Fundamental Aspects of Carbonaceous Particulate Measurements in the Study of Air Pollution in Urban Area

Fundamental aspects of carbonaceous particulate measurements in the study of air pollution in urban area

P. A vino % D. Brocco^, L. Lepore* and I. Ventrone* *ISPESL - Dipartimento Insediamenti Produttivi ed Interazione con I'Ambiente, Monte Porzio Catone (Rome), Italy.

^Istituto Inquinamento Atmosferico - C.N.R., Monterotondo St. (Rome),

Abstract

The continuous measurements of ambient elemental (EC) and organic carbon (OC) concentrations in the total particulate matter of the urban area of Rome are reported. The measurements were carried out by a thermal method in downtown Rome (S. M. Maggiore) from May to December 1999.

During the entire period the results have shown that total carbon (TC) constitutes a significant fraction of the total aerosol mass (~ 30%). The temporal TC evolution has been interpreted by natural radioactivity concentrations as a tracer of the mixing properties of the lower boundary layer. The good relationship between TC and benzene and the strong correlation between EC and OC confirm that the traffic is the most important emission source for the carbonaceous particulate.

1 Introduction

Carbonaceous material is composed of two main fractions, elemental carbon (EC) and organic carbon (OC). The EC has a graphitic-like structure with several adsorption sites which are able to enhance catalytic processes. The OC represents a large variety of organic compound classes such as aliphatic and aromatic hydrocarbons, phenols, alcohols, acids, etc [1,2]. EC is essentially a primary pollutant emitted directly from the incomplete combustion of carbonaceous fuels and OC has both primary and secondary

origin. Primary organic carbon is formed during combustion and it is emitted as submicron particles. Secondary organic carbon has origin from gas-to-particle conversion of volatile organic compounds in the atmosphere either as result of the condensation of low vapor pressure organic compounds or from physical and

chemical adsorption of gaseous species in aerosol surfaces [3]. Production of secondary paniculate organic carbon aerosols has been studied in smog chamber experiments, from light irradiation of gaseous mixtures containing organic compounds [4,5]. Episodes of secondary organic aerosol have been observed in

Los Angeles [6,7]. In this paper we report eight-months continues measurements of EC and OC concentrations in the total suspended particulate (TSP) in downtown Rome. The temporal evolution of TC is discussed using Radon concentrations as tracer to

know the dynamic properties of boundary layer [8]. The relationship between OC and EC is also interpreted.

2 Experimental

The separation and quantification of EC and OC were carried out by means an Ambient Carbon Particulate Monitor (Rupprecht & Patashnik Co Inc., NY). By

means a non-dispersive infrared detector (NDIR) the instrument measures the CO2 amount released when a particulate matter sample collected in a collector is oxidized at elevated temperatures. The instrument cycle is made up of two parts: the collection phase during which the sample is gathered in a collector, and the

analysis phase during which the entire collector with its collected particulate matter is elevated in temperature to perform oxidation. To avoid gaps in the collection in the particulate matter the instrument contains two collectors whereby the monitor uses one for the collection phase of the current instrument cycle while the other is used for the analysis phase of the previous instrument

cycle. The conditions for the collection phase were a collection period of one hour and a collection temperature at the collector of 50°C. The sampling flow rate was of approximately 16.7 L/min. During the analysis phase the collector containing a particulate sample is made part of a closed circulating loop that

includes an afterburner, circulating pump and NDIR CO] detector. The temperature of the collector is then raised to 340°C for a period of 13 min during which the instrument measures the CO] concentrations in the analysis loop. Then, a final burn of 8 min at 750°C takes place to burn off the high-temperature

carbon that was not oxidized at 340°C setting. The instrument measures the organic concentration for the low temperature and the total carbon concentration of the sample. And so, the elemental carbon is calculated as the difference of the TC and the OC. The total mass particulate matter was monitored by a TEOM Ambient

Particulate Monitor (R&P) and the Radon concentration was measured by an ADAM Atmosphere Stability Monitor (Opsis, Sweden). The Benzene measurements were continuously carried out by means of a commercial DOAS (Opsis) with an about 200 m optical path length.

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The "Monitoring Station" is placed at ISPESL building in downtown Rome (S. M. Maggiore) at an height of about 4 m from road level.

3 Results and Discussion

3.1 Temporal evolution of total carbon

In order to interpretate the temporal behavior of carbonaceous particle concentrations we have applied the method based on the identification of key variables able to disclose the variations in TC concentration which are linked to the source and sinks, and the variations which are due to vertical and orizontal mass transport [9]. The temporal evolution of TC concentration at the ground level can be formally described by the following equation:

6Crc/5t= ocO(t) - (3{C| + Adv - Ls. (1) where: C: mixing ratio near the ground

a: stability term O(t): primary emission flux [J{C}: vertical mass exchange due to eddy diffusion Adv: advection term

Ls: removal rate from dry deposition

The term O(t) reflects temporal variability of source intensity and the terms a, (3 and Adv are linked to the dynamics of the lower layers of the atmosphere. The term Adv can be deduced from knowledge of the intensity and direction of the wind at the ground level. The time trend of Radon daughters concentration can be used in order to characterize the terms a and (3. For example, in Figure la are reported the temporal trends of TC and TSP and in Figure Ic the trend of concentration of Radon. It can be seen that the trends of TC and TSP are very similar and that they have the same modulation as Radon concentration. From the quantitative point of view the trend of TSP is larger than the TC one. This is due to contribution of other species (sulfate, nitrate, ammonium, metals, etc.) to total particulate matter. During the high pressure period (December 8-9) the diurnal variations of TC and TSP concentrations exhibit an increasing mixing ratio during all the day and low values during the night until sunrise. During the low pressure period (December 5-7 and 10) the TC and TSP concentrations decrease considerably.

200 140 - - 180 120 - - 160 100 - - 140 - 120 80 - - 100 60 - - 80 40 - 60 - 40 20 - - 20 0 0 0.00 0.00 0.00 0.00 5/12 6/12 7/12 1 1/12

day

Figure la: Temporal trends of TC and TSP.

Figure Ib: Temporal trend of TO and Benzene.

g3 2000 -

||||,..||||...... Illlllllll 0.00 0.00 0.00 5/12 9/12 10/12

Figure Ic: Temporal trend of natural radioactivity.

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Figure Ib shows temporal trends of benzene in gas phase and total carbon particles. Benzene is a primary low-reactive pollutant. The trend of two primary pollutant are similar and they have the same modulation of Radon concentration

(Fig. Ic). The quantitative TC/Benzene ratios vary according the meteorological conditions. In fact, the temporal trends of the two primary pollutants ratio (TC/Benzene) reported in Fig. 2a is not constant but it shows a pattern depending on the relative importance of the term Ls, eqn. (1), for the two pollutants.

Fig. 2a: Daily TC/benzene trend.

•1. 1. ... ll...... 1.1-11 III... ll II III. ll ..lll.l III. I.I ll )() | O.lo 0.00 1 0.00 1 0.00 II " ()lo ' 1 0. .2 6/12 7/12 8/12 1 9/121 I(|l2 1 11

Rado n derivativ e (counts )

SSfeo " ooo ? = o 5 ,jp_ , 000 0

Fig. 2b: Daily Radon derivative trend.

The term Ls, which takes into account the deposition losses, is very important for carbonaceous compounds associated to paniculate matter, expecially during unstable periods, in which turbulent deposition is very effective, whereas the term Ls for Benzene is generally negligible. If we compare the derivative of Radon concentration with TC/Benzene trend, it appears that high values of the ratio occur when stability condition are present

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(December 8-10), instead minimum values occur during unstable periods (December 5-7 and 10). The stability and instability conditions, or phase of transition are shown from the shape of the Radon time derivative trend.

In Table 1 are reported the mean monthly concentrations of TC and TSP monitored during July-December. The TC/TSP ratios range between 30-40% and it is a very significant fraction of carbonaceous material in the ambient aerosol.

Table 1. Monthly concentrations (|ig/nf) of TC and TSP

TC TSP TC/TSP (%)

July 1999 18.0 519 314 August 18.2 512 34.2 September 20.1 51.2 39.2 October 224 57.9 38.7

November 14.6 49.1 29.7 December 15.4 522 29.5

3.2 Relationship between elemental and organic carbon

Table 2 shows the mean monthly concentrations during the entire period investigated. The EC/OC ratios vary between 1.4 to 2 during May and July and they are constant in the other period. These differences can be due to losses of low vapor pressure organic compounds or to chemical degradation of reactive

organic compounds in presence of high ambient temperature and of strong solar irradiation during the summer period.

Table 2. Monthly concentrations (jug/nr^) of OC and EC.

OC EC EC/OC May 1999 5.7 11.7 2.0 June 6.0 10.4 1.7 July 7.6 10.4 1.4 August 8.7 9.5 1.1 September 9.8 103 1.0 October 10.6 11.8 1.1 November 7.0 7.6 1.1 December 7.2 8.2 1.1

Figures 3a and 3b show the temporal trends of OC and EC and the relative correlation curve during the July 9*-13*.

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Fig. 3a: Temporal OC and EC trends from during summertime.

y=134x tf=0.798 U i

10 12 14 16 18 20

Fig. 3b: Correlation curve between OC and EC during summertime.

Figures 4a and 4b report the temporal trends of OC and EC and the relative correlation curve during the December 5*-9*. The temporal trends of EC and OC concentrations either in the summertime that in the winter period have a very similar modulation. During the winter period EC and OC show a good correlation (R? 0.855) which decreases significantly in the summertime (R? 0.798). This can be explained by the decrease of OC concentration during photochemical smog episodes.

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m 30

"So 25 - 20 -

% 15

§ 10

^ 5

0 0.00 0.00 0.00 0.00 0.00 0.00 5/12 6/12 7/12 8/12 9/12 10/12 day

Fig. 4a: Temporal OC and EC trends during wintertime.

25 - 20 -

15 -

10 - y= 1.12x IT = 0.855 5 -

0 10 20 OC

Fig. 4b: Correlation curve between OC and EC during wintertime.

The good agreement between OC and EC trends during the entire investigated period demonstrates that the emissions from traffic are the most important source for carbonaceous particles.

4 Conclusions

The results of this study show that the total paniculate matter in the urban area of Rome contains a significant fraction of carbonaceous material (30-40%). The relationship between TC and Benzene and the good correlation between EC and OC confirm that the traffic is the most intensive emission source. Therefore, the

total carbon concentration can be a specific index for traffic and a fundamental parameter for the knowledge of the state of air quality and for the determination of the strategies on controlling the emission reduction.

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The separation and quantification of OC and EC are very helpful for the study of toxicological TSP effects and to have information on formation and transformation of reactive compounds in the atmosphere.

The measurements of Radon concentrations is a very important approach for the knowledge of the dynamic properties of the lower boundary layer and give a useful help for the interpretation of pollutant evolution.

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