 Relative humidity effect on the chah length of the RO, radical amplifier. Figure 4.5 RH effect on the chernical amplifier for three different reactors Figure 4.6 Absolute humidity effect on the chain length of the RO, radicai amplifier 7 Figure 4.7 Experimental setup for measuring HO2 wall losses for wet or dry conditions 7 Figure 4.8 Rate coefficients for radical wall losses 7 Figure 4.9 Effect of increased PO] on the RH effect on the ROx radical amplifier 7 Figure 4.10 The observed CLwet/CL~,(solid symbols) compared with those predicted by the mode1 (open symbols) for FJO]=12pprnv and [N0]=2ppmv 7 Figure 4.1 1 Humidity effect on the amplifier with 0.3 s residence time in the !4" reactor 8 Figure 4.12 PAN interference for the chernical amplifier with the reactor heated 8 Figure 5.1 Measurements of ground level03, HOBr, Br2 , BrOx and W radiation during PSE97 at Alert, Canada (tick marks represent midnight local 9 tirne). Figure 5.2 Cornparison of 03, Br2, HOBr, BrOx and W radiation during the ozone depletion episode 9 Figure 6.1 ROx radical data for PROPHET 97 (the tick marks correspond to midnight, local time, EDT) 10 Figure 6.2 ROx correlation with UVB*[O31 and square root of UVB*[03] for July 3 1 and August 2 10 Figure 6.3 Sensitivity of [ROJ correlation with ([o~]*uvB)"~ to a and P ratios 1 1 Figure 6.4 PO.] and (UVB*[03])''~ for PROPHET 97 (the tick marks correspond to rnidnight, local time EDT) 11 Figure 6.5 Daily profiles of [ROx], RH and ([o~]*uvB)'" for July 3 1 and August 2 11 Figure 6.6 Mixing ratios of 03,Cs&, CO, PAN, NO and NO2 for July 3 1 during PROPHET 97 12 Figure 6.7 Mixing ratios of 03, CO, PAN, NO and NO2 for August 2 during PROPHET 97 12 Figure 6.8 Mixing ratios of Os, CsHs, CO, PAN, NO and NO2 for August 6 during PROPHET 97 12 Figure 6.9 Modelled (open triangles) and measured RO, (closed circles) for July 3 1, August 2 and August 6 during PROPHET 97; the error bars include the estimated errors associated with RH correction, chain length determination and the measurement of GNO2. 12

xii Figure 6.10 Modeiied (open squares) and measured H202(closed squares) for July 3 1 and August during PROPHET 97. 12 Figure 6.11 ROx sensitivity to isoprene for July 3 1 12 Figure 6.12 Mode1 output for OH, HN03 and HCHO for July 3 1: circles for base case and squares for [CsHs]=O case. 12 Figure 6.13 ROx sensitivity to ozone for July 3 1 12 Figure 6.14 ROx sensitivity to NOx for July 3 1 12 Figure 6.15 Secondary radical sources compared with July 3 1 base case (open triangles) 13 Figure 6.16 Cornparison between measured ROx (dark circles) and ROx calculated using PSSA (open triangles) 13 Figure 6.17 Sensitivity studies of RO, levels estirnated using PSSA for August 7 13 Figure 7.1 Proposed sampIing inlet for the radical detector 14 1 Introduction

1.1 The importance of peroxy radical measurements

In the troposphere, ground level ozone concentrations have been increasiag: measurements performed almost 100 years ago at Montsouris, near Paris, indicated average ozone concentrations of about 10 ppbv (Volz md Klei, 1988), while the current average ozone concentrations in the most unpolluted parts of the globe are in the 20-40 ppbv range. More alarmingly, the atmospheric community is confionted with a severe regional air poliution issue: ground level ozone concentrations fiequently exceed the maximum acceptable air quality objectives (82 ppbv for one-hour average in Canada and

80 ppbv for 8 hour average in USA). These high ozone episodes occur in the summer and are associated with slow-moving, high-pressure air masses.

The toxic effects of high ozone episodes have been extensively documented. It is now known that ozone, a major constituent of smog, is adversely affecting human health.

High levels of ambient ozone are associated with health effects ranging fiom small lung changes to increased hospitalisations for respiratory ailments (Bates, 1995a, 199Sb).

Aiso, high levels of ozone have damaging effects on vegetation. Ozone penetrates the plants through open stomata and then affects a large number of cellular processes that cm fead to foliar pathologies. Since the high ozone episodes occur in the summer, a reduced growth for the plants is observed and severe agricultural losses are caused. For example, crop losses in the Southern Ontario region are estimated as $45M per year (MOE, 1989).

The main process responsible for these regiond high ozone episodes and also for the increase in ozone concentrations in the background troposphere, is in-situ ozone production. In the fust step, hydroxyl radicals are produced by the ozone photolysis at wavelengths shorter than 320 nm and reaction of the excited o('D) atoms with water:

O3 + hv -t o('D) + 02 (RI-1)

H20+ o('D) + 20H (RI -2)

Subsequently, OH radicais start the photooxidation of volatile organic compounds

(VOC), producing peroxy radicals (RO2, where R denotes a part of an organic molecule)

VOC+ OH+ R+Hz0 (R1 .3)

R+O2+M+RO2+M -4)

In the presence of nitrogen oxides (NO,= NO+ NO2) this process ultimately produces ozone:

RO2 + NO + RO + NO2 (RI .5)

RO + O2 j R'CHO + HO2 (R1.6)

HO2 + NO + OH + NO2 (RI.7)

2{NO2+hv~NO+0} -8)

2(0+02+M+ 03+M) (RI -9)

Net: VOC + '402+ 2hv + 203 + R'CHO + Hz0

The above tropospheric ozone production mechanisrn is very simplified. The term R'CHO includes aldehydes and ketones. These compounds (also part of the VOC family) are either fbrther oxidised by OH radicals to finally yield CO2 and H20, or are photolysed (especiaily the aldehydes) to generate more radicals. The VOC oxidation is complete only when the number of CO2 molecules generated is equal to the number of carbon atoms present in the starting VOC molecule. VOC is just a generic term and

includes thousands of chemical species, with different reactivities towards the OH

radicals. A tremendous arnount of work has been done to estimate the yield of ozone per

rnolecule of each individual VOC. These yields vary considerably from one VOC to the

other. Therefore, even if a VOC is present in a srnall concentration in the troposphere, it

may produce significant amounts of ozone.

The precursors (VOCs and NO,) and the oxidation product (ozone) have lifetimes in the order of a few days. As the air mass moves over different land areas, it is enriched in precursors either simultaneously or sequentially, ozone production starts and the damaging effects are more often seen downwind of the sources. This implies that a solid understanding of both ozone production chemistry and transport processes is required to fiilly characterise these high ozone episodes and to design strategies for reducing the occurrences of these episodes.

One possibility to differentiate between chemistry and transport is to study only the fast processes. Due to their high reactivity, ROx radicals (ROX=H02+RO2+OH+RO), have very short lifetimes (minutes or seconds). Therefore, ROX radical measurements dong with 03,NOx and VOCs measurements provide a rigorous test of Our cument understanding of the ozone production chemistry in the absence of transport.

Peroxy radicals (HOz+RO2)also play a key role in the ozone production, being responsible for the net ozone production by oxidising NO to NOz (RI .5 and RI .7).

Peroxy radicals are the main constituents of the ROX radical family ([RO,] s [HO2] +

[ROz]). By measuring the concentrations of RO, radicals, we have a new tool to investigate if the ozone is produced at the sampling site or upwind of the site. In conjunction with NOx measurements, RO. can be used to calculate the local ozone production (Arias and Hastie, 1996).

1.2. The importance of tropospheric halogen radical measurements

1.2.1Halogen radicals in the chemistry of marine boundary layer

In the last decade, the chemistry of the marine boundary layer has received increasing attention. There is mounting evidence that chlorine radicais CIO. (ClOp CIO

+ Cl) and bromine radicals BrOx (BrO,= Br0 + Br) may be active participants in the chemistry of the marine boundary layer. Over the oceans, modelling studies have shown that chlorine radicals may be responsible for 20-40% oxidation of the main non-methane hydrocarbons (NHMC) over the oceans (Singh und Kasting, 1 988). This is because chlorine radicals are more reactive than OH radicals towards hydrocarbons.

The marine boundary layer (MBL) stretches fiom the ocean surface up to about lkm altitude and includes a significant part of the atmosphere. It is characterised by high numbers of aerosols rich in Cl- and Br'. In these conditions, photoactive halogen compounds (HOX, X2,XNO, )(NO2, etc.) could be released from aerosols by one of the various mechanisms presented in section 1.2.2. The rnechanisms for the release of photoactive halogen species in MBL are still under scientific investigation, but measurements of these species have been reported, implying that the halogen chemistry is active in MBL.

Measurements of inorganic chlorine compounds at Virginia Key, Florida (Pszenny et al., 1993) have show the presence of photoactive chlorine compounds in the

MBL. The measured inorganic chlorine species were HCL*(which includes HC1 CINO3,

CINO2 and CINO) and ~12'(CI2 and HOCI). The concentrations of the less photoactive

HCI* were in the range of 144 to 268 pptv, while concentrations of the more photoactive

~12'were in the range of 15 to 254 pptv (reported as Cl). cl; increased during the night and decreased in the moming, while HCI' had an opposite trend. Therefore, the authors concluded that an unknown chlorine source is responsible for the cl; accumulation over night.

More recent night-time Cl2 measurements (Spicer et al., 1998) were recorded at a coastal site in Long Island, NY using an atmospheric pressure chemical ionisation mass spectrorneter. The measured Cl2 concentrations were in the range of 10 to 150 pptv.

Using these Cl2 measurements and a simple photochernical box model, the presence of an unknown Cl2 source that produces up to 9.5' 1o4 molec cm') s-' was suggested.

The halogenated photoactive compounds are then photolysed, releasing halogen radicals, which contribute to the oxidation of hydrocarbons in addition to OH radicals.

Based on the previously mentioned measurements, cldorine radicals may be present in the MBL troposphere at concentrations up to 1.3* 10' molec/cm3(Spicer et al., 1998) or

0.3-1 * 1O' molec/cm3 (Pszenny et al., 1993). Since it is well known that chlorine atoms are more reactive than the OH radicals by at least an order of magnitude for most MC

(Atkinson et al., 1997), the oxidative capacity of the MBL may be significantly affected, or even dominated, by chlorine radicals.

The halogen radicals may also &ct the ozone concentrations in the ME3L. The non-perturbed MBL is characterised by low levels of NOx; in this case, a HOx catalysed

cycle (HO, = HOz+OH) is responsible for ozone destniction in the MBL as shown by the

following reactions:

Net : 203 -+ 302

If halogen radicals are present at levels comparable with the levels of HO,, the

destruction of ozone is further increased by the following reactions:

X+o3+xo+o2 (M.12)

HO2 + XO + HOX + 02 (R1.13)

HOX+hv+X+OH (RI.14)

O3+OH+ 02+Hoa (R1.15)

Net : 203+ 302

On the other hand, coastal sites may be impacted by anthropogenic emissions of NOx and

VOCs. If NO concentrations are higher than 30 pptv, the ozone destruction mechanism is

replaced by the ozone production mechanisrn (Rl.1 to R1.9)and halogen radicals act

only as additional oxidative agents for VOCs.

In conclusion, halogen radical rneasurements in the MBL in conjunction with

ancillary measurements of the trace gases are required to improve Our understanding of the tropospheric photochemistry over the oceans. Given the fact that halogen radicals have very short lifetimes, rneasurements of these specieç can be very useful to study the chemical processes in MBL. 1.2.2 Halogen radical during Arctic ozone depletion episodes

More than a decade ago, the occurrence of the spnng Arctic ozone depletion

phenomenon was observed (Barrie et al., 1988). After sunrise, the ground level ozone

concentrations drop fiom background levels of about 40 ppbv to less than 1 ppbv, on a

very short time scale (1 day or even a few hours). This is a widespread phenomenon in

the Arctic regions which has also been observed at Barrow, Alaska by Sturges et al.,

(1993) and Ny-Alesund, Spitsbergen by Solberg et al., (1 996). Also, airborne

measurements of ozone at low altitudes (below 1 km) confirmed the extent of the ozone

depletion tluoughout the lower layer of the Arctic troposphere (Kieser et al., 1993).

However, a large variability in ozone concentrations was observed, suggesting

that the ozone depletion occurs in specific, localised areas of the Arctic MBL, Using

tethersondes, profiles of ozone, temperature and wind speed have been reported (Mickle

et al., 1989). The profiles indicated that the ozone depletion occurs only in this shallow layer (1 km of the ground) and it appears to be correlated with stable conditions in the

Arctic MBL.

The ozone destruction mechanism must be fast (1-2 ppbv 03per hou), because low levels of ozone have been recorded a day after an Arctic storm (Kieser et aL, 1993).

A storm is very good way of mixing MBL with fiesh air (rich in ozone) fiom the fiee troposphere, and therefore the end of the stom coincides with the beginning of the chemistry that leads to ozone destruction.

Intensive field measurements, modelling studies and laboratory experiments brought a wealth of detail about the most likely chemical destruction scenario for Arctic ozone, but this phenornenon is still not fully understood (Tang and McConnelZ, 1996).

The Arctic tropospheric regions are in the dark for a long tirne, so the

photochemistry is completely turned off. Photoactive halogen compounds are believed to

be released by oceanic sources into a shallow layer, isolated by a temperature inversion

Erom the fiee troposphere. A few mechanisms for the release of photoactive halogen

species in the Arctic MBL have been proposed:

a) halogenated organic compounds, such as bromoform CHBr3, methyl halides CH3X are

emitted directly by the polar oceans (Barrie et al., 1988). For the polar conditions the

lifetime of such compounds is years for CH3X and days for CH& and the measured

concentrations for CHBr3 were 1-5 pptv (Botfenheim et al., 1990).' Thus, the source is

insufficient to maintain halogen radicals at concentrations high enough to destroy ozone

at the observed rate. b) formation of XNO and XNO2 by reaction of NOz, NO3 or N2Os with the sea salt

aerosols rich in NaX (FinZayson -Pitts et al, 1989, Zetzsch and Behnke, 1993). These halogen compounds (XN02 and XNO) are rapidly photolysed. The ozone depletion occurs derthe sunrise, when the levels of NO3 or N205 are presumably low (due to their photolysis) and therefore, the last two species may not be significant in generating photoactive halogenated species. Also, the concentrations of NO, are very iow in the

Arctic (less than 5 pptv), during the low ozone episodes as measured by Beine et al.,

(1998). This implies that this source may be important only for coastal areas impacted by

NOx emissions, but not for the Arctic regions. c) Snow pack oxidation of halides by ozone (Oum et al, 1998a, Oum et al., l998b). Whatever the mechanism for the release of photoactive halogen species in the

Arctic MBL might be, there is evidence about increased concentrations of the haiogenated species during ozone depletion episodes. Simultaneous measurements of filterable bromine (bromine compounds trapped on ceIlulose Whatrnan 4 1 filters) and ozone showed that these species are inversely correlated (Meet al., 1988). During

Polar Sunrise Experiment 1995 (PSE 99,episodic high concentrations of photoactive chlorine species were measured prior to sunrise, whiIe episodic concentrations of photolysable bromine compounds were measured during low ozone episodes (hpey et al, 1997a).

Measurements of C2-C6hydrocarbons were made in the Canadian Arctic during

Polar Sunrise Experiment 1 992 (PSE 92) by Jobson et al., (1 994). It was found that during the ozone depletion episode, the surface concentrations of Cz-Cs hydrocarbons are more consistent with their losses being controlled by chlorine chemistry rather than just

OH chemistry. Furthermore, acetylene concentrations deviated fiom the trend corresponding to light alkanes degradation by OH or Cl, and therefore the first evidence that bromine radicals are involved in the ozone destruction was found. Based on these measurements, the authors concluded that bromine radicals are responsible for more than

90% of the ozone loss during a depletion episode, while chlorine radicals are responsible for about 6%. Estimates of average steady state concentrations of halogen atoms were given: [Cl]=l . 1-7.4* 1~~molec/crn~ and [Br]=0.87-6.1*1 O' molec/cm3. Similar conclusions were drawn fiom a large set of measurements made in the Norwegian Arctic, at al al es und (Solberg et al., 1996). Mer su~se,the photoactive halogen compounds are photolysed to yield haiogen

radicals, which participate in various ozone destruction cycles. One of these cycles

involves only the bromine radicals (Barrie et al., 1988):

2Br + 203 + 2Brû + 202 (RI.16)

BrO+BrO+2Br+02 (Rl. 17)

Net : 203 -+ 302

The second cycle involves bromine and chlorine radicals (Le Bras and Platt, 1995):

cl + 03+ CIO +O2 (R1.18)

Br+03-+BrO+02 (RI. 16)

Br0 + Cl0 -, Br +Cl + O2 (Rl. 19)

Br0 + Cl0 + BrCl +O2 (RI .20)

BrC1-t hv-, Br + Cl (RI .SI)

- - Net : 203 + 3 02

Finally, a combined cycle involving HO, radicals (R1 .12 to RI. 15) could also destroy ozone.

The first tropospheric observations of bromine radicals in the Arctic were obtained using Long-Path Differential Optical Absorption Spectroscopy (LP-DOAS) by

Hausman and PZatt (1994). During PSE 92, Br0 concentrations in the 4 -1 7 pptv range were measured at Alert. Further field measurements using the same technique were conducted at esund und in the spring of 1995 and 1996 (Tuckermann et al., 1997).

During low ozone episodes, halogen radical concentrations as high as 30 pptv were observed for 1995 and 1996, while Cl0 concentrations were more variable (30 pptv in 1995 and less than 3 pptv in 1996). During the periods when ozone concentrations were

in the normal range, average concentrations of Br0 and Cl0 were close to zero.

During the ozone destruction episodes, halogen radicals also react with other

compounds (i.e. hydrocarbons, aldehydes) forming stable species (HBr, brominated

organic compounds). As pointed out by McConnell et al., (1 992) these reactions have the

potential to stop the ozone destruction chemistry and, therefore, a recycling mechanism

must be responsible for converting these inactive halogen species back to fiee radical

precursor species.

One of the first recycling mechanisms (Fan and Jacob, 1992) was based on the

following reactions:

HOBr(,+HOBqaa (R1.22)

mr(g)-+w(@ + Br-(* (RI .23)

HoBr(@ + Br-(ad + +BncW + Hz0 (R 1.24)

Brz(*+Brz(s) (RI .25)

HOBr and HBr are scavenged by the aqueous sulphuric acid aerosols (R1.22 and

RI .23), followed by the Br2 production (RI .24) and the release of Br2 to the gas phase.

The Iimiting factor for this Brzcaaproduction is uptake rate of either HOBr or HBr by the aerosol.

Another proposed recycling mechanism is based on the aqueous-phase oxidation of Br- to HoBr(@ by the peroxymonosulfuric acid , H2SOs or Caro's acid, (Momrkewich,

1995): HSO; (N+ Br-(.@ +SO4' (@+ HoBr(@ (RI .26)

HoBr(* can then produce Br2 (similarly to Fan and Jacob mechanism) which is released into the gas phase.

The haiogen recycling mechanism is critical for the ozone depletion chemistry.

The initial halogen sources may be just the initiators of the ozone depletion chemistry, while the recycling rnechanism may in fact be the tool to maintain high levels of halogen radicals, and ultimately be responsible for ozone destruction.

In conclusion, the spring Arctic ozone depletion is a partially explained phenomenon. Measurements of bromine and chlorine radicals, in conjunction with measurements of photoactive halogenated species, would bring new insights about this phenomenon by constraining the modeis and differentiating between the mechanisms for the release or recycling of the photoactive halogen species.

1.3 Measurement techniques for tropospheric radical measurements

The maximum concentrations of fiee radicals in the troposphere are in the pptv range for HO2, RO2, Br0 and Cl0 radicals, while for the OH radical concentrations are usually less than 0.4 pptv (1 0'molec/cm3). Only in the last decades were successful attempts to rneasure these important trace species reported. Today, various techniques are available for tropospheric radical measurements and they are presented in Table 1.1, along with their main characteristics.

In this work, the chernical amplifier for peroxy radical measurements was used as the starting point. Compared to the other tropospheric radical measurement techniques, measurements rechnique Species Detection limit Principle Representative Reference OH is converted to ~2~~04by 34~~2; Eisele and Tanner, ~2~~~04is then measured by atmosphek 1995 pressure chernical ionisation with NO) [on molecule HO2 and R02 1ob moleclcm3 Chemical amplification by reaction with Reiner et al., 1997 reaction mass for 5 min NO and SOa, to form H2S04, which is spectrometry detected ion-molecule reaction-mass

I 1 spectrometry Radiocarbon 1 OH 1 1o4 molec/cm3 OH radicals react with I4c0to form "~02, Felton et al., 1992 Technique which is then radiocarbon dated Laser induced OH and HO2 . 4* 105molec/cm' OH radicals are brought into the excited HojZumahaus et al, fluorescence after conversion for 5 min usin^ in^ an intense laser bean (h=308nm); 1993 (Lw to OH (reaction the fluorescence (307 -3 1lnm) is rnonitored Hard et al.,, 1995 with NO) with a photomultiplier tube. Brune et al.,, 1995 Differential OH 5-7* 105 Based on measuring the difference between Dorn et al, 1995 optical absorption molec/cm3 for the absorbance at some wavelength where spectroscopy with 3 min target species have a distinct absorption light path> 1km Brû 2-4 ppîv peak (308 nm for OH, 328 or 355 nm for Hausman et al., (LP-DO AS) BrO, 280 nrn for CIO) and another 1994 Cl0 wavelength on either side of the peak. Tuckermann et al, 1997 Matrix Isolation HOz, ~PPW The radicals are trapped in an ice matrix at Mihelcic et al., and Electron Spin NO3, 3 PP~V 77K. the radicals are then quantified by Resonance (ESR) CH3C(0)02 SPP~V ESR spectroscopy Sum of ROz 5pp& Chemical R0,=H02+R02 2-3 pptv RO, produce a large amount of NO2, fuaher Cantrell et al., 1984 Amplification 1 +OH +RO 1 for 30 min measured with a luminol detector Hastie et al., 1991 the chemical amplification is not specific for a radical species: it measures the sum of

HO2, OH and a large fiaction of RO2 and RO radicals. However, the chernical

amplification is relatively simple, inexpensive, with moderate time resolution (10- 30 min). Also, it can be deployed in remote locations and the logistics are not demanding.

1.3.1. Chemical amplifier for ROx radical measurements

The chemical amplifier for ROx radical measurernents was first introduced by

Cantrell and Stedman (1 982). This instrument relies on converting a small concentration of radicals into a large concentration of NOz molecules, which are then easily measured with an NO2 detector.

In a conventional chemical amplifier for RO, radical measurements, the flowing air sample is mixed with large concentrations of NO (2-6 ppmv) and CO (4-1 0%) and the

NO2 produced fiom the following amplification cycle is measured downstream:

HO2 + NO + OH + NO2 (R1.27) ,

OH+CO+H+COz (R1.28)

H+02+M+H02+M (RI .29)

Additional to these reactions, there are loss reactions for the radicals, which Iimit the arnount of NOz produced. The main radical loss reactions are the heterogeneous wall loss reactions for the radicals, the HONO and H02NO2 formation:

HOz ,OH + wdl+ Wall products

OH+NO+M+HONO+M

HOz + NOz + M -+ H02N02 + M Organic peroxy radicals @O2) are also detected by the chernical amplifier, because RO2, in the presence of NO in air, are converted to HO2 radicals (R1.33, R1.34) which are fùrther amplified:

ROz + NO + RO + NO2 (R1.33)

RO + 02+ R'CHO + HO2 (R1.34)

To fiilly described the chemistry of the amplifier a plug-flow mode1 was developed and used to study the sensitivity of the instrument towards the radical loss rates, ROx concentrations and reaction time (HdiFtie et al., 1991). The reactions used in such modelling studies and their rate coefficients are presented in Appendix, Table Al.

One way to turn off the amplification cycle is to remove the chain carrier (CO in this case), so the radicals are involved only in R1.27,R1.30 and R1.3 1. This is conveniently used to measure only the NO2 produced by amplification by operating the instrument in two cycles. In the first cycle, the air sample is mixed with NO only, while

CO is being added at a point situated downstream (more than 1 second after mixing with

NO, suffkient for the removal of the radicals by R1.30 to RI .32, but before the NO2 detector). As a result, the amplifier chemistry is turned off and a baseline signal

(p02]cq,n) results. This baseline signal is due to ambient N02, and NO2 resulting from

O3 titration with NO.

O3 +NO -+ NO2 +O2 (R1.35)

In the second cycle, the ambient sample is mixed with NO and CO, the amplification chemistry is therefore tumed on and a higher signal (~02]co-On) is measured. By subtracting the baseline signal, the amount of NO2 produced by radical amplification is easily calculated:

6NO2= INO2lco-O" - CNO2lco-off (El. 1) The chah length (CL) of the amplifier is defined as the number of molecules of

NO2 produced for each radical that enters the system. The CL is a very important parameter for the chemical amplifier, since it is used, in conjunction with BN02, to calculate the radical concentrations:

[ROJ = BNOz/CL (E 1.2)

The first reliable calibration method was based on PAN (peroxy acetyl nitrate) thermal decomposition (Hastie et al., 1991): known concentrations of PAN (measured by

GC-ECD) were sampled with the chemical amplifier that had a portion of the reactor heated at 200'~. At this temperature, PAN was quantitatively decomposed, yielding peroxy acetyl radicals:

CH3C03N02+ CH3C03 + NO2 (RI.36)

Through reactions with NO in air, CH3C03 radicals were converted to HO2

(R1.37 to R1 AO):

CH3C03 + NO + CH3 + CO2 + NO2 (RI .37)

CH3+02+M+CH302+M (R1.38)

CH3O2f NO + CH30 + NO2 (R1.39)

CH30 + O2 + HCHO + HO2 (RI -40)

The chain length was then calculated using the signal (~N~~,JAN)measured by the chemical amplifier and the PAN concentration.

CL = 6NO~,pm/[CH3C03N021 (El .3) The chah length obtained through these calibrations was 120 (Hmtie et a', 1991).

1.4 Objectives of this research

Objectives for instrument development

ModiQ the conventional amplifier for peroxy radical measurements for halogen

radical measurements. Two radical detectors, able to measure very low

concentrations (Zpptv) were envisaged: one for chlorine radical measurements

and the other one for bromine radical measurements.

Further improve the performance of the ROx chemical amplifier in the field.

Study the water vapour effect on the chemical amplifier.

O Address the calibration issues for the radical source based on water photolysis.

Objectives for field measurements

Deploy the halogen radical detector in the Canadian high Arctic and measure

the concentrations of halogen radicals in the spring.

O Use the conventional chernical amplifier to measure ROx radicals in an intensive

oxidant study, where supporting measurements of other important trace gases are

also perfonned.

Objectives for atmospheric chemistry

O Bring new insights about the spring Arctic ozone depletion phenornenon.

O Compare the ROx measurements with RO, concentrations based on the current

level of understanding of the tropospheric chemistry. 2. Description of the chemical amplifier for ROx radical measurements

In this section, the conventional chemical amplifier, as used for peroxy radical measurements, is described. Presently there are eight operational chemical amplifiers in the world. Although, some minor details differentiate thern, recent laboratory inter- cornparisons have shown that they behave essentially the sarne (HeitZinger et al., 1996).

Since the chemistry behind this technique was already presented in section 1.3, only the instrument configuration for field measurements dong with the calibration routinely performed is discussed.

In figure 2.1 a schernatic of the chemical amplifier, used by our research group in

Air Smple different field campaigns prior

to this work, is shown. The

main elements (the NO2

detector, mass flow controllers,

the reagent cylinders, the PC

used to collect the data and to

control the 3-way valve) are

hosted in the laboratory, while

the sampling inlet and the 3-

way valve are situated well

above the ground (up to 10 m).

For the transportation of the

Figure 2.1. Field configuration for the chemical amplifier. gaseous reagents (CO and NO) to the reactor and the air sarnple to the NO2 detector, %" OD Teflon PFA lines are used.

This configuration ensures that a more representative air sample is obtained and retains the convenience of maintaining the equipment in the laboratoq.

2.1 NO2 detector

The NO2 detector is an LMA-3 mode1 manufactured by Unisearch Associates and has the major advantage that it can measure NOz at sub-ppbv levels even when large concentrations of NO (ppmv levels) are present. It is based on the principle of detecthg the cherniluminescence produced when gaseous NOz reacts with a basic solution of luminol(3Aminophtlhydrazide) and sodium sulphite (Na2S03) on a wetted surface.

This detector has a moderate response time (10 seconds for 95% change in NO2 concentration), is very compact (38 x 20 x 22 cm) and relatively easy to operate.

Although the complete mechanism of this reaction is not completely understood, intensive laboratory studies have shown that this detector has a linear response over a wide range of NO1 concentrations and a non-linear response below 5 ppbv (Drummond et al., 1989, Canfrell et al., 1993). By adding a flow of NO2 in nitrogen to produce a mixing ratio of 10 ppbv just prior to the reaction ce11 of the detector, the LMA-3 was always operated in the linear range. The luminol detector has ambient interferences, mainly Os and PAN, however, as the chemical amplifier is operated in two cycles

(section 2.4) the overall signal of the amplifier is not affected by the above interferences in the NO2 detector.

The luminol solution used throughout this work had the following composition: 10% luminol, 0.1M NazS03, 0.0 1M NaOH, 0.1% isopropanol and 0.1% surfactant.

Through laboratory experirnents, it has been show that this solution is stable for a least

two days.

The calibrations of the NO2 detector were always performed in the presence of

reagent gases (NO and CO), because both gases, especially NO, lower the sensitivity of

the detector. During field rneasurements, fiequent calibrations of this detector were

performed (each day or every other day). During these multi-point calibrations, known

amounts of NO2 were delivered to the detector and the response of the detector was

recorded. A linear regression analysis was then used to obtain the sensitivity of the

detector (LMA,i,pe).

Two methods were employed to generate known concentrations of NOa. In the

first method, NO fiom a 6.4 ppmv NO in nitrogen gas cylinder (Scott Speciality Gases

certified standard) was diluted with synthetic zero air and passed through a chromium

trioxide converter (Cr03 supported on chromosorb), (Levaggi et al., 1974). The

converter quantitatively transforms NO to NO2, then the NO2 concentration was easily

calculated based on the NO concentration and the dilution flows.

A more convenient method involved the use of a commercially available

permeation device (Vici Metronics). In this, a small amount of NO2 is held in liquidvapour phase equilibrium inside a sealed ampoule. This ampoule has a small permeable window through which a constant amount of NO2 is continuously released.

Since the permeation rate depends on temperature, the tube is held in a temperature stabilised housing at 35'~. A small flow of nitrogen (50 cm3 min") carries the NOÎfiom the tube to a larger flow of zero air. By varying the flow of zero air, difFerent

concentrations of NO2 can be obtained. Although the permeation tubes are usually

calibrated by weight loss, 1 calibrated it against NOz generated with the procedure

previously presented (NO and chromium trioxide converter).

2.2 Reagents

CO gas (99,3%, Liquid Carbonic technical grade,) was purified using a trap

containing Iz and activated charcoal. This purification removes the metal carbonyls,

which are usually found in compressed CO cylinders (Steahan el al., 1974). The

presence of metal carbonyls significantly lowers the sensitivity of the luminol detector.

NO for the chemical amplifier was added from a 200-pprnv cylinder of NO in

nitrogen (Liquid Carbonic, working standard). The flow of NO was passed through a

trap containing FeS04 that reduces traces of NOz present in the NO cylinder. It is

important to remove NOz because it Iowers the chain length of the amplifier (Arias,

1998).

The synthetic zero air was obtained using a kADCO Mode1 737-1 2 clean air generator or fiom compressed zero air cylinders (Liquid Carbonic or Matheson).

Since the luminol detector is very sensitive to pressure fluctuations, the

concentrations of NO and CO should be kept constant in the instrument, mass flow controllers (Tylan Corp. and MKS) regulated al1 the flows. The lower range mass flow controllers (1 -200 cm3 min-') were calibrated using a bubble flowmeter (Hastings, mode1

HBM-1), while the larger flow controllers (1-20 1 min-') were calibrated using a Singer "Gas clock" flowmeter (mode1 DTM 1- 11 5).

The gaseous reagents were transported using Teflon PFA tubing (OD=?A")and the

lines were connected using Swagelock compression fittings (stainless steel or Teflon

PFA). The reactor was also made of %" Teflon PFA tubing, with the length of 1Sm.

2.3 HO2 Calibrations

Frequent field calibrations of the chernical amplifier are required, since the chain

length is very sensitive to radical wall losses and the nature of the wall changes during

ambient sampling. The most reliable and convenient radical source is based on

photolysis of water in air at 184.9nm, using a low pressure Hg lamp (Schultz et al., 1995).

At this wavelength, both water and oxygen are photolysed:

H20+ hv (1 84.9nm) + OH + H 1)

O2 + hv (1 84.9nm) + 20 (R2-2)

Hydrogen and oxygen atoms react fùrther with 02to fom HO2 radicals and 03, respective1y:

0+02+M+03+M w.3)

H+02+M+H02+M w.4)

It can be seen that the photolysis of a single Hz0 rnolecule leads to two radicals (one HO2 and one OH), while the photolysis of O2 leads to two ozone molecules. By adding 100-

200 ppmv of CO to the radical source, the OH radicals are converted to HOz radicals:

OH+CO+O2+HOz+C0a (R2.5)

By neglecting the loss reactions for the radicals and assuming the photolytic reactions are the rate-determining step (R2.2R2.4 and R2.5 have high reaction rates), the continuity

equations for the above processes are:

where: 4>184.9is the actinic flux at 184.9nm

m20]and [O2] are the water and oxygen concentrations.

0~20and 002 are the absorption cross section of water and oxygen at 184.9nm.

q03 and q~02are the yields for the formation of 03and HO2 when CO is added

to the ce11 (both equal to 2 in this system).

Finally, by integrating E2.1 and E2.2 and taking the ratio of the HO2 and O3 concentration the following equation is obtained:

Using E2.3, the POz] delivered to the amplifier was easily calculated based on: a) [O2] and [H20] which were known since the air fiow through the photolysis ce11 was

synthetically prepared; b) [O3] was measured; c) o~20and a02 are constants. For the HO2 calibrations performed prior to July 1998,

the values used for the absorption cross sections were o~20=5.546*1 cm2 (Baulch

23 et al., 1982) and om=0.95 * 1u20 cm2 (Washida et al., 197 1). Recent work has shown

that îhese values needed to be revised. Based on the experimental determination of

the ratio 0~2o/a02for Our own radical source (addressed in section 4.1), al1 the radical

data were later corrected by a factor of 1.06.

Figure 2.2 presents the layout for the radical source based on water photolysis at

184.9nm. The main elements are:

- the photolysis ce11 made of Suprasil quartz (transparent for 184.9nm radiation), with a length of 60 cm (to ensure that the flowing regime is laminar) and 33 mm ID (to minimise the wall effects).

- the low pressure Hg lamp (PenRay, mode1 1 1SC-1). - the lamp housing was flushed with nitrogen in order to minimise the photon losses due

Air N2OUT T 7amp housing

Fixed [0,]=20.5% [CO]=20Oppmv b Variable &O]=0-200ppmv

u Photolysis ceIl

Figure 2.2. The HO2 radical source based on water photolysis at 184.9 nm. to the oxygen photolysis in the housing.

- a bubbler filled with distilled water kept at constant temperature. A variable flow of

zero air was bubbled through a fkitted glass into the water, and becarne saturated with

water vapour. By measuring the temperature of the water and using the water vapour

pressure tables (CRC 1974), the water concentration in the flow passing through the

bubbler was easily calculated and then used to detexmine the water concentration in the photolysis region. The dew point was checked using an EG&G dew-allm digital humidity analyser, mode1 91 1 and it was within 10% of the value calculated assuming

100% water saturation. The water concentrations in the photolysis were in the 0-200 ppmv range.

- the mass flow controllers MC)used to measure the flows of zero air through the source (8-101 min-') and through the bubbler (0-100 cm3 min-') and the flow of 5% CO

(3 6-40cm3 min-').

To pefiorm a HOa calibration the following protocol was used. Initially no water was added to the ceIl and the ozone produced was measured using a conventional ozone analyser. Alternatively, the LMA-3 detector was used to quanti9 the ozone by titration with NO and them measuring the amount of NOz produced. By varying the flow through the bubbler different water concentrations (O-2OOppmv)were passed through the radical source, different radical concentrations were generated and different signals were recorded by the chemical amplifier. Plotting the signals recorded by the instrument

(6N02)against the radical concentrations calculated with E2.3, the chain length of the amplifier (CL) was computed as the slope of the regression line. 2.4 Instrument operation and data acquisition

The radical amplifier was operated in two cycles, by adding CO sequentidly at two different points. This resulted in a modulated signai for the NO2 detector (Figure

2.3). In the first cycle (A, C, E), the air sample was mixed with just NO, whiie CO was

added at a further point downstream.

The Iuminol detector measured a

baseline signal (~O~]CO-~,=

[N02]~,[NO&, m02]~,etc.),

was composed of ambient NOa NO2

resulted fiom ozone titration with b Time NO and ambient interferences such Figure 2.3. Data acquisition for the chernical amplifier. as PAN and ozone. In the second cycle (B, D) the air sample was mixed with NO and CO, establishing the amplification cycle. The detector rneasured a peak NO2 signal (FJOZ]CO-~~= [NO& etc.).

This latter signal is made of by the same elements as the baseline signal with the addition of NO2 produced by the amplification mechanism, thus the difference is entirely due to radical amplification.

Since ambient ozone and NOz can be highly variable, it is advisable to have a very fast modulation between cycles. The LMA-3 is very sensitive to pressure variations and the switching of the 3-way valve generates a pressure pulse in the lines. It has been observed that the detector needs 25 sec to stabilise, so the data for this period were rejected. For the next 50 seconds the data, collected at 1Hz fiequency, were valid and were used to calculate the average NO2 concentrations for that specific cycle

pO2lB,etc. The signal produced by chernical amplification of the radicak (6NO2)was

calculated using:

a) two consecutive baseline signals (FOzJA,wz]~,) and the peak signal between them

~OZ]B

b) two consecutive peak signals (mO2]~,PO&) and the baseline signal between them

The radical concentration was then calculated using the measured 6NO2, the slope of

the NO2 detector (LMAslq,) and the chain length of the amplifier (CL) fiom the HO2

calibration:

2.5. Safety considerations

Operating this instrument involve working with CO, which is a highly toxic,

odourless gas. A commercially carbon monoxide sensor (Cosencor International Inc.) was always used to detect levels of CO above 50 ppmv. In addition, the exhaust gas leaving the NO2 detector contained 4% CO. A trap containing Pt deposited on alumina, heated at 200-50'~was used to convert CO to CO2. This trap efficiently removed the

CO levels below the detection limit of the CO sensor. 3 Analytical method development

The efforts to develop two new radical amplifiers capable of independently

detecting low concentrations (pptv levels) of bromine and chlorine radicals are presented

in this section. The importance of tropospheric halogen radical measurements has

already been presented in section 1.1. These instruments are variations of the chemical

amplifier used for peroxy radical measurements (presented in section 1.3 and chapter 2).

3.1 Development of the chlorine radical detector

3.1.1 The principle of the chlorine radical detector

The principle of the chlorine radical detector is to convert chlorine radicals (C10,

= OC10 + CIO + Cl) to HO, radicals through fast reactions with ethane (CH3CH3)and

NO. The HOx radicals produced are then quantified using the chemical amplifier.

Modulating the C10, to HOx conversion wodd eliminate possible interferences caused by

ROx and HOx radicals, bromine radicals and NO3 radicals.

Figure 3.1 shows the proposed configuration

for the chlorine radical detector.

A base (no chlorine) signai is obtained by

mixing the air sample with high

concentrations of CO and NO, while the

ethane is added downstream. The luminol

detector (LMA-3) measures NOz

Figure 3.1. Proposed configuration for the components with various origins: chlonne radical detector. a) ambient ozone which is titrated with NO in the instrument:

Os +NO + 02+NO2 (R3.1) b) HOx radicals that undergo chemical amplification:

HO2 + NO + HO + NO2 W -2)

OH+CO+ H+COz (R3-3)

H+02+M+ HOz+M W -4) c) ROx radicals transformed to HO, (R3.5,R3.6), which are then arnplified by reactions

R3.2 to R3.4:

ROz + NO + RO + NO2 (R.3 -5)

RO + O2 + HO2 + R'CHO (R3 4 d) ambient NO2 e) ambient NO3 converted to NO2

NO3 + NO -+ 2N02 (R3 -7)

By the time that ethane is mixed with the air sample, al1 the chlorine radicals are lost through one of the following processes, while producing at most two NO2 molecules: a) reactions with NO

OC10 + NO + CIO + NO2 (w.8)

Cl0 + NO -, Cl + NO2 (R3 9)

Cl+NO+M+ ClNO+M (W.1O) b) losses to reactor walls

Cl0,Cl + wall + wall products (R3.I 1) in the second cycle, the chlorine detection chernistry is activated by rnixing the air

sarnple with ethane, in addition to NO and CO. Of the possible radicals in the air not

contributing to the baseline signal, only CIOx radicals will be transfonned into HOx

radicals, through the following reactions:

OC10 + NO + Cl0 + NO2 W -8)

Cl0 + NO -, Cl + NO2 W.9)

Cl + C2H6+ HCl + CZHS (R-3-12)

O2 + C2H5+ M + C2HsO0 + M (M.1 3)

CzHsOO i-NO + C2H50 + NO2 (R3.14)

C2Hs0 + O2 -+ CH3CHO + HO2 (R3.15)

The HOx radicals are then amplified through the nomial amplification cycle (R3.2 to R3.4) and the difference between the NO2 signal obtained in the second cycle and the

NO2 baseline is due to the chlorine radicals.

The chlorine radical detector is not sensitive to bromine radicals, because they are not converted to HOx radicals: Br atoms react very slowly with ethane with a rate coefficient for this reaction of only kZg8K = 3.1 * 1 O-'' cm3 molec-' sa' (Russell or al.,

1988). In the instrument, bromine radicals are converted into stable products, such as nitrosyl bromide and wall Ioss products, similar to chlorine atoms in the absence of ethane (R3.9 to 3.11).

The agent for CIO, to HOx conversion can be any saturated hydrocarbon, since these compounds are not very reactive towards bromine atoms. Ethane was preferred as the conversion agent, as CH3CH302has the lowest production of organic nitrite: CzHSO+ NO -+ C~HSONO+ M (R3 16)

and consequently the highest conversion efficiency to HOx among the organic peroxy

radicals originating from saturated hydrocarbons (Cantrell et al., 1 993).

3.1.2 Modelling the chemistry of the chlonne radical detector

A plug flow model was used to test whether the proposed modifications would

produce a chlorine radical detector that could be used to measure very low concentrations

of chlorine radicals. The model was also used to investigate if other ambient trace gases

are likely to interfere with the chlorine radical measurements. The model was developed

based on the early version of the ROx radical amplifier (presented in Appendix, Table

Al). Reactions specific to the chlorine chemistry were added; their chemical equations

and their temperature dependent rate coefficients are shown in Appendix, Table A2 This

led to a total number of 84 reactions and 52 chemical species for the model used to study

the chlorine radical detector.

Since chlorine radicals are present in the troposphere at such low concentrations

(up to a maximum of 20 pptv) the amplification process (the chah length) must be maximised. Modelling studies of the radical amplifier chemistry (Cantrell et al., 1993) have shown that an increase in the CO leads to an increase in the chain length.

Considering that the explosion limit of CO in air is 12.5% (CRC,1974), al1 the modelling runs were done with a CO mixing ratio of 10% in the detector. The modeIling runs were performed with an input concentration for [ClO]i,pt= 4pptv and for varying concentrations for NO and ethane. Using the NO2 concentrations calculated by the model and [CIO]inpusthe chain length of the chlorine radical detector (the number of NO2 molecules produced for each cblorine radical that enters the system) was then computed for each pair of NO and ethane concentrations. The results are presented in Figure 3.2 and, similar to the ROx radical amplifier, chah lengths in order of 250 are obtained at

FJ0]=6-8 ppmv.

The optimum ethane concentration is in the range 5-20 ppmv. It was expected that

an increase in ethane concentration

would lead to higher chain lengths since

R3.12 will dominate over R3.10,and

therefore the fraction of chlorine radicals

that are converted to HO, radicals would

be increased. This behaviour is observed

for the 1 to 20 ppmv, but at higher

ethane concentrations, lower chah

lengths resulted. This is because at

higher concentrations of ethane, the

amplification cycle is perturbed as some of the OH radicals react with ethane rather than CO:

OH + C2H6 + C2Hs (R3.17)

This is not a total loss for the radicals since C2Hs is converted through R3.13-R3.15 to an

HO2 radical. However, due to the formation of ethyl nitrate by reaction (R3.16), a significant nurnber of radicals are lost and the overall chain length is lower. With respect to the variation in the NO concentration, the model predicts an increase in chah length as the WO] is increased fkom 2 to 7 ppmv, followed by a decrease in the chah length as PO]is Merincreased. The behaviour is also observed when the model for the ROx amplifier is run (Cantrell et al., 1993). Increasing WO]is responsible for increasing the rates of two reactions with opposite effects: R3.2, which is part of the amplification cycle, and R1.3 1, which is a loss process for the radicals (OH radicals are converted to HONO). By increasing PO]fkom 2 to 7 ppmv speeding up the amplification cycle dominates over increasing the radical losses, while at higher NO concentrations, the opposite occurs.

Another conclusion was that ozone is a minor interference for CIO, measurements

(ozone decreases the chain length by O. 16%/lppbv of ozone). This is similar to the ROx radical detector and is atüibuted to increasing the loss rate of HO2 radicals by reaction with NO2 produced fiom ozone titration by NO to yield H02N02 (R1.32).

Ambient concentrations of methane (1.7 ppmv) also affect the chlorine radical measurements by increasing the baseline signal. A small fraction of the chlorine radicals

(about 3%) will be converted to CH302radicals in the off cycle by reaction with methane:

Cl + Ch+ HCl + CH3 (R3.18)

0t+CH3+M+CW302+M (R3.19)

Methane concentration is almost constant in the troposphere and therefore ambient C10, radical measurements would have to be corrected by 3%.

The modelling studies revealed that the chlorine radical detector should be linear for the expected range of radical concentrations (1-2Opptv), even when 4 pptv of HO2 radicals were present. This indicated that the chlorine detection chernistry does not interfere with the HOx amplification.

The highest chah lengths (about 250) for the chlorine amplifier were obtained for

[CO]=lO%, wO]=7ppmv and [CH3CH3]=lO ppmv. These chain lengths are similar to the chah length for the ROx amplifier and indicated that C10, radical measurements by this modification of the chernical amplifier may be possible.

3.1.3 Calibration source for chlorine radical detector

Before using the chlorine radical detector to collect data, the instrument must be calibrated against known concentrations of chlorine radicals. The system also needs to be tested against interferences, since the modelling studies are just an indication of the characteristics of the instrument. This demanded a chlorine radical source.

One possibility developed in this work was to use chlorine dioxide, which, in the presence of NO, generates chlorine radicals:

NO + OC10 -, Cl0 + NO2

NO + Cl0 + Cl + NO2

3.1.3.1 OC10 Preparation

A first attempt to develop a dynarnic system to generate sub-ppbv concentrations of gaseous OC10 was based on the following reaction: Sub-ppbv concentrations of Cl2, obtained by dilution of a known standard of Cl2

(50 ppmv in nitrogen, Scott Speciality Gases), were passed over a moist bed of sodium chlorite NaCl02 (BDH)similar to Derby et al., (1953). This method proved unsuccessful: no detectable OC10 was produced. The absence of the OClO in the reaction products cm be explained by a closer look at the reaction mechanism (Kiefler et al., 1968). A first step is the formation of a highly reactive intermediate [C1202](R3.2 I), which cm react Merin two ways: disproportionation to yield chlorine dioxide (R3.22) or reaction with water (R3.23), which does not produce the desired product:

Cl2 + Cloz- + [Cl2021 + Cl' (R3.21)

2[C1202] + Cl2 + 20C10 (R3 .22)

[C1202]+ H20 + Cl- + CIO,' + 2~' (R3.23)

At low concentrations of Cl2, the amount of [C1202]formed is quite small; therefore R3.22, which is second order in [C1202], is less important than R3.23 and no significant OC10 is forrned. Thus the above method cannot be applied to produce sub- ppbv concentrations of OC10.

A second method was used to prepare an aqueous solution of OClO by adding dilute H2S04 to a saturated solution of sodium chlorite (NaC102), similar to Wellington et al., (1965).

5NaC102+ 4I-J? + 40C10 + 5~a++ Cl- + 2H20 (R3.24)

A Stream of zero air was used to purge OC10 along with other volatile components (mainly Cl2)out of the reaction mixture. Chlorine impurities (Cl2) were removed by passing the air Stream through two bubblers filled with saturated NaCl02 solutions. Finally the OClO was trapped in deionized water and the OClO solution was

stored at -20'~, in the dark, because OClO is not stable in light. The concentrated OClO

solution was later used to prepare more dilute OClO solutions.

An OClO pemeation source was made by imxnersing a porous tube (Imperial

Eastman Poly Filo 22 P 1/8") in an OClO dilute solution, kept in an ice bath. As a small flow of zero air (50cm3min-') was passed through this tube, OClO molecules pemeated through the walls into the zero air flow (Figure 3.3). By rneasuring the output of the

OClO source (the procedure is described in 3.1.3.2), it was found that this source cm be used to generate gaseous stable concentrations of gaseous OClO for at least 9 hours.

Vent

Figure 3.3. The Cl0 source and the experimental setup for rneasuring [OCIO] by NO titration. 3.1.3.2 Quantification of OClO in the gas phase

A simple method to quanti@ OC10 in the gas phase for concentration in the 0.3 to

10 ppbv range was based on the titration of OC10 with NO (R3.8 and R3.9) which produce 2 NO2molecules for each OC10 moIecule reacted. The experimental configuration is shown in Figure 3.3.

The NO2 produced fiom OClO reacting with NO was measured with the luminol detector. Since the detector is non-linear below 5 ppbv, first a baseline signai [NO2lbase was obtained by adding a known amount of NO2 fiom a NO2 permeation source and NO fiom the same system as used for the radical detector. When the air Stream containing

OClO was sampled an increase in NO2 due to the reaction of OClO with NO was measured, [N02]oclo,and then [OC101 was easily determined by:

[OC10]=1/2* ([N0210~10- m021base) (E3.1)

A second independent method to measure OClO concentration in the gas phase was based on the conversion of OC10 into HCl through R3.8, R3.9 and R3.12 with added

NO and CH3CH3. The HCl produced and the Cl2 impurities (from the OC10 solution) were quantified based on the protocol developed by Keene et al., (1993). First al1 the chlorine species (HCl produced and Cl2 impurities) were measured by bubbling the gaseous mixture through a set of two bubblers containing bicarbonate-bisulphate solution which reduced al1 the chlorine species to chloride ion(C1'). In a second part of the experiment, the same gas mixture was bubbled through deionized water, where al1 of HC1 was trapped, but only a fraction of Cl2 (10-20%) was absorbed and.disproportionated to

Cl-. The chloride ion in both solutions was then quantified by Ion Chromatography (IC). The chloride concentration in the first set of bubblers was 15% higher that the

chloride concentration in second set. Based on 20% collection efficiency for Clz in the

deionized water, the estimated Cl2 impurities in respect to OClO were less than 8.5%,

while for a 10% Cl2 collection efficiency, the Cl2 impurities were less than 7.9%.

A cornparison between the two caiibration methods was undertaken for gaseous

OClO at a mixing ratio of 6.8 ppbv and the agreement was within 10% (considenng that

Cl2 impurities were 8.5%). The good agreement between the two methods suggested that

we can use OClO as a reliable source for calibrating the C10, radical detector.

3.1.4 Instrument adaptation for chlorine radical measurements

The chlorine radical instrument is similar to the radical amplifier for ROx radical

measurements, described in chapter 2. An additional mass fiow controller was used to

deliver a uniform flow of 1000 ppmv ethane in nitrogen (Praxair, certified standard), as

shown in Figure 3.1. For the first set of experiments, the amplifier was operated with the following reagent concentrations: [NO]=2 ppmv, [CO]= 4% and [CH3CH3]=10 ppmv.

These are not the optimum conditions, but [CO] and [NO] were the same as routinely used in the ROx amplifier.

To improve the performance of the chlorine radical detector by increasing the modulation fiequency, the 3-way solenoid valve was replaced by a 4-way valve (Figure

3.4). In this case an additional flow of zero air was used to balance the ethane flow. This served to balance the pressure changes on switching the valve and to rapidly flush the valve. This valve replacement was done because it has been observed that ethane sticks on the Teflon walls of the valve 1 Zcro Air Zao Au and reactor, and a longer dead

time was needed, thus

increasing detection limit for Baseline Signal Chlorine Detection Signal field measurements, as will be Figure 3.4. The 4-way valve for ethane modulation used shown in section 4.3. I for the chlorine radical detector

3.1.5 Experimental evaluation of the chlonne radical detector

A blank test was perforrned by adding OClO to the conventional amplifier for the

ROx radical measurements. No signal was expected, since chlorine radicals should not be converted to HOx radicals in the absence of ethane. The test had an unexpected result: a significant signal was obtained.

A second set of experirnents was done in order to quanti& the source of this signal. Known concentrations of OClO were sampled with the amplifier operated in three different configurations:

by constantly adding NO and by modulating the point at which CO was added, the

amplifier was operated in the ROx mode and a signal 6wO& was obtained:

ap02]~=~OZ]A,PEAK ' [NOZ]A,BASELME

by constantly adding NO and CO and by modulating the ethane, the amplifier was

operated in the proposed CIO, mode. This produced a different signal:

Qp02]~= [N~Z]B.PEAK' ~OZ]B,BASELME c) by constantIy adding NO and ethane and by modulating CO, the amplifier was

operated in the ROx + CIOx ,i.e the NO2 produced was due to both C10, and ROx

radicals. A higher signal was obtained &~02]C=~02]C,~EAK- m02]C,BASELME

The following observations were recorded:

~~~]B.BASELINE= ~OZ]APEAK

~O~]C.PEAK=[N~~]B,PEAK

m02]A, BASELINE = [NO~IC.BASELME

and the following relations between these signals were obtained:

6[N02]C = g[N02]~+ ~~OZ]B

6m02]~=(0.55 I 0.05)*6[N02]c

6m02]~=(0.45 10.05)*6~02]c

These experiments led to the following conclusions:

- a significant increase in the baseline of the chlorine radical detector was observed; this

poses serious questions about the sensitivity of this instrument for field measurements.

- ROx radical amplifiers respond to chlorine radicals; other~ise&~O~]~should be zero.

Therefore, this issue was Merpursued.

Known concentrations of OC10 were sampled by the chernical amplifier operated

in three modes, as previously described and different 6N02 signals were recorded. By plotting 6N02vs. [OC101 for each mode, the chah lengths were detemined as the dope

of each line. The results are shown in Figure 3.5 and showed some interesting features.

The chah length of the proposed configuration for the chlorine radical detector was about

45. This means that the sensitivity of the instrument is low and this detector cannot provide reliable ambient chlorine C: RO, + CIO, mode : Slope~CLRo,,,,=l 10 radical measurements. Furthemore, B: CIO, modc : Slope,=CLao,=45 ,.A

A: RO, mode: Sl0pe,=C~~~=60,' chlo~eradicals are detected by the A ROx amplifier with a chah length of

about 60, or in other words the ROx

amplifier detects C10, with a 50%

efficiency compare to the ROx Figure 3.5. Chain length deteminations for the OC10 source radicals

These experiments led to the following conclusion: either there was a CIOx to

HOx conversion by some hydrocarbon impurities present in the working gases, or our understanding of the chemistry specific to the chlorine detector was not complete. The following experiments were designed to test if the hydrocarbon impurities were responsible for converting C10, radicals to HOx radicals. The major impurity in the CO cylinders (CP grade) is methane, which may be present at concentrations up to 3000 ppmv (Matheson). This high concentration of methane is enough to convert 74% of the chlorine radicals into HOx radicals. A better grade of carbon monoxide was purchased

(Research Purity with [C&]

6[NO2]A/6~O2Jcratio for both grades of CO was obtained and the hypothesis that the hydrocarbon impurities present in the CO cylinders caused the increase in the baseline of the chlorine radical detector was rejected.

The working gases (NO, CO and zero air) were passed through a cold finger immersed in liquid argon (boiling point of argon is bpk=-1 86'~). At this temperature the hydrocarbon impurities were expccted to condense (bpcHp-i 64Oc, bPCM=88Oc). The finger was brought to ambient temperature and the sample was analysed for hydrocarbons by GC-FID in Dr. Niki's laboratory. The concentrations of hydrocarbons in the working gases (mainly methane) were too low to account for the magnitude of the signal 6wOzlA.

The sensitivity of the detector to changes in the NO concentration was used to test the hypothesis of CIOx to HOx conversion by some hydrocarbon irnpurities. The fate of chlonne radicals is determined by two processes: a) C10, are removed fiom the system by termination reactions, mainly represented by reaction with NO to form ClNO (R3.10). b) C10, radicals may be transformed into HOx radicals by some hydrocarbon impurities present in the working gasses:

C10, + NO, Hyd + HOx (R3.25)

Assurning that the limiting step for the rate of reaction R3.25 is the reaction between the chlorine atoms and the hydrocarbon irnpurities, the fiaction of C10, radicals transformed into HOx radicals is given by:

Knowing that 6~02]A=0.556~02]cimplies that foo,+Hox=0.55 or in other words kIu.2s[Hyd]=1.2 kIu.iomO]for these conditions. If BO] is then increased by a factor of

2, the fraction of CIO, converted to HOx will be: Thus, increasing the NO concentration by a factor of two should decrease the ratio

6FJO2JA/6FJ02]cfrom 0.55 to 0.38.

A set of experiments to test this prediction were done for J?JO]=2ppmv and

FJO]=4ppmv and for [CO]=4% and [CO]=8%. The results (presented in Table 3.1) showed that an increase in [NO] did not affect 6j?J02]A/6~02]c.This, dong with the previous resuIts, definitely ruled out the hypothesis of C10, to HOx conversion by hydrocarbon impurities and indicated that there is an unknown mechanism responsible for the increase in the baselin? of the chlorine detector (6pO2lA).

Table 3.1. 6[N02]A/6m02]c for different NO and CO concentrations

3.1.6 New advances in understanding the chemise of the chlorine radical detector

A possible explanation for the experimental observations described in section

3.1.4 is based on the following cycle of reactions.

Cl+CO+M+ ClCO+M (R3-26)

ClCO + 02+ M * C1C002 +M (R3.27) C1C002 + NO -+ Cl + NO2 + CO2 (R3.28)

If this cycle is operative, chlorine radicals can directly complete an amplification cycle with NO and CO, without requiring the production of HO,. When the initial modelling studies for the chlorine detector where done, there was no experirnental evidence for the last two reactions, therefore only the reversible reaction R3.26 was included. Recently, kinetic studies (Hayitt et al., 1996) have shown that the rate coefficient for R3.27 is kR327 = (4.3&3.2)* 10-13 ~m~rnolec'~s-l. R3.28 is similar to the reaction of CW3C002(peroxy acetyl radicai) and FCO02 with NO:

CH3C002+ NO + CH3 + NO2 + CO2 (33-29)

FCO02 + NO + Products (R3-30)

Assming a rate coefficient for R3.28 similar to the rate coefficient for R3.30

= 2.5 * 10" cm3molec" s-' as measured by Wallington et al., 1994), the above chemistry (R3.26 to R3.28) was included in the model. Successive modelling runs confirmed that R3.26 to R3.28 are able to generate a large nurnber of NO2 molecules, and therefore the increase in the baseline of the instrument can be explained.

A method to overcome the effect of reactions R3.26-R3.28 was tested by adding

ClNO (nitrosyl chloride) to the instrument. CINO reacts with chlorine atoms

(l~~~.3~=8.1* 10" l ~rn~rnolec-~ s", Atkinson et al.. 1997) much faster than with OH radicals

1* 10-13 cm3molec-' il, Afkinson et al., 1997).

ClNo + CI +B Cl2 + NO (R3.3 1)

ClNO + OH + HOC1 + NO (R3.32)

Therefore, it was expected that ClNO will teminate the Cl-CO cycle, eliminating the increase in the baseline, while the C10, to HO, transformation in the presence of ethane and NO and the HOx amplification process will be slightly affected.

To test the use of ClNO in the chlorine radical detector, a ClNO pemeation source was prepared in the same way as for OC10, with the exception that the OC10 solution was replaced with a mixture 1: 2 (molar ratios) of NaN02:(CH3)3SiCI

(chlorotrimethylsilane) (Chowdhury et al.,1994). Another series of experiments were performed by constantly adding CINO to the reactor (concomitent with NO) and by sampling different amounts of OC10. The instrument was ernployed in the same configurations as in section 3.1.5. The results are show in Figure 3.6.

S~02].&~02]c=60/110=0.55

30 A: RO, mode CL-16 when no CINO was added to the , . B: CIO, Modc CL46 4 , ' A C:CIO, + RO, Modc CL43 reactor, compare to

6[No2]A/6[No2]c=16/83=O. 1 9 w

ClNO was added to the reactor.

However, the sensitivity of the

chlonne radical detector was not Figure 3.6. Effect of CINO addition to the 1 chimical amplifier operated in 3 different modes. 1 comparable with the sensitivity of the ROx radical detector. This can be easily seen fiorn the following calculations.

Assurning [CIOx]=lOpptv, when no ClNO is added, CL for CIO, is 45 (as in figure 3.5): so the signal produced will be 450 pptv NO2. When ClNO is added, the CL for CIO, is

66 and the signal will be 660 pptv NO2 (about 40% increase in the sensitivity). However, the chah length for the ROx detector is about 120, a factor of two higher than the chain length for the chlorine radicai detector. Therefore, ambient CIO, radical measurements detector even when ClNO is added, may not have the sensitivity required.

Recent laboratory research efforts (Arnold et al., 1998) have confirmed that the

ROx radical amplifier responds to chlorine radicals. Fwthermore, a radical amplifier was deployed at Ny-Alesund in the spring of 1996 and 1997 and radical measurements were

collected by operating the instrument conventionally (modulating CO). The main ROx

radical source was assumed to be ozone photolysis, so the ROx diurnal variation was

superimposed with the UV solar radiation. A good agreement wa's observed for most days, but deviations were also noted. These deviations were attributed to the presence of

C10, radicals in the Arctic troposphere.

3.1.7 OClO as a ROx radical source

OC10 can be used as a non-photolytic ROx radical source to calibrate the conventional chemical amplifier. The advantages of this new ROx radical source are: a) OClO in the gas phase is easily quantified (subsection 3.1.3.2). b) a reliable ROx calibration can be perfomed (Figure 3.5, slope C). c) different ROz radicals can be generated just by substituthg ethane with other

hydrocarbons; d) the radicals are generated inside the reactor, so other substances (i.e water) can be rnixed with OClQ, and this mixture can be then sarnpled by the amplifier. Thus the effect of various substances on the response of the chemical amplifier can be studied.

3.1.8 Conclusions for the chlorine radical detector

The attempts to develop a chlorine radical detector by modiQing the ROx chemical amplifier were not successful. The sensitivity of the chlorine radical detector was significantly reduced due to an increase in the baseline by a cycle of reactions involving CO, NO and chlorine radicals.

The chemical amplifier for ROx measurements also responds with 50%

efficiency to C10, radicals, so previouç ROx measurements in the marine boundary layer,

where chlorine radical may also be present, may not be reliable.

A reliable chlorine radical source based on OC10 was developed. This source

can be converted into a non-photolytic ROx radical source.

3.2 Development of the bromine radical detector

3.2.1 The principle of the bromine radical detector

The bromine radical detector is based on transfonning the bromine radicals to

HOxradicals, that can be measured using the chemical amplifier. This instrument

measures the sum of Br + Br0 = BrO,, since OBrO is not stable at temperatures higher

than 233 K(Kirk-Othmer, 1993). To convert Baxradicals to HOx radicals, compounds

that react relativeîy fast with Br atoms must be used. Good candidates for this conversion

are ddehydes and alkenes. The proposed configuration for the bromine radical detector

is shown in figure 3.7.

Possible interferences (RO,and HOx radicals, CIO, and NO3 radicals) are eliminated by modulating the BrO, to HOx transformation. In the first cycle, a NO;! baseline is obtahed when the air sample is mixed with high concentrations of CO, NO, ethane and the zero air balance flow. In this cycle, the bromine radicals are not converted into HO, radicals, and the luminol detector measures the NO2 produced by R3.1 to R3.15.

In second cycle, the conversion agent for bromine radicals is added dong with CO, NO and ethane and the bromine radicals are

converted into HOx radicals, and a higher NO AU Reactor Sample concentration of NO2 is produced. Details p***..LZ about the methodology are in the

following sections.

Bd- AgatfaBrO. Air to RO. ~IIt'wsi~n

Figure 3.7. Proposed configuration for the bromine radical detector

3.2.2 Bromine radical source

Bromine radicals were produced by photolysing gaseous Br2:

Br2 + hv + 2Br (R3.33)

A Br2 permeation source was prepared by immersing a porous tube into an aqueous Br2 solution (simila. to the OC10 permeation source). Using a halogen larnp

(Canadian Tire, mode1 52-4080-0) bromine radicals were produced by Br2 photolysis at

A2330 nrn with the

experimental

configuration shown in

figure 3.8. The

photolysis cell, 25 cm

Br2 phoiolysis cell long and with a 1.5 cm Figure 3.8. Br source based on Br2 photolysis at hl 330 nm. ID, was made of glass. 3.2.3 Conversion agent for BrOx to HOx transformation

It is known that Br atoms react rapidly with aldehydes and it was assurned that

aldehydes would be suitable agents for conversion of BrO, to HO, radicals. The first

choice was formaldehyde (HCHO) because the chemistry is well studied. Unfortunately,

sufficiently high gas phase concentrations of formaldehyde are difficult to obtain because

of the HCHO polymerisation process. The next choice was acetaldehyde CH3CH0.

Gaseous acetaldehyde was obtained by bubbling nitrogen through an aqueous solution of

CH3CH0.

Bromine radicals were sampled with the bromine radical detector. It was found

that the time needed for one cycle had to be increased from t min to 3 min, because

CH3CH0 sticks on the walls of the valve and reactor and more time is needed to flush the

lines. Increasing the sampling time has a negative effect on the performance of the

instrument (it decreases the signal to noise ratio), as shown in section 4.3. Therefore acetaldehyde is not suitable for the bromine radical detector.

Alkenes, specially propene (CH3CH=CH2), also react relatively quickly with Br radicals to generate brominated ROx radicals, which ultimately lead to HO, radicals in the presence of NO. The chemistry that leads to HO2 radicals fiom propene and bromine radicals in the presence of Os and NO (Figure 3.9) is not completely understood (the yields of the final products are not known). It is known that the addition of Br atoms at the double bond is the major pathway, since the hydrogen abstraction is not common for the bromine atoms. One of the major final compounds is bromoacetone, but there are several termination reactions for the radicals. Such termination reactions forrn stable IËq+"o BrNO

BrCHCH, + HCHO I

1 J BrCHO-CH3 + HO, 1 Figure 3.9. Br to HOx hansformation using propene as conversion agent

compounds such as BrNO, CH3CHBrCH20N0 and (CH3)(CH2Br)CHON0.

Bromine radicals were generated as described above and merexperiments confirmed that propene is a better conversion agent than acetaldehyde because it does not stick on the walls and a higher sampling fiequency is obtained.

Also, further tests were performed to test if the radical amplifier for ROx radical measurements is sensitive to brornine radicals. The radical amplifier was operated in three different configurations (RO,,BrO,, and ROx + BrO, respectively), sirnilar to experiments described in section 3.1 S. The experiments indicated the BrO, radicais are not detected by the ROx amplifier and no increase in the baseline of the bromine radical detector was observed. 3.2.4 Linearity of the bromine amplifier

Previous studies have shown that the ROx amplifier is linear in the 0-200 pptv range (Arias,1 998). According to indirect estimates and recent measurements (section

1.I), the levels of BrO, radicals are lower than 50 pptv, so, provided the BrO, to HOx conversion is complete, the bromine radical detector should be linear.

An indirect method was used to test the Iinearity of the bromine radical detector, as shown in Figure 3.1 0. A constant concentration of Br atoms [Bris was generated using

the source based on Br2 photolysis

(described is section 3.2.2). An

additional flow of zero air Fadd

was added with the non-

modulated reagents (NO, CO and

ethane). Propene was modulated

and an additional flow of zero air

(Fbai)was used to balance the Figure 3.10. Dilution method for bromine radicals. propene flow, similar to the chlorine detector (section 3.1.4). Initiaîiy, when Fadd=O,the total flow through the reactor

Fm,(which was always constant and equal to 1.6 1 min-') was composed by the flow of reagents F,, (NO, CO, ethane, propene), Fbal and the flow sampled fiom the Br source

(Fscontaining a constant bromine radical concentration [Bris). The bromine radical concentration entering the reactor was [Brio, given by: The signal recorded by the bromine radical detector was GFJO2Io.If Fdd>O, Fs will be

smaller, and the bromine radicai concentration sampled will be also smaller and given by:

Fs Fot -F,, -LI -FLi 401 - Fmog - 40, [Br]= [Br],- = [Br], = Pr], -[BrJS-F' = F,, Co1 601 60,

401 -- Fu& = [Br10 -[Br10 - [Br]O (1 - 1 Fot - Fkg - 401 601 401 - - FL and the instrument will record a different signal SFJ02]. If the instnunent is linear the predicted 6/?IO2lcd,fiom the instrument will be given by:

and will be independent of the absolute calibration of the detector.

The method described above was used to test the linearity of the bromine amplifier and

the results are shown in Figure 3.1 1, where the

' measured signal was plotted against the

calculated signal. It can be easily seen that the

instnunent is linear for signals in the range 0-7

ppbv NO2. Assuming a chah length of 100

this translates into a linear behaviour from O to

70 pptv Br.

Figure 3.1 1. Linearity of the bromine radical detector. 3.2.5 Caiibration of the bromine radical detector

The bromine radical source was used to generate different concentrations of

bromine radicals. Bromine radicals were sampled using two different methods: with the

bromine radical detector and by converting Br atoms to bromoacetone by reaction with

propene (C3H6), in the presence of NO (Figure 3.9). The bromoacetone was collected on

Porapak cartridges, extracted with benzene, and quantified by GC-ECD.

The calibration curve for the Photoactive Halogen Detector (PHD),(Impey, 1998)

was used to find a correspondence between the bromoacetone collected and bromine radicals. To calibrate the PHD,known concentrations of Br2 were introduced in the celI, then photolysed in the presence of [NO]=0.5 ppmv and [C3H6]=0.5 ppmv using a Xe

larnp (Impey , 1998). The bromoacetone produced was collected and was quantified

similarly to the method described above. A calibration curve was then constructed by plotting the quantity of bromoacetone trapped on the cartridges against the concentration of Br2 introduced. Through laboratory experiments, the rate coefficient for Br2 photolysis

(JBr2) were measured (Impey, 1998) and used to prove that the rate of Br2 photolysis in the ce11 surpassed 99%. Therefore, the concentration of bromine atoms can be calculated based on the bromoacetone collected.

The results are presented in table 3.2 and show that the precision in calculating the chain length for the bromine detector is very low. The cause of this low precision is not known. Also, since this calibration method is very time consuming (1 point every

1hr), it is not suitable for field calibrations. Table 3.2. Chain length determination for brornine radical detector Pr21 @PW [Br] (PPW 61NOzI~p(PP~V) CL fiom PHD calibration measured by BrOx 6[NO2lavg/[Br] cuve [Br21 *2 detector

0.0495 0.0990 8.44 85.23 Avg = 113.9 Std.dev=84.1 N=7

In the absence of a direct BrO, radical calibration source, a different approach

was tested. In the bromine radical detector, bromine radicals are converted first into HOx

radicals, with a yield ïleax-~ox,which is less than unity. The chain length for the bromine

detector CLBm is then given by:

CL~r0x=~B~OX-HOX CLHOX

The conversion yield ~B~o~-HO~should be constant, being affected only by the

concentrations of the reagents (NO and propene) but not by the conditions of the wall of

the reactor. Therefore the conversion yield qe*.~~~was detennined experimentally and

in the field HOa calibrations were performed to determine CLHO,, and then CLsflx was

calculated using E3 S.

The conversion yield ~B~o~-HO~was measured with the folfowing protocol. The

HO2 radical source based on water photolysis was used to generate constant concentrations of HOz radicals that were sampled by the instrument, operated in different modes. A Br2 penneation source was prepared as described in section 3.2 and used to

add 1-2 pprnv gaseous Br2 to the instrument.

In the first mode, the instrument was used to measure the radicais from the source

in the ROx mode: no propene was added and CO was modulated. Initiaily, no Br2 was

added and a signal 6[N02]0 was measured. Then, Br2 was added and a smaller signal

6pJO2I1was obtained, since a fraction of HOx radicals reacted with Br2:

Br2 + OH + HOBr + Br (FU .34)

The amount of HOx radicals converted into Br atoms, or in other words the amount of

BrO, radicals formed, is given by:

[B~OX]=(~F102Io-6 INOZ]I)/CLHOX (E3 -6)

In the second mode, the instniment was operated in the proposed configuration for rneasuring bromine radicals and the gaseous Br2 being added with NO. A different signal

6wO2l2 was measured:

~[No2]2'CL~rOx*[B~~X~TB~OX-HOX*CLHO~ [Bdxh310x0~-HOX* (~~o2]oc6[N02] i ) (E3-7)

Using 6FJ02]0, 6FO2]1 and 6m02]2 were then used to calculate the yield for the BrO, to

HOx conversion:

A set of three experhent to determine ~~B~o~-Ho~was performed for the following conditions: wO]=2ppmv, [CO]=4% and [CH3CH=CH2]=lppmv. The results led to qBr0x-HOx=0.9&0.1 (95%CL). Therefore tropospheric bromine radical measurements are possible, since the sensitivity of the bromine radical detector with the proposed configuration is 90% cornpared to the RO, amplifier.

3.2.6 Conclusions for the bromine radical detector

The BrOx(Br + BrO) radical detector was developed by modifiing the chemical amplifier for ROx radical measurements. Baxradicals are converted fust into ROx radicals by reaction with propene and NO. The ROx radicals produced are then quantified using the chemical amphfier.

The ROx radical amplifier does not detect bromine radicals.

The BrO, radical detector is linear in the range of 0-70 pptv.

An indirect method was used to calibrate the instrument in the field based on the conversion yield ~B~o~-HO~(measured in the laboratory) and the chah Iength of the ROx amplifier detennined with the water photolysis source. 4 New improvements for the ROx radical detector

In this chapter the efforts to Merimprove the performance of the chemical amplifier for ROx radical measurements are presented. Aspects related to a better characterisation of the radical source based on water photolysis, the ambient water vapour interference for the ROx radical measurements, and signal to noise enhancements are discussed.

4.1 Measurement of the GH~~GEF,~ratio for the radical source based on water

photolysis

4.1.1 The importance of measuring oH2&EF,02ratio

The radical source based on water photolysis at 184.9 nm has been described in section 2.4. This radical source has received a great deal of attention because it provides a reliable and inexpensive way of delivering known concentrations of radicals. The radical concentrations are detemiined using the following equation:

where the concentration of ozone produced [O31 is measured, the water and molecular oxygen concentrations ([H20]and [O2]) are calculated based on dilution flows, and the absorption cross sections (G~~~and oo2)were thought to be independent of the experirnental conditions and known at 184.9nm: om&.546* 1O-" cm2 (Baulch et al.,

1982) and c~~~=0.95*10"cm2 (Washidaet al.. 1971). Recent work has shown that omo and 002 needed to be revised.

The absorption cross section for water vapour at 184.9nm hm been recently remeasured by two groups. The values obtained were similar, 7.14f 0.20*1 0'20cm2

Cantrell et al., (1 997) and 7.1010.1 O* 10'20cm2 Hofumahaus et al., (1 997). These values are 30% higher than the value recommended by Baulch et al., (1982).

The effective absorption cross section for moiecdar oxygen (oEFm)depends on the particular operating conditions for each radical source and must be well characterised

for each particular source (Lanzendorfet

al., 1997). The reason for this is shown in

Figure 4.1, where the absorption cross section

for molecular oxygen was superposed on the

emission spectra for a Pen Ray lamp operated 1949 45 1849 55 Wavelength (A) in two different conditions: less cooling with section and lamp emission with less cooling (A) or more nitrogen (trace A) and the cooling was ooling and plasma voltage reduced by = 3 (B) (Lamendojet al., 1997). dramatically increased and the plasma voltage was reduced by = 3 (trace B). Clearly, the low-pressure Hg lamp is not a line source: the emission spectrum has a red-shifted tail, probably due to fluorescence of the envelope of the lamp, formation of dimers of Hg or line broadening phenomena (Cantrell et al.,

1997). Furthemore, a line reversal was observed (Lanzendorf et al, 1997). This can be a major problem as the real absorption cross section for O2 varies over the width of the emission line.

The effective absorption cross section for02 (cTEF,~~)mut be used in E4.1 instead of the absorption cross section for 184.9nm. This can be obtained by using a weighted lamp spectrum (Lazendorfet al., 1997):

Based on the above considerations, it was crucial to detemiine the a~~da~~,~2 ratio for our own radical source based on water photolysis at 184.9 nm. Usually, figh- resolution spectra of lamp are taken, then local absorption cross sections are measured and E4.2 is used to calculate ~EF,OZ.The difference between GEF,OZAand ~EF,O~Bwas about 30% (Lanzendorf et al., 1997), which means that the [HO2] calculated with E4.1 has a large uncertainv, if GEF,~~is not measured for each individual source. A somewhat simpler method used to experimentally measure this ratio was deveIoped and is presented in the following sections.

4.1.2 Theoretical aspects for the a~zda~~,~~determination

A mixture of water in synthetic zero air is passed through an illuminated region of

184.9 nm radiation. As detailed in section 2.3, the following reactions dominate the photochemistry in this illuminated region:

H20+hv (1 84.9nm) + OH + H

H+02+M+ HOz+M

02+hv (184.9nm) + 20

0+02+M+ 03+M By neglecting the loss reactions for the radicals and by assuming that the photolytic reactions are the rate-detennining step (R4.2 and R4.4 have high reaction rates, since the [02j=2 1%), the production rates for 03and HO2 are:

where 0184.9 is the actinic flux at l84.9nm

OEF, 02 is the effective absorption cross sections for 02

am0 is the absorption cross sections for H20

qo3=2 and q~02=l are the yields for the formation of o3and HO2 respectively;

(qH02=1 since [CO]=O, the OH radicals are not converted to HO2 radicals).

By integrating E4.3 and E4.4 and recognising that At is the time interval that the air parce1 flows through the illuminated region, which can be related to the air flow and the volume of the illuminated region by At= VIFair,the following relations are obtained:

V [HO2 f = 01a.9* a~2,* [H20] * At = * anlo* [H20]*- Fa,,

Then, if the 03and HO2 are measured for different airfiows with constant and known concentrations of water and oxygen, plots of [O3]and BO2] vs. l/Fairshould be linear and the slopes of the plots would be: Then, c~~O/

Another way to analyse the results is by plotting [HO21 against [O31 for each of different flows, computing the slope rn~o~o3,which is then used to calculate Q~z~G~~,~~:

The advantages of this novel method are: it is relativeIy easy to perform by measuring the output concentrations of HO2 and o3and GEF,~~can be easily detennined under field conditions by measuring mo3.

This method to determine CTH~~/~EF,~~is based on the assumptions that the HO2 losses are negligible. Therefore, the HO2 losses should be estimated and the experimental conditions must be chosen accordingly.

The main HO2 losses are represented by the following reactions:

OH + HOî + H20+ 02

HO2 + HOz -+ H202+ 02

HOz + HO2 + M+ H202+ 02+M O3 + HO2 + OH + 202 (R4-7)

The rates of production for OH and HO2 radicals are equal; assuming that the other losses for OH radicak are also comparable to HOz losses leads to [HO2]z[OH].

Also, [O3] was about a factor of 30 higher than [HO2].

The rate coefficients for the these reactions (Atkinson et al., 1997) are: kR4.2=1. I * 1 O-'' ~rn~rnoiec~~s-~ kR4.6a+ kR4.6b=3 .O* I 0-I2cm3mokc-' S" kR4.,= 2.0~1~~'~crn~ molec-' s"

By comparing the ratios between various losses for the HO2 radicals

we can see that the major loss for the HOz radicals in the illurninated region is R4.5.

Integrating we obtain:

Imposing that HO2 losses to be less than 3%( [H02]20.97[HO2]0) Ieads to Ats28ms for

[H0210= 10'' molec/cm3=0.4pptv.

This will be an upper limit as there are additional losses for the OH radicals, the main one being OH+OH+M + H202+M (R4-8)

Due to this additional OH loss, [OH]

This method also requires that the synthetic zero air should contain minimum amounts of hydrocarbon impurities, since OH radicals react with these impurities and a small amplification cycle can be established:

OH+m+O2-+R02+H20 (R4.10)

NO + R02+ RO + NO2 (R4.11)

RO + O2 + HO2 + R'CHO (R4.12)

NO + HO2 + NO2 + OH (R4.13)

In conclusion, the requirements for an accurate determination of crH2&EF,02are:

[HO21 in the 0.1-0.5 ppbv range, short transport times (At 5 28 s) between the photolysis region and NO addition point, and synthetic air with minimum concentrations of VOC.

4.1.3. Experimental aspects for G~~G~~,~~determination

[O3] and [HO2] were rneasured with the LMA-3 detector, derconversion to NO2 in the presence of NO (no CO is added, since OH radicds are converted into HONO and no amplification occurs):

03+N0 +NO2+02 (R4.14)

HO2 + NO + NO2 + OH (R4.15)

OH+NO+M jHONO+M (R4.16) The experiments for GH~&~~,~~determination were sirnikir to a HO2 calibration

(figure2.2) and were conducted as follow:

a) flow of zero air (at lest 30 1 min-') was passed through the photolysis ceIl, (not

illuminated) sarnpled by the luminol detector, in the presence of [NO] -3 ppmv. A

constant concentration [NOl]base was added from a NO2 penneation tube, to bring the

luminol detector in the linear regime (section 2.1).

b) the Hg-lamp was turned on, allowed to stabilise for at least 1 hr and then different

flows of zero air (30 to 40 Ipm) were sampled and corresponding fNO2IO3signals were

recorded. The ozone produced for each flow was then calculated by subtracting mO2lbase.

c) A small fraction of the zero air flow (up to 8%) was saturated &th water vapour by

passing through a series of bubblers with distilled water, kept at constant room

temperature (2 1OC). This "wet" flow was then mixed with the rest of the airflow and the

total flow adjusted to be equal to the previous "dry" flow (b). The absolute water

concentration through the photolysis region was calculated assuming 100% saturation

(this was checked using an EG&G dew-dlTMdigital humidity analyser, mode1 91 1). By

passing this flow through the photolysis region, the lurninol detector measured a higher

signal [N02]fi02. By subtracting FJO& from p02]~02,the [HO21 produced was then

detennined.

The inlet was modified to minimise the time interval spent by the air sarnple between the photolysis region and the NO addition point. This time interval was calculated based on the geornetrical characteristics of the photolysis ce11 and inlet, along with the flows of zero air, and was always less than 28 msec. This method was used to determine aH2dbEF,02for two radical sources; both sources have been used for HO2 calibration in previous laboratory and field radical rneasurements (Arias, 1998). One was described in section 2.4 and the other one was similar, but with different dimensions (the inner diameter was 15 cm and the length was

30 cm). For the second cell, the flows were smaller (10-15 1 min-').

4.1.3 Results for CH~~CTEF,~~detennination

The results of a Spica1 o~~&~~,~~determination are shown in figure 4.2. The experiments were performed three times for each photolysis cell.

The overail resuits for to bH2&EF,02detennination are presented in Table 4.1 and show consistent results. Our radical sources are similar in design with those developed

Figure 4.2. Determination of CJH20/~EF,02 for [Hz0]=2100ppmv using the 33 mm ID photolysis cell; the remession lines were forced through zero. and used by various groups involved in peroxy radical measurements. Recent experiments were conducted to determine oEF.02 for these systems (Voh-Thomas et al.,

1999). Combining the obtained range of values for 0~F.02(1.1-1 S*10-~~crn~) with the

QEF,~Oas measured by Canhe~~et al., (1997) GW~=~.14M.20 * 10-20cm2, leads to a range of values for the ratio G&EF,~~~ ranging fiom 4.8 to 6.5. Our Q~&~~.~~ measurements are consistent with the above range.

Table 4.1. Determination of bH2&EF,02 33 mmID 15 mm ID photolysis ce11 photolysis ce11

[Ha ~EF.H~~GEF,O~ [Ha GEF.H~~GEF,O~ ppmv ppmv 2100 6.26 1300 6.49 2040 6.09 1500 6.08 1920 6.32 1600 6.17 Avg: 6.2 Avg: 6.2 95%CL: 0.3 95%CL: 0.5

For the radical measurements performed pnor to these experiments (July 1998), the value for the ratio owdoozwas 5.546' 10-l0cm2 /0.95*10"~cm2 = 5.84. Therefore, al1 the radical measurements (including BrO, measurements) were corrected by a small factor (6.215.84-1.06). 4.2 The effect of ambient water vapour on the RO, radical amplifier

The laboratory and field calibrations for the radical amplifier are performed using synthetic zero air with ody 0-200 ppmv of water, whiIe arnbient water concentrations are in the percent range. Laboratory studies on the effect of ambient levels of water vapour on the radical amplifier were conducted, and the results are presented in the following sub-sections.

4.2.1 Methods for studying the humidity effect on the ROx radical amplifier

Two radical sources, in two groups of experiments, were used to study the effect of humidity on the radicaI amplifier. For the first group, the radical source based on water photolysis was used. The experimental configuration is shown in Figure 4.3.

Hg Lamp CO l * 0-200 ppmv H20/Air Reactor

I<.. Ad ytional DRY

'T'.--L MFC I

I : I LMA-3 Distilled Water 1 Zero Air 1 ' Pump at constant temperature 1 MFC i -r-.4 Exhaust 'ïgure 4.3. Experimental configuration for studying the ambient water vapour effect on the RO, radical mplifier using the radical source based on water photofysis at 184.9 nrn. A supplementary flow of zero air, which was either dry or water saturated, was added along with NO, while the amplifier was sampling radicals fiom the water photolysis source. The procedure is similar to that used to test the linearity of the Bax radical detector (section 3.2.4). Knowing the total flow through the reactor (Fm), the NO and CO flows (FNOand Fco) the flow added with NO (Fada)and the initial signal

(6m02]o)when Fadd=O,the expected signal based on dilution is obtained similarly to

E3.7:

and 6pJO2]wetwhen Fsdd was saturated with water vapour. 6plO2Io, was in agreement with 6[N02]~,but 6[NOz]wet was lower than 6[NO2Icd,. This led to the conclusion that humidity lowers the response of the chernical amplifier.

For the second group of experiments, peroxy radicals were generated using the chlorine dioxide source (section 3.1 -3). OClO in the gas phase is easily quantified by titration with NO (section 3.1.3.2). Since gaseous OClO can be diluted with either dry or wet air, the radicals are generated inside the reactor, after reaction with NO and ethane:

OC10 + NO + NO2 + Cl0 (R4.14)

Cl0 + NO + NO2 + Ci (R4.15)

Cl + CH3CH3+ HCl + CH3CH2 (R4.16)

CH3CH2+ O2 + M + CH3CH202 + M (R4.17) 4.2.2 Results for the humidity effect on the chain length of the radical amplifier

Experiments were performed using both procedures presented in section 4.2.1.

Radical concentrations of 10 to150 pptv were used, so the radical amplifier was always

kept in the linear regime. The radical amplifier was configured as described in section 2.

The relative humidity (RH) was calcdated as ]RH=(F,dd/Ftol)* 100. The results, shown in

Figure 4.4, are reported in terms of the ratios of the chab lengths for wet and dry conditions CLwet/CL~,. This is because the radical concentrations sampled were

identical for both dry and wet conditions, and the ratios of the signals recorded were in fact the ratio of the chain lengths:

~0]=2ppmv,[C0]=4% Reactor :1/4" Teflon PFA

I r RO, generated using OC10 source Li

O HO, generated by %O photolysis

O 20 40 60 80 1 O0 RH(%) Figure 4.4. Relative humidity effect on the chain length of the RO, radical amplifier. The results proved that the humidity significantly lowers the chah length of the

radical amplifier, consistent for both radical sources. Because the radical source based on

water photolyis is more stable and easier to use, this source was used for Mer

experiments.

To test if this is common problem to al1 the radical amplifiers, similar experiments

were perfomed by replacing the %" OD Teflon reactor with two other reactors: a !4" OD

Teflon reactor and a 3 cm OD, 30cm long glass reactor. CLDrywere deterrnined for al1

three configurations using the calibration procedure described in section 2.3 and were

120 for the '/4" OD Teflon reactor, I r Glass Rcactor IL2 " Teflon Rucmr 1 160 for the ?4" OD Teflon reactor and & 114 " Tcflon Rcactor 100 for the glass reactor. The results

(Figure 4.5) show that the radical

l 1 0.2 i, / amplifiers behave essentially the 1 1 0.0 same in spite of their different reactor O 20 40 60 80 RH(%) geometries or materials. Figure 4.5. RH effect on the chernical amplifier for three different reactors

Since the calibrations are usually performed under dry conditions (0-200ppmv of water), the ambient ROx measurernents may be greatly underestimated: for example at

40% RH, the Clwetis a factor of two lower than ClD,, which means that the actual radical concentrations are in fact two times higher than those given by the radical detector. At higher relative humidities, the radical concentrations may be underestimated by an even larger factor. In addition, the chah length is very sensitive to changes in the relative humidity: at about 40% RH it changes by about 1% for every 1% change in RH.

The chain length decrease with humidity may be caused either by a heterogeneous

process or by a homogeneous effect on the chemistry of the amplifier. Water may be

adsorbed on the walls and given the affinity of HO2 radicals for water, the heterogeneous

radical wdl losses could be higher at high relative humidity. Alternatively, water may

have an adverse effect on the reactions involved in the chernical amplification cycle or it

may increase the rate for radical termination reactions.

If the effect is predo&nantly due to gas phase kinetics, the absolute water

concentration inside the reactor should be the dominant factor. By increasing the

temperature of the reactor, for the same water'input, the absolute water concentration was

kept constant, but ]RH was varied, A cornparison between two different temperatures

would provide clues about the main cause for lower chain lengths. Expenments were

perfonned at room temperature (21'~)and by heating the walls of the glass reactor at

60°c,which heated the gas so that the air temperature leaving the reactor was 37'~. The

results (Figure 4.6) show that, for the same absolute humidity, there is less impact when

the reactor is heated. Thus the absoIute

water concentration is not the main cause

for the lower chain lengths. The main

process responsible for lowering the chah

length appeared to be the heterogeneous

I radical wail losses. Figure 4.6. Absolute humidity effect on the chain Ilength of the ROx radical amplifier. 4.2.3 Measurement of the heterogeneous wall losses for peroxy radicals

Up to this work, the wall loss rates for peroxy radicals have been estimated by

fitting the plug-fïow model of the chemical amplifier (Table Al) to match the chah

lengths detemined through calibrations (Hastie et al., 1991). The model was also able to

reproduce the sensitivity of the chemical amplifier in respect to NO and CO

concentrations. Also, despite their different geometries and construction materials for the

reactors, most of the chernicd amplifiers have similar chah length (120-200); therefore

the wall losses rates appear to be comparable. Based on these modelling studies, a rate coefficient for first order HO2 wall losses of 2.5 s" was estimated.

4.2.3.1 Experimental setup for measuring the wall losses for HOz radicals

The experimental configuration for measuring heterogeneous wall losses for HO2 radicals is show in Figure 4.6. The components were: the radical amplifier; the water photolysis source for HO2 radicals; the humidifying air suppiy, where a measured flow of zero air is passed through a temperature stabilised bubbler; and tubing for wall losses determination. Al1 measurements were made on the same 114" OD Teflon PFA tubing used as the reactor in the chemical amplifier. A typical experiment, dry or wet, was performed by measuring a signal 6[NO2Iofor a minimum length of tubing, followed by measuring several signals 6FJ02] for different lengths of tubing. The reaction time was calculated using the airflow through the tubes, the interna1 diameter and the length of the tubes. Based on the rate coefficients for the radical self-reactions, for wet or dry conditions, it was estimated that these processes are responsible for removing less than Chernical Arnpl ifier Radical Source

Tubing for b, CO l determination

il' JI/-

--

1

Water at constant temperature ! 1

1 -- HurnidiSring Air Supply

.- .- -Exhaust : >

Figure 4.7. ExperimentaI configuration for measuring HO2 wall losses for wet or dry conditions.

2% of the radicals and were not considered.

The radical wall losses are assumed to be first order with respect to radical concentrations:

A plot of ln([H02]/[H02]0) vs. reaction time should be linear and the slope cm be used to obtain the rate coefficient for walI losses. The absolute values for the radical concentrations are not needed to determine kWaissince the instrument was operated in the linear range (10-1 50pptv) and the chah length is constant:

4.2.3.2 Sources for CH302 and CH3CH202 radicals

Organic peroxy radicals (Rot) can be produced by replacing CO in the radical source witb another hydrocarbon, so OH radicals are converted to R02(i.e Cawas added to produce CH302 and CH3CH3 for CH3CH202).

Initial experiments, performed by applying this procedure, showed that additional radicals are fonned: higher concentrations of radicals were measured when CO was replaced with methane or ethane. The origin of these radicals was attributed to the photolysis of impurities present in the hydrocarbon mixture. However, by adding the parent hydrocarbon after the photolysis region, a consistent mixture of HO2 and R02 radicals was obtained.

During the course of this work it was found that the wall losses for HOz radicds are much higher than the losses for the organic peroxy radicals. This provided a good opportunity to remove the HO2 radicals, by passing the radical mixture through several meters of tubing. To find out the minimum length required for the HO2 removal from the radical mixture the parent hydrocarbon was replaced with CO. Increasing lengths of

Teflon tubing were introduced between the radical source and the amplifier, until the HO2 radical concentration was below the detection limit of the radical amplifier (0.5pptv). By using this procedure, only the HO2 radicals were removed from the radical mixture. 4.2.3.3 Wall loss rate coefficients for HO2, CH302and CH3CH202 radicals

The variation in kdt with relative humidity for the HOz, CH302and CH3CH202

radicals is shown in figure 4.8. For HO2 radicals the rate coefficient for dry conditions is

2.8k0.2 s-', in good agreement with the value of 2.5 s'l found to give the best fit to chah

10 - PFA Teflon 1/4" nq

Figure 4.8. Rate coeficients for peroxy radical wall losses. length measurements. While the relative humidity seems to have almost no efi

kWailfor CH302and CH3CH202 radicals, it drarnatically enhances the rate coefficient for the HO2 radical wall losses fiom 2.810.2 s-' at RH=O to 7.41 I .O s" at RH=65%. This

different HO2 behaviour is consistent with the fact that HO2 and water form an adduct (Li et al., 1982), while there no evidence for an adduct fonned by organic peroxy radicals and water. This adduct formed with the water condensed on the walls rnay be responsible for the increase in the rate of HO2 losses on the reactor walls. 4.2.4 The effect of increased NO concentrations on the sensitivity of RO, radical amplifier to relative humidity

The wall losses represent one of the main losses of the radicals during the amplification cycle. Another main radical loss is the OH radical removal by NO (RI.32).

One way to reduce the instrument sensitivity to increased wall losses due to the water condensation on the walls, is to increase the rate of gas phase removal of the radicals by increasing the NO mixing ratio in the reactor.

Sirnila.experiments to those

described in section 4.2.2 were performed

for a higher NO concentration of 12 ppmv

(the CO mixing ratio was kept constant at

4%). The experimental results are compared

with the results for m0]=2 ppmv in Figure

4.9. For the high FJO], the instrument

sensitivity to water is less pronounced

Figure 4.9. Effect of increased WO]on the compared to the lower PO]. RH effect on the radical amplifier

At mO]=12 ppmv, the lumirio1 detector is not linear and the chah length of the amplifier is lower. Increasing NO concentration for ambient sarnpling has these two disadvantages:

- the quadratic equation for calibration curve for the luminol detector must be used.

- the sensitivity of the chernical amplifier is lower. 4.2.5 Modelling of the radical amplifier sensitivity to water vapour

Modelling studies were conducted to test whether the increase in the radical wall losses could explain the observed sensitivity of the radical amplifier to water vapour. In addition to the "dry chemistry" of the radical amplifier presented in Appendix 1, the water dependence of the third order reactions of OH with NO (Overend et al., 1976), HOi with HO2 (Li et al., 198 1) and HO2 with NO2 (Sander and Peterson, 1984) were included. The measured rate coeficients for the radicai losses to the wall for different relative humidities were used and the results, reported as CLw&L~ly, are shown in

Figure 4.10. For [NO]=2 ppmv (right panel) the experimentai decrease in the chain length as the relative hurnidity increases is almost reproduced by the model. At pO]=12

~0]=2ppmv 1 Experimental Experimental Calculated O Calculated

Figure 4.10. The observed CLwet/CLDW(solid syrnbols) compared with those predicted by the model (open symbols) for [NO]=12ppmv and [NO]=2ppmv. ppmv (lefi panel) the agreement between the experiment and the model worsens.

The discrepancy between the model and the experiments cannot be explained by our current understanding. It points to an additional water dependence of the gas phase chemistry. Water vapour may lower the rate of one of the reactions that form the amplification cycle or increase the tennination reactions for the radicals. Since the HO2 radicals are present at higher mixing ratios than OH radicals and HO2 radicals form an adduct with water in the gas phase (Li et al., 198 1, Sander and Peterson. 1984):

HO2 + H20+ H02H20 (R4.18)

H02H20+ HO2 + H20 (-R4.18)

It is expected that the water vapour must impact the HO2 chemistry.

The equilibriurn constant for the adduct formation R4.18 is &,-,=3 * 1O-'' cm3 molec-' (Sander and Peterson. 1984) which means that at 2 1OC and 40% relative humidity ([~~0]=2.5*10"molec cmJ) 7.5% of the HO2 nidicals are in the adduct form.

This limits the impact on the chah propagation chemistry. Even if the adduct HOzH20 was unreactive, this would lower the rate coefficient for HO2 oxidation of NO to NO2 by a maximum of 7.5%, but there is no such evidence in any kinetic experiments.

Furthemore, based on model calculations, this 7.5% decrease in the mentioned rate coefficient will reduce the chain length by only 1% for wO]=12 ppmv. This is not consistent with the 40% decrease in the chah length experimentally determined.

One possibility for the impact on the chain tennination chernistry is that the water adduct (H02H20)reacts Merwith NO to form peroxynitrous acid HOONO or nitric acid HN03: HOtH20 + NO + HOONO + H20 (R4.19)

H02H20+ NO + HONO2 + H20 (R4.20)

The HOONO could Merisomerise to nitric acid (R4.21) or decompose (R4.22):

HOONO -+ HONO2 + Hz0 (R4.2 1)

HOONO + HO2 + NO (R4.22)

In order for the mode1 to simulate the observed decrease in the chah length, a rate coefficient for R4.19 or R4.20 (in this case the rate of R4.21- the rate of R4.22) of about 7* 1 O-" cm3 m01ec'~s" is needed. At 40% relative humidity, this rate of this tennination reaction represents about 1% of the rate of the propagation reaction (R1.28).

Kinetic experiments, carefully design and conducted, could provide evidence about this loss mechanism for both radicals and NOx because it may have possible atmospheric implications (Mihele et al., 1998) the major radical and NOx loss in the atmosphere is nitric acid formation fkom OH and NO2:

OH + NO2 +M +HONO2 + M (R4.23)

The proposed reaction R4.20 may be an additional significant sink for NO, and radicak and its possible impact can be estimated assuming: a) the rate of R4.20 is rnuch higher than the rate of R4.21 or the rate of R4.21 is much

higher than the rate of R4.22; b) [H02]/[OH]=100; c) ~02]~IN01=3;

The rates of loss for HOx and NOx through the two pathways cm be compared: . Loss -to - nitric - acid - R4.23 - - kR4.23 ' 1. [OH 1 Loss - by - proposed - R4.20 kR4,*,K, [H20]* [NO] [HO,]

1.4*10'" - kR4.23 .-CN02] [OH] kR,2,.K,-[H20] [NO] [H023-7~104'3~3~10-19~2.5-10"1 100

Thus, the reaction of HO2 with NO in the presence of water may be minor, but significant

loss for radicals and NOx (up to 12% of loss through nitric acid formation).

4.3 Optimisation for the operation of the RO, radical amplifier

4.3.1 Enhancement of the Signal to Noise Ratio

The signal obtained by the chernical amplifier for ROx radical measurements is

measured on top of a NO2 baseline with the main component being the NO2 produced

fiom ozone titration with NO (section 2.2). Usually the ROx signal is quite small

compared to this baseline. For example for [ROx]=10 pptv and [03]=50ppbv, the ROx

signal will be (for a chah length of 120) 6m02]=1.2ppbv, while the baseline is 50 ppbv.

A change in baseline for two consecutive baseline measurements of ody 1% (50*0.01 =

0.5 ppbv) would introduce an error of 4.2 pptv (or 42%) in the ROx radical measurements. Based on these considerations, one way to improve the performance of the radical amplifier is to reduce the variations in the baseline. This could be done by a higher sampling fiequency or by reducing the baseline to lessen the impact of slow variations in ozone concentration.

A higher sampling fiequency was achieved by minimising the dead volumes and by replacing the 3-way valve with a 4-way valve (Figure 3.4). The limiting factor is the defay time (20 s) for the flow stabilisation through the luminol detector. Implementing

this change reduced the RO, samplhg time fkom 75s (Arias, 1998) for one measurement

to 60s.

As shown in figure 2.1, the NO2 detector is typically situated in the laboratory, so

the gases travel about 9 s fiom the reactor to the detector. During this tirne, ozone is still

being converted to NO2 (R4.11). Based on a [NO]= 2 ppmv =5* 10') molec/crn3 and

k~4,11=1.8*10-14 cm3 molec-' i' ,the lifetime of ozone will be 1.1 sec. Moving the NO2

detector closer to the reactor will reduce the NO2 baseline if the transit time can be reduced to 0.5-1 s, as less NO2 is produced by this reaction.

In this case, the 3-way vaive used to modulate the carbon monoxide must be replaced with a 4-way valve and an additional flow of zero air (equal to the flow of CO) needs to be used. A combination of two 3-way valves cm be used instead of a 4-way valve. This change is important, since 3-way valve configuration contributes to negative signal when no radicals are present. To illustrate this negative bias the following conditions are assumed: a) total flow through the reactor ~y1600cm~/min; b) reagent flows: CO ~co=64cm~/minand NO flow ~~0=16crn~/min(for a cylinder of

[N0]=200ppmv); d) the time ailowed for radical rernoval (the time after the CO is added to the reactor in

the chemistry off cycle) is tr=1sec and the residence time between the reactor and the

NO2 detector t2=l S.

The NO mixing ratio in the first cycle when CO is not added in the reactor is [NO]co-ott-2.08 ppmv in the reactor and mO],=2.00 ppmv after the reactor. In the second cycle, CO is added in the reactor and the NO rnixing ration is [Nolco -,,,,=2.00 ppmv. NO2 produced in the first cycle fkom ozone titration will be:

For [03]=5O ppbv, ,Sm02]= -0.29ppbv [ROX]=-2.9pptv (for a chah length =1 00). This negative signal could be completely removed if the NO concentration is kept constant in both cycles by replacing the CO flow with an equal fiow of zero air.

4.3.3 Minimisation of the water vapour interference

The radical amplifier is sensitive to ambient water vapour as shown in section 4.2.

One way to obtain accwate ROx radical measurements is to perform calibrations as a function of humidity for various temperatures. The data would then be corrected using ambient hurnidity data collected simultaneously. However, given the magnitude of this humidity correction, this procedure will decrease the precision of the ROx measurements. A second approach is to redesign the radical amplifier to minimise the effect of ambient water vapour.

The most obvious improvement is to heat the reactor. However, this would introduce another interference, since PAN-type species could decompose inside the reactor. To counteract this, the air sample residence time in the reactor would need to be reduced. Modelling studies (Hastie et al., 1991) have shown that the radicals are removed in about 0.03sec in the absence of CO, so decreasing the residence tirne in the reactor to

0.1-0.3s from 1s would be enough to inactivate the radicals in the off cycle and to reduce the decornposition rate of the PAN-type products. This tirne interval is not sufficient to complete the amplification process, (since the time needed for the amplification chemistry based on modelling studies is 0.8-1 .O s). This process proceeds in the heated tubing leading to the NOz detector, where the temperature of the air is higher than ambient temperature and the relative humidity is much lower, and therefore the sensitivity of the detector towards humidity will be significantly reduced.

To test this hypothesis, experiments were performed with a %" Teflon PFA reactor (instead of the %" Teflon PFA reactor). The experiments were perforrned at room temperature (21 OC) and then the reactor walls were heated to 60'~and 80'~.The residence time for the air sample in this reactor was 0.3sec, the reagent concentrations in the reactor were [N0]=3 ppmv and [C0]=6%. Dry HO2 calibrations showed that the chain lengths were higher (1 80) than the previous configuration (described in section 2) and about 10% higher than the chain length calculated for [NO]=3 ppmv and [CO]=6%., most probably due to lower wall losses for this reactor. Further experiments were performed to test the humidity effect on this systern and the

results are shown in figure 4.1 1 where

Cl,d CLdryis plotted against the relative

humidity of the air that enters the reactor.

The results showed that the humidity

effect is less pronounced if the reactor

walls are heated, similar to the results

already presented in figure 4.5.

Figure 4.11. Humidity effect on the amplifier with 0.3 s residence time in the W' reactor

The same configuration was used to test the PAN interference for the radical measurements. Liquid PAN was prepared similar to Nielsen et al., (1982) by nitrating peracetic acid in an inert solvent (dodecane). Constant concentrations of gaseous PAN were obtained by bubbling a small flow of nitrogen through diluted solutions, of gaseous

PAN were obtained. Gaseous PAN was quantified by measuring the amount of NO2 produced fiom the PAN thermal decomposition PAN at 200'~ in the presence of BO]=

3 ppmv:

CH3C03N02+ CH3CO3 + NO2 (R4.24)

CH3C03+NO + CH3 + CO2 + NO2 (R4.25)

CH3 + O2 + M 3 CH302 + M (R4.26)

CH302+NO + CH30 + NO2 (R4.27) CH30 + O2 + + HCHO + HO2 (R4.2 8)

Net : CH3C03N02+ 4N0 + 02+ 4N02 + HCHO+ HONO

Gaseous PAN, in the O-SOOpptv range, was sampled by the amplifier, with the reactor walls heated to three different temperatures (2 1°c, 60'~and 80'~respectively).

The results are show in figure 4.12,

where the amplifier response is plotted

against PAN concentration. These

laboratory experiments showed the

following PAN interference:

twali=210~,O. 1IO. 1%;

tWdi=6o0c,O.6kO. 1%; Figure 4.12. PAN interference for the chernical amplifier with the reactor heated. tWdi=8o0c,O.8+0.l%.

In conclusion, despite the improvement described here, the radical amplifier should be conf~guredfor field measurement based on characteristics of the sampling site.

For example, for RO, measurement in non-polluted marine boundary layer (high relative humidity and low levels of PAN), the reactor walls could be heated at 80'~or even higher. For studies where the PAN concentrations are expected to be high, the reactor walls could be heated to a lower temperature. Usudly, ROx radical measurements are made simultaneously with measurements for other important trace gases, including PAN. Thus, the ROx radical data cm then be corrected based on laboratory experiments for

PAN interference.

For the bromine radical measurements, the reactor can be heated at higher temperatures. The radicals generated fiom PAN thermal decomposition will be incorporated in the NO2 baseline, dong with the NO2 produced fiom ozone titration, RO, amplification and arnbient NOz. However, the instrument must be operated in the linear range (0-150 pptv) and care must be taken in choosing the operating conditions 5 Polar Sunrise Experiment 1997

The main objective of this Polar Sunrise Experiment 1997 was to bring new

insights about the spring Arctic ozone depletion phenomenon. Photolysable bromine and

chlorine species were measured using the Photoactive Halogen Detector (PHD)

(presented in section 5.2), while bromine radicals were measured using the bromine

radical detector. Since the sensitivity of the chlonne radical detector was reduced by

about 50% (as shown in section 3.1) and the expected chlorine radical concentrations

were expected to be in the low pptv range, no chlorine radical measurements were

attempted during this filed carnpaign.

5.1 Site location

The Polar Sunrise Experiment 1997 (PSE97) took place at Alert, Northwest

Territones, Canada (82.3% and 62.2'~)fiom March 25 (Julian Day 84) to April 11

(Julian Day 101). Alert is situated at the northem edge of Ellesmere Island, on the shore

of the Arctic Ocean, only 800 km away fkom the North Pole. Two laboratories, situated about 1 km apart and about 6 km away fiom the Canadian Forces Station, were used during this experiment. In the Special Studies Laboratory (SSL), the bromine radical detector, the PHD and the W radiometer were set-up, while the BAPMON laboratory was used to host the ozone monitors and the meteorological instrumentation. To avoid local contamination, no utility vehicles were driven close to the laboratories during sampling. 5.2 Instrumentation deployed

Meteorological data (wind speed, wind direction, temperature, relative humidity) dong with in-situ ozone measurements were collected continuously before, during and

&er PSE97 by Atmospheric Environment Service (AES). The bromine radical detector, presented in section 3.2, was operational from March 29 to April 1 1 and the PHD collected sarnples fiom March 27 to Apnl 1 1.

The PHD (Zmpey et al., 1997b) relies on drawing the air sample into a cylindrical cell, where the photoactive halogenated species (mainly X2 and HOX, where X = Br, Cl) are photolysed, yielding halogen atoms. These halogen atoms react fûrther with added propene (CH2=CHCH3) and NO in air according to reactions R5.1 to R5.5, to forrn the stabIe halogenated compounds (XCH2COCH3), bromoacetone and chloroacetone. The bromoacetone and chloroacetone formed are collected on Poropak cartridges, extracted in benzene, separated and quantified by GC-ECD.

HOX, X2+ hv (h>330nm) -P X (R5.1)

X + CH2=CHCH3+ XCH2CHCH3 (RS .2)

XCH2CHCH3+ O2 + M + XCH2C(OO)HCH3+ M (R5-3)

XCH2C(OO)HCH3+ NO + XCH2C(O)HCH3 + NO2 (R5.4)

XCH2C(0)HCH3+ O2 + XCH2COCH3 + HO2 (R5.5)

As shown for bromine atoms in Figure 3.9 (the chemistry for chlorine is similar),

XCH2COCH3is not the only product formed, and therefore the yield of XCH2COCH3 formation in not unity in respect to the halogen atoms. The formation yields for bromoacetone and chloroacetone were determined in the laboratory (Zmpey, 1998). For PSE 97, the Pl33 was fùrther modified in order to differentiate between Brz and HOBr and between CIz and HOC1 (lmpey, 1998). Since HOX and X2 have quite different photolysis rate coefficients, two photolysis cells with different residence times were used. The residence tirne in the first ceil was about 17 s, while for the second ce11 the residence time was 13 1 S. The photolysis rate coefficients were detennined experimentaliy for each ceil (Impey, 1998). For ambient sampling, when the difference between bromoacetone or chloroacetone for the two cells was small, X2and HOX could not be differentiated, and then the ambient concentrations of total photoactive halogenated species are reported as (Xz),.

The upward looking radiometer is sensitive to UV solar radiation (285-385 nm).

This radiometer was not calibrated, and these data were just used just as a relative measure of the UV solar radiation.

5.3 Field operation for the bromine radical detector

The bromine radical detector was configured similarly to the RO, radical detector

(Figure 2.1): the reactor and the 4-way valve were placed outside the laboratory (about 10 m away from the NOz detector). The valve and the initial 1/10 portion of the reactor were kept in a temperature stabilised housing at 10'~. The reagents (NO, CO, ethane and propene) were fed to the reactor via W Teflon PFA tubes. The istrument was operated with the following mixing ratios inside the reactor were: mO]=2 ppmv, [CO]= 4%,

[C2&]=5 ppmv and [C3&]=1 ppmv. A BrO, measurement was collected every minute

(20 s dead time and 40 s acquisition tirne) and the data are presented as 30-min average. At an early stage of the experiment, it was found that the NO line became blocked

with ice, probably due to water present in the NO cylinders. Even the use of a water

absorber (Dryente) to trap the water in the NO cylinder was unsuccessful in eliminating

the blocking of the NO line. To avoid this, a heated line was used for transporting NO to

the reactor. One diagnostic tool used to check if the NO line was blocked or not, was to

compare the ozone data with the baseline signal obtained with the BrO, detector, since

the ozone reaction with NO is the major component of the NO2 baseline. When these

values were different, this indicated that no NO was deIivered to the reactor, and these

BrOx data were rejected.

Daily NO2 and HOs calibrations were performed by bringing the inlet of the instrument into the laboratory (286K), using the methods described in chapter 2. The reagent cylinders, dong with the synthetic zero air and nitrogen tanks, were kept outside the laboratory.

As previously described in section 4.2, the chah lengths of the radical detector are affected by ambient water vapour. In the Arctic troposphere, water is present at very low levels, but the relative humidity is quite high. However, given the low temperatures, it was assumed that water does not condense on the reactor walls to increase the radical wall losses significantly. Taking these into consideration, no correction was applied to bromine radical data for PSE97, but fiirther laboratory studies at low temperatures

(around 250K) need to be performed to test the validity of this assumption.

The detection lirnit for the bromine amplifier during PSE97 was estimated using the bromine radical data collected during Julian Days 89-90. The detection lirnit, calculated as 3*0, was 4pptv.

5.4 Results and Discussion

Based on ozone levels, PSE 97 was divided into five distinct periods:

1) days 86-89 when ozone was at normal levels (30-40 ppbv) ;

2) days 89-91 when a minor ozone depletion occurred

3) days 9 1-96 when ozone recovered back to normal levels

4) days 96-1 00 when a significant ozone depletion episode took place

5) days 100-101 when ozone levels were again 30-40 ppbv.

Measurements of the photoactive chlorine species during the whole expa were below or just above the detection limit of the PHD (14 pptv as (Cl2),) and individual

Cl2 and HOC1 mixing ratios could not be calculated. It should be noted that photoactive chlorine species were present at levels above the detection limit of the instrument when ozone was partially depleted. Given the very low levels of photoactive chlorine species and the inability to differentiate between Cl2 and HOC1, no definite conclusions were reached and the photoactive chlorine data are not presented.

Measured ground levels of ozone, Br2 and HOBr dong with the BrO, and UV solar radiation for the entire experiment are shown in Figure 5.1. For penods 1,3 and 5, both BR and HOBr were below the detection Iimit of the PHD. However, for the ozone depletion events (the periods 2 and 4), measurable amounts of Br2 and HOBr are present in the Arctic troposphere.

Concentrations of bromine radicals above the 4 pptv detection limit of the instrument were observed under 'normal' ozone conditions (for exarnple penod 3 and 03 O Br,

40 -

30 -

20 -j

PHD DL

'"-DBi--'O ...... UV Rad

1s -

10 -

5 - Br0,DL

92 94 96 9 8 100 IO2 Julian Day

Figure 5.1. Measurements of ground level 03, HOBr, Br2 , BrOx and UV radiation during PSE97 at Alert, Canada (tick marks represent midnight local time). 5), especially after Day 95. A maximum of about 16 pptv in BrOx levels was recorded

when ozone was recovering fiom a depletion episode in period 4. Measurable BrOx

levels were present even when the UV radiation was at its minimum (during the nights of

Day 95, Day 99 and Day 100).

To attempt to explain the BrO, radical concentrations and their temporal

behaviour, the following reactions are assurned to represent, to a significant extent, the

bromine radical chemistry in the Arctic:

Br2 + hv + 2Br

HOBr + hv + OH + Br

Br + R=C + Products

Br+03 +BrO+02

Br0 + NO + Br + NO2

Br0 + HO2 + HOBr + 02

Br0 + Br0 + 2Br + 02

HOBr + aerosols + aerosols(H0Br) (R5.13)

BrO+hv+Br+O (M.14) where R=C represents the unsaturated compounds that react with Br atoms. For Arctic springtirne conditions they are mainly fonnaidehyde, acetaldehyde, ethylene, acetylene

and propene. Assuming steady state for Br atoms we obtain: Similarly, assuming steady state for Br0 radicals:

Since [BrO,]=Pr]+[BrO]

The main losses for HOBr are the photolysis, with a midday coefficient

JHosr+.5* IO%-' (Tang and McConnelZ, 1996) and the heterogeneous loss to aerosol phase (R5.13) with rate coefficient khet=1.4* 10 4 s-1 (Shepson et al., 1996). The lifetime of HOBr under these conditions is in the,order of 28 min (Impey et al, 1998), so a steady state assumption for HOBr is appropriate:

which leads to:

Substituting E5.5 in ES.3, we obtain: Finally [Br] can be expressed as:

[Br1 = JR5.6 CBr2 1 - kher [HoBr] zkRS.8 [R= Cl

For [Br0]<10 pptv and w]=5 pptv, R5.10 has a much faster rate than R12:

molec s

The rate coefficients for R5.10 and R5.f 2 were calculated for 250 K (Atkinson et al.,

1997). Thus, the steady state equation for [BrO] (E5.2) becornes:

The steady state Br0 concentration is:

Finally the Baxrnixing ratio is given by:

2JB,, [Br2 1 - k,,, [HoBr] [BrO,] = [O3I~RW ) (E5.10) C k,,, [R = Cl

As shown by the above equation, [BrO,] is dependent on a number of parameters such as

Brz, HOBr, 03,NO, R=C,as well as solar radiation. Br2 absorbs radiation in the visible and some solar radiation was always present at

the site: Figure 5.1 shows that W radiation is not zero at midnight after Day 95. The JBrZ

at night could be just 0.5% of the midday value of 0.015~(McConnell et al., 1993).

Assuming that JBIO= O and that HOBr and HO2 are present only at negligible

xkR5.8[R=C]can be estimated using the values presented in Table 5.1

Table 5.1. Unsaturated compounds present in the Arctic troposphere Compound Mixing ratio k~5.8 h5.8[RC=]

RC= (PP~ (10"2cm3mole~~'s~1) (10%')

HCHO 200 0.70 0.4 1

CH3CH0 50 2.90 0.42

c2H4 200 0.16 0.10

C2H2 1O00 0.15 0.44

C3H6 50 2.70 0.39

ZkR5.8 [RC=] 1.76

Knowing that k~1.9=6.9*1 ~m~rn01ec~~s~'at 250K (Atkinson et al., 1997), the expected steady state concentration of BrO, can be estimated using E5.10:

According to this estimation, [BrO,] of about 5 pptv could be present even when [Brz] levels are 5 pptv, well below the detection limit of the PHD. This may explain why some measurable levels of Baxradicals are present during the night as seen for Day 95, Day

99 and Day 100.

For the partial ozone episode @ay go), there are considerable amounts of Br2 (1 O-

20 pptv), but BrOx and HOBr are below the detection limits of the instruments. This is

not consistent with E5.10. One possible explanation for this is local NOx contamination,

which not unusual for this site (Bottenheim et al., 1990). If the NO concentration is increased by a factor of 20 (mO]=100 pptv) the expected concentration of BrO, will be lower by a factor of 5 (to about lpptv) even if the [Br2] levels were increased by a factor of 4 (to 2Opptv as measured by the PHD). Furtherrnore, E5.10 was derived on a limited number of reactions and other bromine radical losses were not corisidered. For example,

Br0 radicals react relatively fast with NO2 to fom bromine nitrate, which rnay be an important loss for both NOx and bromine radicals (Beine et al., 1997):

Br0 + NO2 + M -+ BrON02 + M (R5.15)

The data for the ozone depletion episode that occurred in period 4 are presented in

Figure 5.2. For the last part of Day 96 and the whole of Day 97, the ozone concentrations are close to the detection limit of the ozone monitor (0.5 ppbv) and there are a few BrO, data points above 4 pptv. Interestingly, the BrO, data seems to be correlated with the solar radiation for this period.

Since the ozone concentration is very low at this moment, the concentration of

Br0 should also be low, according to E5.9. One source for these BrO, radicals could be the photoIysis of Br2, present at concentrations of about 20 pptv (Figure 5.1). This could generate up to 22 pptv of BrO. for the noon time (Jer2= 0.01 s"): 95 96 97 98 99 100 101 Julian Day

Figure 5.2. Cornparison of 03,Brz, HOBr, Baxand UV radiation during the ozone depletion episode.

2J,,[Br2] - 2*10-~[~r,] [BrO, ] = [Br]= - = 1 .l[Br,] C~,,,[R= CI 1.7*10-~

Also, the correlation between the BrO, concentrations and solar radiation can be

explained since [BrO,] is directly related to JBr2in this case.

Another intriguing observation for the low ozone days (JD96 and JD97) is the

presence of about 20 pptv Br2. Since Brz has a very short lifetime (1-3 min), the obvious

conclusion for this will be that a local Br2 production mechanism operates in the absence of O3 (Zmpey et al., 1998). However, laboratory experiments (Fan et al., 1992, Oum et al., 1998b) suggest that ozone must be present in order for Br2 to be either regenerated through a FancJacob type mechanism or directly produced fiom O3 and sea salt. Recent

measurements performed during PSE 98 (Sumner et al., 1998) have show that a flux of

formaldehyde is ernitted hmthe snow pack. They postuiated that formaldehyde

production occurs on the ice surfaces and the formaldehyde diffiion fiom the snow into

the gas phase is the determinhg step. The sarne assumptions could be appiied for Br2,

where the Br2 regeneration mechanism takes place in the presence of ozone, but the Br2 is

slowly released into the gas phase coincidentally at the same tirne when ozone levels are

low.

Bromine radical concentrations were generally low throughout the experiment.

This suggests that the ozone destruction chemistry was not occurring at the sampling site.

If an air mass rich in bromine compounds and low in ozone was being sampled and the

BrO, and HOBr concentrations were low, the ozone depletion chemistry had dready taken place over the Arctic Ocean, before the air mass reached the site.

The highest bromine radical concentrations were observed at the end of period 4, simultaneously with the maximum in HOBr concentration. The BrOxlevels reached 16 pptv, consistent with previous Br0 measurements (Hausmann and Platt, 1994). Because

HOBr is a direct product of Br0 + HO2 (R5.1 l), this simultaneity is not surprising.

However, the HOBr maximum of about 260 pptv is higher than the values infen-ed fiom modelling studies. Sander et al., (1997) predicted a [HOBr] of only about 2 pptv. Based on the measured concentrations of HOBr and Ba,, the HO2 concentration was estimated

(Zmpey et al.,1998) using the following assumptions: a) HOBr is formed by R5.11; b) The main losses for HOBr are the photolysis with a coefficient JHos,=4.5* 10%-' and heterogeneous losses to aerosol phase by R5.13; c) al1 of the Baxradicals are in the Br0 fom, since ozone is present.

Thus using assumptions a) and b) and E5.5, we obtain:

And the HO2 concentration will be given by:

For [HOBr]=260 pptv, [Br0]=16 pptv, ES. 12 predicts [H02]=6 pptv. This HOz concentration is not consistent with a 2 pptv concentration as calculated by Sander et ai.,

(1 997) and implies that important HO2 sources are present in the Arctic troposphere.

Given the low solar elevation angle and the dryness associated with the Arctic, the ozone photolysis is not expected to be an effective radical source. Another potential source for

HO2 is the chlorine reaction with hydrocarbons, but given the low NOx levels, this source will produce mostly R02 radicals. HO2 radicals could also be produced by the photolysis of formaldehyde since recent measurements indicate that HCHO is present at 200-300 pptv at this time of year (Shepson et.al., 1996).

Recent laboratory studies (Aranda et al., 1997) have shown that there could be an additional channel for HOBr formation by the reaction of CH302with BrO: Br0 + CH302+ Products (RSW

At 298, k~.i6=(5.7*0.6)* 10"~cm3molec"s" and the branching ratio for HOBr formation

is q4.8f0.2. The rate coefficient for R5.16 is lower that the rate coefficient for R5.11

o<~~.~1=5.~*10.' ' ~rn~rnolec~'~~~),but simila. reactions may occur between other R02 (potentially forrned fiom halogen radical attack on hydrocarbons) and BrO. If so, the

HOBr concentration would be given by:

and the HO2 concentration would be:

Estimations of total organic peroxy radicals (RO2) of about 6 pptv (Aranda et al., 1997) and assuming that the branching ratio for WOBr formation is similar for al1 ROz radicals

(q=0.8) still leads to a HO2 mixing ratio of 5.4 pptv.

Direct measurements of the ROx (ROx z HO2 + R02) radicals in the Arctic troposphere using the chemical amplification have been attempted at ~y-Alesundin 1995 and 1996 (Arnold et al., 1998), but the ROx amplifier used in these field campaigns also responded to chlonne radicals similar, to our ROx radical amplifier (section 3.2).

Measured ROx radical concentrations up to 6 pptv were reported, but the ROx chernicd amplifier is not able to distinguish between HO2 and RO2. Assuming that HO2 radicals are the main component of the ROx radicals ([HO21 = [RO,]), then the [Hot] denved

based on BrO, and HOBr measurements is consistent with these ROx measurements.

Clearly, there is discrepancy between the measurements and the modelling studies.

5.5 Conclusions for the BrO, measurements during PSE 97

The bromine radical detector was deployed at AIert in the spring of 1997, dong

with the photoactive halogen detector. BrOx concentrations above the detection limit of the bromine radical detector were recorded. Reasonable assurnptions were used to

attempt to explain the observed concentrations of bromine radicals when the solar radiation was low or during an ozone depletion episode.

The highest BrO, concentrations (16 pptv) were recorded at the end of an ozone depletion episode, simultaneously with a maximum in HOBr concentration (260 pptv).

These values were used to calculate [HO21 = 5-6 pptv, higher than the modelling studies have suggested.

The Arctic chemistry is not active at this site, except for the time when two different air masses are mixed. To study Merthe ozone depletion mechanism, experiments may have to be conducted away fiom the shore, over the Arctic Ocean, where the ozone depletion chemistry is more Iikely to take place. 6 PROPHET 97 field campaign

The main goal of PROPHET (Program for Research on Oxidants:

Photochemistry, Emissions and Transport) is to characterise the regional air pollution

episodes in the mid-west of the United States. Researchers fFom over 20 institutions

across North Amenca are actively involved in this program.

6.1 Site location

The experimental site for PROPHET is located near Pellston, MI (45'33'33" N

and 84°42'53" W), about 5km eastward of the University of Michigan Biological

Station. The sampling site is situated in the middle of a forested area, with beech and

maple being the dominant species. With a few exceptions, the trace gases are measured

fiom the air sample drawn through the glass manifold. The air sample is drawn through a

gIass manifold fiom the top of a 3 lm high tower, about 15m above the canopy. The residence time for the air sample in the manifold is less than 2 seconds. For the very

short-lived species, such as the peroxy radicals, the instruments are mounted on the tower and the sampling was performed at the top of the tower.

PROPHET is characterised by a long tenn component (mixing ratios of CO, PAN,

O3 and meteorological parameters are measured continuously at the site) while other trace species like ROx radicals, NOx and NOy, VOCs, organic nitrates, peroxides, aerosol physical properties and chemical composition, are measured during seasonal intensive field campaigns. One of these intensive campaigns took place fiom July 24 to August 18, in the summer of 1997. 6.2 Instrumentation deployed

The instrumentation deployed during PROPHET Summer Intensive 1997

(PROPHET97) is presented in Table 6.1, dong with the sampling point, the detection limit and theresolution specific for each instrument.

Table 6.1. Instruments used during PROPHET Summer Intensive 1997 1 Trace gas 1 Sarnpling 1 Measurement technique 1 Detection 1 Time 1 measured point Limit resolution O3 manifold UV photometer 0.Sppbv 1min CO manifold Gas-filter correlation 50ppbv 1min IR absorption NO manifold . Chemiluminescence IPP~ 1min NO2 photolysis 8pptv NO, manifold Reduction to NO with a Au ~PPW 1min converter/ Cherniluminescence H202,ROOH Own lines Enzyme fluorimetry lOOpptv Smin !4" PFA Aerosol - Lidar 75 m 30min backscatter resolution ( PAN, PPN, 1 manifold 1 GC-ECD 1 Spp~ 1-20 min 1 MPAN Isoprene flux Relaxed eddy accumulation 30 min Isoprene, manifold GC-MS 50pptv 40 min MWK, aldehydes UV radiation - Eppley radiometer 285-380nm - 1min WBradiation - Upward Iooking radiometer - 3min 285-320 nm Ra On the tower Chemical amplification Gpptv 1Omin

6.3 Configuration of the radical amplifier for PROPHET 97

The ROx radical amplifier was configured as described in section 2 with a few minor modifications. The reagent cylinders, the mass flow controllers and the data acquisition module were kept in the Iaboratory, while the luminol detector was placed on a platfom, about 3 m (2 s) below the sampling idet. Radical data were collected every minute (with a 200s dead tirne and 400s acquisition time) and are reported as 10-min

block average. Since the relative humidity effect on the radical amplifier was not known

during the field campaign, the ROx data were later corrected to take into account this

effect. This correction was based on laboratory experiments performed a few months

after the field campaign, when the state of the reactor walls may have been modified.

Therefore this correction may induce large errors to the radical data, especially when the

relative humidity was hi&.

The detection limit of the radical amplifier was calculated as 30using ROx radical

data for several days fiom 0:00 to 6:00 am,when the radical levels should be close to

zero. However, this detection limit (6pptv for 10 min) corresponds to high relative

humidity (90-1 00%) conditions. The main factors governing the detection limit are the

noise in the NO2 baseline and correction due to RH effect. The detection limit for

laboratory experiments was 0.5 pptv (for 6 min), since no correction was applied (the

relative humidity is Iess than 0.2%) and the noise (a)in the NO2 baseline was only 0.16

pptv. Therefore the detection limit for daytime radical data (when the RH was lower than

100%, but the noise in the NOz baseline was still present) should be less than 6 pptv.

6.4 Data analysis for ROx radical measurements made during PROPHET 97

6.4.1 ROx radical data for PROPHET 97

The ROx radical data for the entire field carnpaign are shown in Figure 6. The

highest radical concentrations were about 70 pptv (July 3 1 and August 2), a factor of two higher than previous ROx measurements made by our group. This is largely because of the RH correction applied to the ROx radical data.

The maximum radical concentration was recorded in the mid-afternoon (3-4pm), well after the local noon (1 :40pm EDT), while the minimum in radical concentrations was recorded after midnight. In addition, it appears that radicals persist late in the evening and this will be discussed in section 6.4.3.

Penkett et al. (1997) have shown that, for the moming hours, [ROJ is well correlated with Jo3 * [O31 or the square root of Jo3 * [O3], depending on the NO concentrations present. This relationship was tested for the PROPHET site. The UVB radiation (285-320nm) was used 2iq a measure of Js3 rathcr than the UV radiation measured with an Eppley radiometer (285-380nm) since it better matches the ozone photolysis at AI320nm. Measured ROx, O3 and WBRadiation data collected on July

3 1 and August 2 before the local solar noon were used to find if the [RO,] are better correlated with [O3]*wB Radiation or with the square root of [O3] UVB Radiation. The results of this cornparison are shown in Figure 6.2.

For July 3 1, the correlation coefficient (R~)for ([03lrUVB radiation)lR vs. [RO,] is identical with R~for the [O3J* WBradiation vs. [ROJ and very high (0.99), showing that both correlations are valid. However, for August 2, R~is low for both cases (0.67 for

[03]*WBand 0.63 for ([03]*UV~)lR).On this day, the RH was higher than 65%

(Figure 6.5) and this might have induced some errors in the ROx data. One other possible explanation for the poor correlation obtained on this day is that RO, radicals could be sensitive to other parameters, beside [O3]or UVB. Therefore, a more detailed analysis was performed, and is presented in the foltowing section. July 3 1 =O 0

O 20 40 60 O 20 40 60 80 WxI(PPW [ROxI(PPW Figure 6.2 RO, correlation with UVB*[03]and square root of UVB*[03]for July 3 1 and August 2

6.4.2 Theoretical considerations for ROx correlation with the square root of [O3]*WB

Radiation

Following the analysis performed by Penkett et al. (1997), the following reactions were considered. First, the ozone photolysis is the only radical source:

O3 + hv(h< 320nm) + o('D) + 02 (R6.1)

o('D) + H20-+ 20H (R6.2)

o('D)+M-+o+M (R6.3)

0+02+M-+ 03+M (R6-4)

The fraction o(~D)that is converted into OH radicals, is given by: The rate of production for OH radicals is given by:

Then, the OH radicais generate ROx by reacting with CO and VOCs (R6.5 and

R6.6) or are lost by reaction with NO2 to fom nitric acid (R6.7):

0H+CO+02+ H02+CO2 (R6.5)

OH + VOC + O2 + R02 + H20 (R6-6)

0H+N02+M+HN03+M (R6.7)

The fraction of OH Iost through R6.7 is given by:

HO2 and R02radicals react with NO to refonn OH or are lost by self-reactions

(R6.12 to R6.14) or ROx+NOx processes, represented by R6.11:

ROz +NO + RO+NOz (R6.8)

RO + O2 + HO2 + Products (R6.9)

HO2 +NO -+ OH +NO2 (R6.10)

RO, + NOx + M + Products (PAN,RON02) + M (R6.11)

HOz + HO2 + Hz02 + 02 (R6.12)

HO2 + R02+ ROOH + 02

1O9 R02 + ROz+ Stable Products (R6.14)

The following four assumptions were made: a) the absolute water concentration is constant during the day; using E6.1, we can conclude then that foiMHis also constant during the day. b) the oniy net ROx radical production path is by ozone photolysis; then the rate of ROx production is equal to rate of OH production:

P(RO* [O, = P(OW = 2J03 lf"l,, (E6.4) c) since R6.14 is at least an order of magnitude slower than R6.12 and R6.13 (as shown in Table 6.2 for some particular cases), the organic peroxy radicals lost through this reaction can be neglected.

Table 6.2 Rate coefficients for the self- reactions of the peroxy radicals Radical A Radical B Rate coefficient

d) the main radical losses are given by: - radical loss through self-reactions:

L~~ = kR0.11[Ho212 - kRI.II[Ho2 l[Ro2 1 - radical loss through reaction with NOx: In their analysis, Penkett et al., (1997) considered two scenarios: the baseline case when NO concentrations are negiigible (LSR>> LN&) and polluted case, when mO]>SOpptv (LN~~>> GR). The analysis was performed for marine environments, where the concentration of methane, the dominant hydrocarbon, was constant throughout the day. For this field carnpaign, these conditions were not met since the site was surrounded by strong isoprene emitting species. Tt is known that the isoprene emission rate is dependent on the leaf temperature and the intensity of the photosynthetically active radiation, PAR (Guenther et al., 1993). Isoprene emission is dependent on PAR up to a saturation point. After this point, the isoprene emission is dependent only on the leaf temperature, and therefore isoprene concentration maximises in the afiernoon, about 2 hours after the local solar noon, at the same time as the maximum in temperature. This diurnal variation in isoprene concentrations was also observed during PROPHET 97

(Figures 6.6 to 6.8). In this case, and LSRare variable during the day. Also, neither of them is negligible compared to the other. For a VOC/NOx ratio of 20, Carter and

Lurmann (1991), found that about 53% radicals are lost through self-reactions and 47% radicals are lost by reacting with NOx. Therefore, to fmd a relationship between [ROJ and [03]*UVB, 1 assumed that the ratio between the two main radical losses is constant during the day:

The rate coeficients for reactions R6.12 and R6.13, k~6.12and kR.~.I3are comparable (kR6.12 = k~6.13)as shown in Table 6.2, where k~6.12was calculated for a relative humidity of 50% and a temperature of 298K. Knowing that [RO,]n m02]+[H02] and introducing cx = [H02]/[ROX],E6.5 becomes:

LNOi = PLsR = -12 [ROI I2

Assuming steady state for ROx radicals, we obtain:

Assuming that the ratios folD-OH,a and P are also constant throughout the day,

[RO,] is expected to be correlated with the square root of [03]*Jo3. However, the above correlation is based on a few critical assumptions (ozone is the onIy radical source, kR6.I2

= kR6.13and ~OID~H,ayP = constant) and therefore diurnal variations in fol~a~,a and P may impact the correlation between [RO,] and the square root of [03]*Jo3.

To study the sensitivity of [ROJ correlation with ([03]* WB)'~in respect to folDaH, a and P ratios, E5.11 was rearranged as:

WX1 = and ÿ foi^.^^)'^, (ll~)'~and (1/(l+p))ln were calculated for 0.W folDOHIO.3, O-QXSI

and O

Figure 6.3 sensitivity of [RO,] correlation with ([03]*WB)"L to folDOH,a and B ratios

ranges studied, this correlation is more sensitive to a than to P or foiwH. Since foiMH, and especially a and B could vary during the day (for example if NOx concentrations are increased, a and p would decrease), the correlation of [ROX]with ([03]*Jo3 )ln must be used with caution.

The correlation of [R0,] with the square root of [03]*Jo3 is a primary tool to understand the day to day variations in the ROx radical data. The peroxy radical data, dong with the ([o~]*wB)~~are shown in Figure 6.4 for the whole sampling penod. It seems that [RO,] is quite well correlated with the (WB * [O>])lRas the highest mOX]

are recorded when (UVB * [03])'" is also high (July 3 1, AugustZ), and lower [R0,] were

measured when (UVB * [03])'" was lower (August 3 to August 6). There are a few

discrepancies (notably August 8 and August 9), but the previous analysis has shown that

ROx radicals may also be affected by other factors, not just (UVB * [03])".

6.4.3 Interpretation of the evening radical levels

Daily plots of BOx] reveaied that the radicals seem to persist late in the evening,

decaying much slower than ([03]* UVB)'~and even remaining after the nuiset

(UVB=O). This is clearly seen in Figure 6.5, when daily plots of RO,([03]* UVB)In

and RH are shown for two different days: July 3 1 and August 2.

E6.11 and the ratio a =[HO2]/Wx] could be used to explain this behaviour by

assuming that a is high in the morning and becomes Iower in the afternoon. This can be visualised if we compare the production and loss rates of HO2 radical with the same rates for the organic peroxy radicals ROz.

For these low NO, conditions, the steady state concentration for OH radicals can be calculated using R6.1 to R6.7 and R6.10 and assuming fhatfloNx0:

The production rate for HO2 radicals can be calculated considering the processes July 31

.*._...... -...... c.- '.. August 2

...... nm RO,

0:OO 6:OO 12:OO 18:OO 24:OO Local time (EDT)

Figure 6.5. Daily profiles of [ROJ,RH and ([o~]*uvB)''for July 3 1 and August 2. responsible for producing HO2 radicals (R6.5 and R6.9) and assuming steady state for

RO radicals (R6.8 and R.9):

Similarly, the production rate for R02 radicals is estimated considering that only

R6.6 produces ROz and assuming steady state for OH radicals:

As show by E6.13,the OH concentration is sensitive to isoprene: an increase in the isoprene leads to a decrease in OH concentration. Therefore, P(H02) is lower when the isoprene concentration is higher, while P(RQ9 is higher when isoprene concentration is higher.

To compare the loss rates for HO2 and R02 radicds, the radical self-reactions

(R6.11 and R6.13) must be considered. If kR6.1 = kR6.12 then the loss rate for HO2 radicals is at Ieast the same as loss rate for R02radicals for [HOt]«[RO,] and three times higher if [HO2] = [R02]. It cmbe seen that R02 radicals are much longer lived that the HOz radicals. Taking these into consideration, a should decrease in the afternoon and, therefore,

the radical concentrations should be higher that infemd fom the (0,' UVB radiation)lR

This is consistent with the radical measurements.

Another possible explanation for the radical deviation for the square root of

[O3]*WB is that a secondary radical source maximises in the afternoon. A number of

aldehydes (especially formaldehyde and methacrolein), are produced through isoprene

oxidation and therefore their concentration maximises in the afternoon. Therefore this

secondary radical source is negligible compared with ozone photolysis in the morning,

but may become significant in the afternoon.

PAN decomposition could also generate significant levels of radicals in the

aftemoon, when the temperatures are higher. This behaviour should be more obvious for the days when PAN levels are higher, but the observations indicate that is a common feature for al1 the days during PROPHET 97, even when PAN concentrations are relatively Iow (100 pptv or less).

6.5 Modelling the PROPHET 97 RO, data

The correlation between ROx and the square root of [03]*UVB is usefil in understanding the shape of the radical diurnal profile and to differentiate for the day to day variations in the radical concentrations. However, this correlation does not allow cornparison of the measured radical concentrations with values inferred fiom our current understanding of the tropospheric chemistry. This is mainly because the above correlation is based on a limiteci number of reactions and some questionable assurnptions

(i.e. ozone photolysis is the only source of radicals a and P are constant). A more

rigorous approach is to atternpt to model the KO, data for this field campaign, based on

the good supporting measurements that are available for PROPHET 97. The lifetime of

ROx is much shorter that the time scales associated with the transport of air masses.

Therefore, a box model should be appropriate to probe the measured radical levels.

6.5.1 Mode1 desniPtion

The software developed by Atmospheric Environrnent Service of Environrnent

Canada called CREAMS (Chemical REActions Modelling System) was used to nui a box model incorporating the Master Chemical Mechanism (MCM) proposed by Jenkins et al., (1 997). The mechanism consists of 486 reactions and 177 chemical species. It includes the chemistry of methane, ethane, formaldehyde, acetaldehyde, PAN, isoprene along with the inorganic tropospheric reactions. The oxidation of isoprene is treated explicitly, since the isoprene is the dominant hydrocarbon for this site. At the time when this modelling study was performed, the remainder of the hydrocarbon data for this field campaign was not available, so no other hydrocarbon chemistry was inciuded in this modelling study. Previous studies in rural and forested areas have shown that the hydrocarbons included in this modelling account for most of the hydrocarbon reactivity

(Taylor, 1997).

The model was run for a period of 24 hours starting at midnight. The initial concentrations of methane and ethane were 1.7 ppmv and 400 pptv, respectively. Within the model, the concentrations of CO, 03, NO,NO2, isoprene and PAN were set to

measured values every 40 min.

6.5.2 Modelling the total radical concentration

Modelling studies were performed only for the days when measurements of al1 the

above trace gases were available and when the photochemistry was active (the sky was

not covered by clouds). For the missing data reasonable assumptions (trends or data fkom previous days) were used to estimate these data. Modelling runs were conducted for July

3 1, August 2 and August 6 (the sky was partly covered by clouds for this 1st day). The

input concentrations for these days are shown in Figure 6.6, Figure 6.7 and Figure 6.8.

The ROxoutputs of the model are compared with the measurements in Figure 6.9. Not only does the model reproduce the absolute values of the total radical concentrations within the experimental errors, it also duplicates the diurnal variation of the radicals, including the late evening decay of radicals.

The only other trace gas that was measured, and was not used in model initialisation, was hyàrogen peroxide. Its lifetime is sufficiently short to provide an additionai test of the model performance. Hydrogen peroxide measurements are only available for July 3 1 and August 2 and these are compared with the model output in

Figure 6.10.

The model was able to reproduce the morning rise in the H202(especially for

July3 l), but the late-aftemoon and evening measured H202data are a factor of two lower than the model output for [H202].This suggested that the H202losses were not well NO, ..*;k 2000 -a. 4.

6:OO 12:OO 18:OO Local Time (EDT)

Figure 6.6. Mixing ratios of03, C5H8, CO, PAN, NO and NO2 for July 3 1 during PROPNET 97 0:OO 6:OO 12:OO 18:OO 24:OO Local Time (EDT)

Figure 6.7. Mixhg ratios of 03,C5Hs, CO,PAN, NO and NO2for August 2 during PROPHET 97 PAN

6:OO 12:OO 18:OO Local Tirne (EDT)

ïigure 6.8. Mixing ratios of O,, C5Hs, CO,PAN, NO and NOz for August 6 during PROPHET 97 60 4 August 6

Local The (EDT) Figure 6.9. Modelled (open triangles) and measured RO, (closed circles) for July 3 1, August 2 and August 6 during PROPHET 97; the error bars include the estimated errors associated with RH correction, chah Iength determination and the measurement of 6N02. July 3 1 August 2

0:OO t 2:00 0:OO 12:OO 24:OO Local Time (EDT) Local Time (EDT)

Figure 6.10. Modelled (open squares) and measured H202(closed squares) for July 3 1 and PROPHET 97. described in the model. The main loss for H202during the evening hours is dry deposition. The first order rate constant for H202decay used in the model was estimated with the following equation (Shepson et al., 1992):

kd(H202)=2*Vd/H (E6.16)

Where H is the mixing height of the boundary Iayer (H=l km) and Vdis the deposition velocity for Hz02(Vd=l cm/s). This led to k4~202)=2*1 O-' S-' . The justification for H=lkrn is based on the observation that there did not seem to be a stable noctmal boundary layer irnpacting the daily ozone concentrations on these days (Figure

6.6.and 6.7).

The H202is a very soluble compound and wet losses for H202may be higher

when the relative humidity is higher. When the relative humidity is high, the rate of

water condensation on the particles is increased and H202can be very efficiently

scavenged by the aqueous aerosols (Nusfie et al., 1993). Thus, the wet losses of H20:

may become significant during the evening hours and may explain the disagreement

between the model and measurements.

The agreement between the model and H20z measurements in the morning

indicated that we have a reasonable understanding of a major sink for the HO2 radicals.

6.5.3 Modelling studies for RO, sensitivity

The mode1 is able to reproduce the measured radical concentrations based on the

measurements of ancillary species. Modelling studies were conducted to study the ROx

sensitivity towards isoprene, ozone and NOx to determine the main species affecting the

radical concentration. AIso, the input for the concentrations of these species was

estimated when measurement data were not available and ROx model output may be very

sensitive to one of these species.

6.5.3.1 Modelling studies for ROx sensitivity to isoprene

Since isoprene dominates the hydrocarbon chemistry at this,site, the model was used to study the ROx sensitivity to isoprene using the July 3 1 data. The base case refers to the run when the mode1 was 60 A Base Case constrained according to

measurements. In a second run, the

isoprene chemistry was completely

removed ([C5Hg]= 0) and the results

are shown in Figure 6.11. Isoprene

increases the maximum [RO,] by

30% and also affects the RO, diurnal 0:OO 6:OO 12:OO 18:OO 24:OO Local Time (EDT) profile by delaying the evening decay Figure 6.1 1 RO, sensitivity to isoprene for July 3 1 of the radicals. The causes for lower 1~+7r 1 [ROx] for the no isoprene case are:

a) an increase in the [OKJ, since

when [Cs&]= O its lifetime is longer.

This increases the radical losses

towards nitric acid: flOH (see section

6.4.1) is higher when [CsH8]=O;

b) less VOC oxidation products (i.e.

formaldehyde) are fomed and so the 0:OO 6:OO 12:OO 18:OO 24:OO Local Timc (EDT) strength of these secondary radical

uly 3 1 : triangles for the base case and squares for the sources is diminished.

The mode1 outputs for OH, HN03and HC [O for the base case and for [C5H8]=0 are plotted in Figure 6.12 and show the expected variations. For [C5H8]=0,the maximum in OH concentration is a factor of two higher, the HN03 formed is a factor two higher,

while HCHO is a factor of two lower.

The radicals persist later in the evening for the base case as shown in Figure 6.1 1,

since the majority of radicals are hydrogen peroxy radicals (HO2) and their lifetime is

shorter that the lifetime of organic peroxy radicals as shown section 6.4.3.

6.5.3.2 Modelling studies for ROx sensitivity to ozone

Modelling nuis were further perforrned to study the sensitivity of RO, to ozone.

The ozone levels were increased by 50% over the ambient and the mode1 output for ROx

radicals is shown in Figure 6.13, dong with those for the base case. Increasing ozone

concentrations by 50%, increased the

afiemoon radical concentrations by

only about 13%. Based on the

square root dependency of ROx vs.

[03]* WBradiation (E6.1 l), the

concentration of radicals was

expected to increase by a factor of

(1 .5)IR=l.22 if ozone photolysis was

the only radical source. As shown Local Time (EDT) Figure 6.13 RO, sensitivity to ozone for July 3 1 in section 6.5.5, the ozone photolysis

is not the only radical source: the photolysis of aldehydes and PAN thermal decomposition are significant sources of ROx. Also, higher ozone concentrations were responsible for an increased w02]/FJO] ratio and higher OH concentrations. This led to higher radical losses through nitric acid formation similar to the [CsHsJ=û case.

6.5.3.3 Modelling studies for ROx sensitivity to NOx

The levels of NOx (NO+ NO2) were increased by 50% from the modelling and another modelling run was performed. The results are shown in Figure 6.14 and show

that radical concentrations were

reduced by less than 2%. This low

decrease in radical levels can be

explained with the help of Figure 6.3.

Increased NOx has an effect on

increasing the radical losses trough

reaction with NOx (LNOx)and

therefore decreasing B, but [RO,] is 0:OO 6:OO 12:OO 18:OO 24: O0 Local Timc (EDT) not that sensitive to B.

Figure 6.14 RO, sensitivity to NO, for July 3 1.

6.5.4 Modelling studies on quantification the secondary radical sources ,

Further modelIing studies were conducted in order to assess the importance of secondary radical sources: PAN thermal decomposition, aldehyde photolysis, isoprene ozonolysis and ROOH photolysis (including Hz02 and al1 organic peroxides). By switching off the photochemistry for each individual case and comparing the results with loNo PAN decompositior 0 No aldehyde photolysi 0 No isoprene ozonolysi

0:OO 6:OO 12:OO 18:OO 24:OO 0:OO 6:OO 1200 0:OO 6:OO 12:OO 18:OO 24:OO Local Time (EDï) Local Time (EDT) LocaI Time (EDT)

Figure 6.15. Secondary radical sources compared with July 3 1 base case (open triangles) the base case, the following conclusions were reached: a) al1 three processes impact the radical levels (Figure 6.15). PAN decomposition and and the photolysis of aldehydes contribute each to about 15% to the tobl. radical concentrations, while isoprene ozonolysis is responsible for ody 5% of the radicals. b) eliminating the peroxides photolysis (results not shown) lowered [ROx] by less than

OS%, indicating that this is not a significant ROx source.

In conclusion these secondary radical sources have a significant impact on the

ROx radicals. This cm be an additional cause for the late evening decay of the radicals, as postulated in section 6.4.3. 6.6 Radical levels estimated using Pseudo Steady State Approximation

A common method to estimate the radical levels is the Pseudo Steady State

Approximation (PSSA), (Parish et al., 1986). This was undertaken in this work for cornparison with the ROx measurements. The PSSA method assumes that nitrogen chemistry is dominated by the following reactions:

N02+hv+NO+0 (R6.15)

NO + 03+NO2 + 02 (R6.16)

R'Oz + NO + R'O + NO2 (R6.17a)

R'02 + NO + M + RON02 + M (Rd. 17b)

R'02 + NO + Products (R6.17)

For R6.17, R'O2 includes HO2 and organic peroxy radicals ROz and the rate coefficients for these reactions are dependent on the relative percentages of each peroxy radical.

Considering that this site is characterised by low NO, emissions and the lifetirne of NO in the presence of 40 ppbv of ozone is only lmin (NO reaches steady state firly fast), the steady state equation for NO is:

Solving the above equation, we obtain:

For this site, the organic peroxy radicals represent about 60% of the total radical levels as show by modelling studies (section 6.3). The rate of reaction R6.17b increases with the number of carbon atoms present in the dkyl radicaI and can lead to about 10% nitrate formation fiom the RO2 radicals originating fiom isoprene and therefore must be included in these calculations. Finally, knowing that [ROJ = [RYO2]we obtain:

The mixing ratios of NO, NO2 and ozone were measured continuously, the rate coefficient for reaction R6.16 is well known kR6.16=1.8* 10-~~ex~(-1370/')cm3 molec-' s-l

(Atkinson et al., 1997) and the rate coefficient for reaction R6.17 was estimated as

8.0' 10-l2 cm3 molec" s'l . The NO2 photodissociation rate coefficient JNo2 was not measured during PROPHET 1997. JN~~was estimated using the UV radiation measured by the Eppley radiometer with the following expression (Madronich et al., 1987):

where z is the elevation above sea level in km;

sza is the solar zenith angle; E is the UV irradiance measured by the Eppley

radiometer and is expressed in W cm'.

E6.19 is applicable for sza< 60°, for moderate regional albedo (0.15) and for clear

sky conditions.

The ROx concentrations were calculated using E6.18 for three days: July 3 1,

August 2 and August 6, but only for the time intervals when NO, NO2, O3 and UV radiation measurements were available. The results are shown in Figure 6-16. Where July 31 a

August 2

August 6

12:OO 15:OO Local lime (EDT)

Figure 6.16 Cornparison between measured RO, (dark circles) and RO, calculated using PSSA (open triangles) available, the RO, concentrations calculated using PSSA are very variable. This variability can be attributed to the fact that a relatively small number @Ox concentration) is computed fiom the difference of two relatively large numbers. Another intriguing aspect is that the [RO,] calculated using E6.18 are higher than the measured ROx concentrations, which are well reproduced by the model (section 6.5). The PSSA was

Merused to estimate [RO,] for August 7 (more ancillary data were available for this day) and compared with the measured [ROx]. Since the photolysis rate coefficient for

NOz is calculated with an equation based on several assumptions (clear sky conditions, regional albedo of 0.19, sensitivity studies were also conducted for JNO2 lowered by

20%. The results are shown in Figure 6.17 and showed that PSSA ROx levels are very sensitive to JN02: reducing JNo2 by 20% is responsible for a 40% reduction in [ROJ fkom about 100 pptv to about 60 pptv, still high compared to measurements (40 pptv). This suggests that PSSA is not a reliable tool to estimate the radical levels, unless accurate measurements of NO, NO2, O3 and JNoZare available. Furthemore, the rate coefficient for reaction R6.17 must be accurately estimated based on relative contribution of type of peroxy radicals to the RO, budget.

These results are consistent with the findings of Volz-Thomas et al., (1 988). In this study, in addition to PSSA, a chemical box model, constrained according to NO,

NOz, 03, CO and VOC rneasurements was to calculate the peroxy radical concentrations.

Also, [ROJ were measured by two independent techniques: chemical amplification and

MIESR. A poor agreement was obtained between the [RO,] calculated by the model and

[RO,] inferred using PSSA, while the MIESR measurements agreed fairly well with the 1 Caldaîed RO, usïng PSSA fa 0.8 JN02

8: 00 1QOO 12:OO 14:ûû 16:W 18:ûû Local Tirne

Figure 6.17. Sensitivity studies of ROx levels estimated using PSSA for August 7 model calculations. The [RO,] rneasured by chernical amplification were much lower than the model calculations, most probably due to the relative humidity effect on the chah length of these instruments (Volz-Thomas et al., 1988).

6.7 Local ozone production for PROPHET 97

The ozone diurnal profile for days when the sarne air mass is being sampled has a few characteristics, partly dictated by the nochunal boundq layer (NBL). The minimum ozone levels are recorded at night due to ozone dry deposition. In the morning (8-10 am), the ozone concentration may have a shqincrease due to the break-up of the

NIBL, followed by more graduai increase. This second increase is attributed to the local photochernical production of ozone and the rate of increase can be detennined fiom ozone observations (Arias and Hastie, 1996).

The only net ozone production in the troposphere is fiom NO2 photolysis and the rate is given by:

This relation allows the calculation of the ozone production using NO and ROx measurements and this can be compared to that obtained fiom the ozone profile. Based on a distribution for RO, of 40% HO2 and 60% C5Hg03(organic peroxy radicals fiom

OH addition to isoprene) b,l~awas estimated as 5.8* 10"~cm3 molec-' S-'. The result of this analysis is presented in Table 6.3.

It can be easily observed that the local ozone production for this site is quite low, surpassing 1 ppbvhr just in two cases. The reason for this is that NOxare present only at low levels. With a few exceptions (notably August 7),the observed ozone increase is matched by the calculated values, considering the possible large errors associated with this comparison: a) the relative humidity correction for ROxradicals b) the estimation of k.17a c) the dificulty in determining small ozone increases for short periods of time.

In conclusion, the measured RO, concentrations were used to calculate the local ozone production for 7 days. Ttie values obtained were similar to the values calcuIated fiom the ozone diudprofiles, suggesting that the small increase in ozone cm be explained by the local photochemisûy.

Table 6.3. Cornparison between observed and calcdated net ozone production for 'ROPHET97 1 1 Date ( Time Interval 1 Observed ozone 1 Calculated ozone 1 CalculatedlObserved increase production ozone increase ppbvfi ppbv/hr (%) Aug 2 13:15-18:OO 1.35 1.07k0.05 74 Aug 3 14:OO-17:OO 0.54 0.77kO. 19 143

Aug 6 1 12:OO-17:OO 1 0.67 1 0.61k0.43 1 91 Aug 7 1 11:30-13:40 1 0.62 1 1.30k0.40 1 210

1.20 0.7210.5 1 60 Average 130

6.8. Conclusions for PROPHET 97

ROx radical measurements have been made in addition to other chernical species that are invohed in the tropospheric ozone production.

The concentration of ROx radicais showed a maximum late in the afternoon (3-

4pm) afler the local solar noon ( I :4Opm) and a minimum dermidnight.

ROx radicals were correlated with the ([03]*WB)'~ except for the evening, when ROx decayed slower than ([03]*WB)~". A box model was used to simdate RO, measurements. The agreement between the model and the measurements suggest that our current understanding of the nual tropospheric chernistry is quite advanced.

ModelIing studies showed that the main source for ROx radicals is the photoIysis of ozone; other secondary sources (PAN decomposition, aldehyde photolysis and isoprene ozonolysis) are responsible for producing minor but significant levels of radicals.

The estimated ROx concentrations using PSSA were not reliable.

The local ozone production is low due to the absence of NOx and it can be accounted for using ROx and NO measurements. 7 Conclusions and Future Directions

7.1 Conclusions a) Attempts were made to develop a chlorine radical detector capable of detecting very low concentrations of CIOx radicals (CIOx = OC10 + Cl0 + Cl). The principle of this detector was based on converting CIOx radicals to HOxradicals through a fast reaction with ethane and NO, followed by the quantification of the HOx produced, using the chernical amplifier. These attempts proved unsuccessfd and, furthemore, it was found that the ROx radical amplifier responds to chlorine radicals. Therefore, the reported

ROx measurements in the marine boundary layer may not be reliable. The most probable cause is that chlorine radicals are able to participate in an amplification reaction cycle that involves only CO and NO. b) A convenient chlorine radical source was developed based on OC10; this source can also be modified to generate various ROx radicals. c) A bromine radical detector was developed by modifjring the chernical amplifier for ROx radical measurements. Through fast reactions with propene and NO, brornine radicals (BrO, = BrO+ Br) are first converted into ROx radicals, which are measured using the chernical amplifier. The response of the bromine radical detector is linear up to

70 pptv. The ROx chernical amplifier does not respond to brornine radicals. d) The chah length for the bromine radical detector CLBa (the number of NOz molecules generated per initial bromine radical that enters the system) could not be detennined, because a reliable brornine radical source was not yet developed. An indirect method was used to calibrate this detector in the field: the BrO, to HOx conversion yield (qBIOK-HOX)was measured in the laboratory. The chah length for the ROXamplifier

(CLRox)was detennined using the HO2 calibration source based on water photolysis at

184.9nm,and then CLBm was calculated by multiplying ~B~-HO~by CLHa. e) The bromine radical detector and the Photoactive Halogen Detector were deployed at Alert, NWT during the Polar Sunrise Experiment 97. Bromine radicals were observed above the detection limit of the instrument (4 pptv) for normal ozone conditions, even when the solar radiation was low. Also, measurable BrO, (presumably

Br atoms) were present when ozone concentrations were less than 0.5 ppbv. Reasonable assumptions based on Br2 photolysis were used to explain this behaviour. f) The low radical concentrations measured during PSE 97 led to the conclusion that the ozone depletion mechanisrn was not active at the sarnpling site. When an oceanic air mass, low in ozone and rich in photoactive halogen species, was sampled, the ozone depletion chemistry had already taken place, and the bromine radicai concentrations were low. The Arctic chemistry was active only when two different air masses were mixed.

To study further the spring Arctic ozone depletion phenornenon, experiments may have to be conducted away fiom the shore, over the Arctic Ocean. g) The highest BrOx concentrations (1 6pptv) were recorded when ozone was recovering fiom a depletion episode, simultaneously with a maximum in HOBr concentration (260 pptv). Based on the current understanding of the Arctic chemistry this was not surprising, since HOBr is produced by the reaction of Br0 with HOz.

Pseudo steady state calculations for HO2 radicals, based on measured HOBr and BrO, concentrations, led to [HO2]= 6 pptv, which is not consistent with modelling studies. g) The calibration source based on water photolysis relies on a critical parameter: the ratio between the absorption cross section of H20and the effective absorption cross

section of 02(~~zdaEF.02). A new method to determine this ratio was developed and

Q~~&~~,~~was measured for two different sources and the obtained value (6.2) was

similar for both sources. This led to a relative minor correction (6%) for al1 the radical

data collected by our group in various field campaigns in which the water photolysis

source was used for instrument calibration.

h) Laboratory studies have show that the chemical amplifier for ROx radicals is sensitive to ambient water vapour. The chain length of the instrument is greatly reduced

(i.e. by a factor of two at 2 1°c and 40% relative humidity) and very sensitive to relative

humidity fluctuations (about 1% for every 1% change in relative humidity). The main

cause of this effect is an increase in the wall loss rates for the HO2 radicals, but also some unknown gas phase chemistry plays a significant role especiaily when the amplifier is operated at higher NO concentrations. i) The first order rate coefficients for wall losses have been measured for HO2, and C2H502radicals for various levels of relative humidity (fiom O to 65%).

These measurements indicated that the relative humidity is not affecting the wall loss rates organic peroxy radicals, but it has a significant impact on the wall losses for HO2 radicals. j) The signal to noise ratio of the radical amplifier was enhanced by a reducing the dead time durhg sampling and by moving the NO2 detector closer to the reactor. Also, laboratory experiments indicated that heating the reactor partially and reducing the residence time in the reactor is a viable solution to decrease the water vapour effect and

to minimise the PAN interference during ambient sampling

k) The RO, radical amplifier was deployed in the summer of 1997 in Northern Michigan, as part of PROPHET. The RO, radical data collected during this field

campaign were corrected to take into account the water effect on the chah length. The

measured radical concentrations were found to correlate relatively well with the square

root of ozone concentration and UVB radiation. A deviation for this correlation was

observed for in evening and it was attributed to a decrease in the H02/R0, ratio and to

secondary radical sources that maximises in the afternoon.

1) Measurements of 03, CO, NO, NO2, PAN, isoprene, were used to run a box mode1 to calculate peroxy radical concentrations. A good agreement was obtained

between the measured and calculated radical concentrations, suggesting that the current understanding of the tropospheric oxidant photochemistry is quite advanced. m) The measured radical concentrations were compared with the radical concentrations calculated using the PSSA, but the conclusion was that this approximation is not a reliable tool to estimate the radical levels, unless more accurate measurements of

NO, NOz, O3 and JNO2are available. n) The local ozone production calculated based on RO, and NO measurernents was in agreement with compared with the observed photochernical ozone production at the site. The local ozone production was relatively low (1 ppbv/hr or less) due to the low

NOx concentrations present at this site. 7.2 Future directions

7.2.1 Further testing of the bromine radical detector

Laboratory experiments are needed to indicate if the bromine radical detector is af3ected by water vapour at low temperatures (240-260K).

7.2.2 Further development of the CIOx detector for tropospheric measurements

Further experirnents may reveal if combined CIOx and ROx radical measurements are possible with the following configuration. In the first mode, the instrument will be conventionally operated (CO is modulated, while the ethane is added at the end of the reactor) and the instrument records a signal given by the ROx amplification by a factor

CLI (the chah length given by cycle of HOx,NO and CO) and CIOx amplification by a factor CL2 (given by chlorine cycle involving Cl, CO and NO):

S[N02],= [RO,]* CL, +[CIOx]* CL, (E7.1)

In the second mode, CO and ethane are modulated: C10, radicals are converted into ROx radicals with a conversion yield approaching unity (~CIOXXRO~l)and then amplified dong with the existing ROx radicals. The signal recorded by the instrument will be:

6[NO,], =qcrox-mx*[CQJ*CL, +[RO,]*CLl~[CZOx]*CLl +[ROx]*CL, (E7.2)

Then [CIOx] can then be calculated with:

[ClOJ = wo2 1, - SIN02 II CL, - CL, and ~O,]by:

This method will require accurate calibrations for detennining CLI and CL2. To optimise the system, the experimental conditions (mainly the concentrations of the reagents) must be set so that the dif5erence CLI- CL2 is maximised.

7.2.3 Further developments of the chernical amplifier for HO2 and R02 measurernents

The wall loss rates for HO2 radicals are higher than those for organic peroxy radicals (R02). A method to differentiate between HO2 and R02radicals will be to have two inlets, with different lengths:

- a very short inlet (as is now used); the radical amplifier will sample most of the radicals

- a longer inlet, when most of the HO2 radicals and a fraction of R02 radicals will be lost through wall reactions; therefore only a part of the R02radicals will be sampled. Using laboratory experiments, the fraction of R02lost would be detennined.

7.2.4 Modelling studies for PROPHET 97

The isoprene decay in the aflemoon is an intriguing phenomenon: the isoprene concentration drops from a few ppbv to less than 100 pptv in a few hours. Isoprene flux measurernents showed that the isoprene fluxes (downward or upward) are close to zero after sunset. This decay cannot be explained by chemistry, since OH, NO3 or ozone concentrations would have to be rnuch higher than the observed or inferred levels. Alternatively, vertical transport, &er isoprene emissions have ceased for the day, could be responsible for de observed decay. A l-D mode1 with a fine grid resolution could be used to obtain the rates of upward vertical transport to test the importance of this mechanism.

7.2.5 Improvement of the ROx radical detector

Future experiments may focus on improving the performance of the chernical amplifier for ROx radical measurements. One possibility is to use the sampling configuration shown in Figure 7.1

The air sample is mixed with the reagents (CO and NO) into a pre-reactor ( a tube with lager ID than the reactor). The residence time in the pre-reactor must allow the complete destruction of the radicais by reaction with NO (t = 0.1-0.3sec). The reactor

(where the amplification cycle is completed) could be heated to decrease the humidity effect. Heating the reactor should not cause a PAN interference, since the radicals formed through PAN decomposition will be amplified, but the NOz produced dlbe included in the baseline. This configuration may have the following advantages: a) the relative humidity effect could be decreased significantly, without any PAN interference. b) the sampling fiequency could be increased, since the luminol detector will be not affected by the pulse in pressure. The limiting factor will be in this case the mixing of the reagents after the valve switching; this could be in the order of 1-2sec. Re- Reactor

NO Residence time=0.3s

Air Sarnple - w TO LMA-3

Heated Reactor

Exhaust CO

Figure 7.1. Proposed sampling inlet for the wdical detector Appendix The radical detector chemistry was modelled by using the FACSIMILE software

(Curtis and Sweetenham, 1987). The rate coefficients for the reactions included in these modelling studies were taken form DeMore et al. (1 994) and updated according to

Atkinson et al. (1997).

The rate coefficients for the termoIecuIar reactions (klIl)were calculated using the low pressure limit k~ limit and the hi& pressure limit k, as:

k,,(T) - M J1+[.".%j]l k,= 1 + 5(nM km (Tl Table Al. Rate coefficients for the reactions needed to mode1 the chemistry of the ROx radical amplifier \JO Reaction kir k, (cm3/(molecs) ko (cm6/(molec2s) 1 HOz + NO +OH + NO2 k11=3.7*1 O-" exp(240Iï) 2 OH + CO -+H+ CO2 kIi =2.4* 10-l3 k=7.5* IO-' (~/300)O" 3 H+02+M+HOz+M b=S .4* 1 (~/300)" F=0.55 k=4.5* 1O-' ' 4 OH + NO +M -,HONO +M b=7.4* 1 ' (~1300)-~*~ F=0.90 5 Rad + Wall -,Wall products kWdi=variable (units a') tk,=4.7* 10'12 6 HOz + NO2 +M+m04 + M b=1 .8* 1o5 ' (~1300)-~.~ F=0.60 k=6.7* 1O-" (~/3OO)~'.~ 7 OH + NOz +M+HN03 + M k0=2.6* 1Po (~13 OO)~~.~ F=0.43 L=~.o*1 0l4exP(-1 0000 /'ï) 8 HN04 + M +HO2 + NO2 +M b=5* 1o4exP(- 10900/1) F10.60 -- 35 2CH300 +2CH3O + 02 kII=8.0*10-l4 36 2CH300 +CH30H + HCHO kri =2.5*10 "' 37' 2CH300+CH300CH3+02 kn =3.0*10-'~ 38 OH + HCHO +HO2 + HCO kII=8.6* 10'' exp(2Orï) 3s CHO + 02+CO + HO2 kn =5.5* 10-l2

Rad = OH, HO2, CH302, CH20, CH3C03

Table A2. Rate coefficients of the reactions added to the mode1 for the ROx radical amplifier to study the chemistry of the C10, radical detector No Reaction k~,k, (cm' /(molec s) ko (cm6 /( mo1ec2 s) 1 C2H6 +C1 +HCI+ C2H5 ni1 =8.1* 1 O-' ' exp(-95/T) k=7.8* 1 O-'' 2 C2Hs +O2 + M+C2HsOO+M k&5.9* 1 (~/300)'~.* F=0.54 3 C2H500+ NO+C2HsO+N02 kir =1.3*10-l3 4 C2HsO0 + NO+CzHsON02 k11 =1.3*10"~ 5 C2H500+ HO2 +C2HsOOH + 02 k11=6.5*10 '13 exp(650îï) 6 C2H6 +OH +H2O + C2H5 k11 =2.6*10'12 exp(-35OLï) k~4.4*1 O-' ' (~/300)-' F=0.60 24 Cl + HO2 +HC1+02 k11=1.8*1 O-' lexp(l 70IT) 25 Cl0 + HO2 +HOC1+02 k11=4.8*1 O'"exp(-700/~) 26 Cl + O3 +C10+02 kil=1.2' 10" 'exp(-260/~) - 27 Cl + CO+ M +ClCo + M b=1.1 * 1 0-~~exp(81 OIT) - 28 ClCO + M +Cl+ CO + M W.1 * 1 0-'~ex~(-2960/T) 29 HC1+ OH +Cl+H20 k11=8.7*1 0"~exp(-l070lT) ~=1.9*1 0~~~(~/300)-'.' 30 2CIO +M +C1202+M b=7.0.* 10-l2

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