Environ. Sci. Technol. 1997, 31, 2869-2872
Toluene-Benzene Concentration Ratio as a Tool for Characterizing the Distance from Vehicular Emission Sources A N D R AÄ S G E L E N C S EÄ R , † KRISZTINA SISZLER,‡ AND JO Ä Z S E F H L A V A Y * ,‡ Air Chemistry Group of the Hungarian Academy of Sciences, P.O. Box 158, H-8201 Veszpre´m, Hungary, and University of Veszpre´m, Department of Analytical Chemistry, P.O. Box 158, H-8201 Veszpre´m, Hungary
An attempt was made to assess the “medium distance” of sampling sites from vehicular emission sources. Two parameters, the “site ratio” and the “traffic-equivalent distance”, were calculated from the time-weighted average (TWA) concentrations of two selected volatile air pollutants, benzene and toluene. The concept of the model was based on the considerable difference between the rate constants of benzene and toluene with OH radicals. It was shown that these parameters are more reliable indicators of the distances of emission sources than the absolute TWA concentrations which are greatly influenced by the intensities of the emission sources, the location of the sampling site, and the duration of sampling.
Introduction In the industrialized countries the primary source of volatile non-methane hydrocarbons (NHMC) in the atmosphere is traffic, which accounts for more than 70% of their emission (1). These compounds were shown to play a key role in the formation of secondary atmospheric pollutants, particularly ozone. According to a recent study (2), the composition of NMHCs is quite different in various cities, but the predominant species are alkanes and aromatic hydrocarbons. Among these, the aromatic compounds are more important than their relative share on a carbon basis: the majority of them are toxic themselves and produce toxic intermediates in photochemical reactions. There have been relatively few articles published on the concentration of NMHCs in rural atmosphere, in particular from Central-Eastern Europe. In addition, most of these papers consider the NMHCs as a collective noun in terms of sampling, analysis, and interpretation of results. There are, however, considerable differences among the individual NMHC compounds, such as (a) toxicity: ranging from the nontoxic alkanes to the highly carcinogenic polynuclear aromatics; (b) reactivity toward atmospheric radicals and, consequently, their atmospheric lifetime; and (c) significant differences in volatility enabling different sampling techniques: grab sampling in general, active or diffusive sampling for C5 or higher. Total NMHC analysis would only facilitate grab sampling which allows instantaneous concentration measurement. * Corresponding author fax: +36-88-423-203; e-mail: hlavay@ anal.venus.vein.hu. † Hungarian Academy of Sciences. ‡ University of Veszpre ´ m.
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1997 American Chemical Society
Furthermore, most studies have been focused on the determination of absolute concentrations. Emission is often measured directly, but for the studies on urban atmosphere, sampling sites are usually selected to be away from point sources and in well-mixed open air. The latter is, however, not of too much use for global atmospheric studies as instantaneous absolute concentrations are affected by a number of local factors which are impossible to take into account. More valuable information can be gained from measurements in which the time-weighted average (TWA) concentrations are determined because these are much less susceptible to instantaneous variations of local conditions. Diffusive sampling is an ideal method to determine TWA concentrations. Its use, however, is restricted to hydrocarbons having five or more carbon atoms. Diffusive sampling is a method in which volatile organic compounds are taken from the atmosphere at a rate controlled by diffusion. Its advantages over active (pumped) samplingspossibility of unattended operation, ease of handling, and low costsmakes this technique ideally suited for the prolonged sampling times usually required in outdoor applications (3). Although TWA concentrations are more significant in terms of atmospheric modeling, the relative ratio of TWA concentrations of certain compounds can be even more useful. While absolute concentrations are affected by vertical mixing and other atmospheric processes, the relative ratios are solely dependent on differences in the mechanisms and rates of photochemical reactions provided that the compounds are emitted from the same sources at nearly constant emission ratio. Aromatic hydrocarbons are ideal candidates for such studies, as they are less reactive in the atmosphere, have life times of several days (4) and are subject to midrange atmospheric transport, and, in addition, they can be diffusively sampled. The two most abundant aromatic hydrocarbons, benzene and toluene, are especially worth studying, as there are quite significant differences between their reactivities (toluene is five times as reactive as benzene in its reaction with OH radicals). Therefore, the change of their relative concentration ratio can be more easily observed. In our study the relative concentration ratio of benzene and toluene is used to determine the “distance” of the sampling site from traffic-originated emission sources, assuming that transport is the predominant source of these compounds. This is a problem of topical interest since for NMHCs the conventional distinction between “urban” and “rural” sites can hardly be supported any longer: the traffic accounting for 70% of the emission cannot be localized to the downtown of the cities as a dense network of busy highways covers the countryside.
Theoretical Considerations The principle of the model is as follows. First, the relative concentration ratio of toluene and benzene, expressed in µg/ m3, is determined at the site of the emission for a number of vehicles representative of the car fleet currently in use (hereinafter referred to as the “source emission ratio”). The reliability of the method is strongly dependent on the accuracy of this measurement. The ratio of the TWA concentrations of these compounds over a period of several hours is then measured at the sampling site to minimize the effects of changes in local factors (hereinafter referred to as “site ratio”). Knowing these parameters, the rate constants and mechanism of predominant photochemical processes, it is then possible to calculate the time required for the transformation of the
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source emission ratio into the site ratio. The parameter thus obtained can be used to characterize the “distance” of the sampling site from traffic-originated emission sources. Nonetheless, it can only be referred to in broad qualitative terms and should not be taken literally.
The concentration of toluene is reduced by the reaction with OH radicals as well as by air mixing:
Determination of the Emission Ratio. For the purpose of the present study it is possible to determine the emission ratio indirectly in a well-established tunnel experiment. Besides being incomparably cheaper and simpler than a series of emission measurements on vehicles representative of the car fleet mix, it is more reliable as it provides the data under actual driving conditions from a large number of vehicles. If diffusive sampling is used over a period of 4-6 h the changes in driving conditions and other factors are averaged in the TWA concentrations. This is highly desirable as the emission profile is strongly dependent on the technical conditions of individual vehicles, the driving skill of the drivers and the driving conditions as well as the type and quality of the fuel (5). The differences in technical conditions and driving skills are even more significant in this respect than the differences resulting from the type of fuel. Fortunately, in the tunnel experiment many of these effects are averaged due to the great number of vehicles, the only parameter remaining is the average speed which varies between 20 and 50 km/h. In a recent study (6) it was shown that though emission data given in mg/km unit decreases as a function of increasing speed, emission per unit time and especially emission profile shows little variations over the full speed range. It implies that the emission ratio obtained in the tunnel experiment is characteristic of most driving conditions. Thus a tunnel experiment by diffusive sampling over several hours may establish a good approximation of average emission ratio of the vehicles (5).
where cT is the time-weighted average concentration of toluene at the sampling site in µg/m3, cET is the time-weighted average concentration of toluene in µg/m3, as determined in the tunnel experiment, f(t) is the time-dependent mixing factor, kT is the rate of reaction of toluene with OH radicals at 25 °C, cOH is the average daylight concentration of OH radicals at the sampling site during the sampling period in molecule/cm3, and t ) time in seconds. The depletion of benzene in the atmosphere can be described, mutatis mutandis, by the same form of equation:
Atmospheric Transformation of Toluene and Benzene. Benzene and toluene, as monoaromatic hydrocarbons react only very slowly with ozone and NO3 radicals, their rate constants being in the order of
