Conversion of SO2 to sulfur particulate in the Los Angeles atmosphere.

Gas phase and particular phase sulfur have been measured at various locations in the Los Angeles basin to determine atmospheric conversion rates and mechanisms. A new technique was developed for the measurement of particulate sulfur. From measurements of the particulate to gas phase sulfur ratio near the major stationary sources and far downstream and from estimates of travel time determined by air trajectory analysis, it is possible to estimate gas-to-particle conversion rates for sulfur. Such calculations show that automobiles presently contribute a major part of the total sulfur as measured at a receptor site such as Pasadena, while contributing only a small amount to the particulate sulfur loading. The introduction of oxidation catalyst-equipped vehicles may add significantly to the particulate sulfur at downwind receptor sites; predictions of particulate sulfur concentrations near freeways show substantial increases due to such vehicles.


Introduction
There have been many laboratory studies of SO2 to aerosol sulfur conversion (1) but data on the actual rate and/or conversion mechanisms in an urban atmosphere are scanty. Most of the atmospheric studies to date have been made in power plant plumes (2). In this paper, we report the results of measurements of the particulate to gas sulfur ratio near sources and far downstream. From estimates of travel times based on air trajectory analyses, rates for conversion of gas to particulate sulfur can be calculated. These calculations show that the automobile presently contributes relatively little to the particulate sulfur loading; however, the additional particulate sulfur resulting from catalytic conversion in automobile reactors may add significantly to the aerosol sulfur burden at local and downwind receptor sites.

Model Development
In Los Angeles, the major stationary sources of SO2 are power plants and refineries located along the coast. When a stable marine wind is established during the late morning, sulfur oxides are transported from these sources to downwind receptor sites. Additional quantities of sulfur oxides are emitted from industrial and automobile sources along the trajectory.
For a detailed understanding of the time and emission history of aerosol samples taken at a receptor site, it is convenient to use an air trajectory technique such as that suggested by White,Husar,and Friedlander (3). From such an analysis, it is possible to determine the average position-time history of that air parcel and to calculate pollutant inputs during movements of the air from major source to receptor site. Once this has been done, such data can be used in the Eulerian diffusion model which will be developed next.
First, we write the diffusion equation for both SO2 and total sulfur (ST) in an Eulerian system [eqs. (1) and (2)], where we have assumed zero wind in the y and z directions and have neglected dispersion in the direction of transport. Here, SO2 where we have assumed zero wind in the y and z directions and have neglected dispersion in the direction of transport. Here, SO2 and ST are both concentrations expressed as sulfur, u is the steady wind in the x direction, Wso2 and WST are the area source emission terms for SO2 and total sulfur, H is the mixing height, and KV and KZ are the eddy diffusivities. Also, k is the pseudo first-order rate term for the SO2 to aerosol sulfur con-aS02 -S02 a S02 0i at + _ ) + a dt Ox 'ay ay Oz version, vg is the deposition velocity for loss of SO2 to the ground, and va is the deposition velocity for loss of aerosol sulfur to the ground. The area source emission is assumed to become uniformly mixed in the vertical direction quickly and is thus treated as a volume source term. Likewise the gas and aerosol sulfur ground loss terms are treated as volume loss terms. Georgii (4) has shown that the SO2/ST ratio is not a function of z at low altitudes. Assuming also that SO2/ST = g(x), we can carry out the substitution of eq. (3).
On substituting into eqs. (1) and (2) and rearranging, the result is eq. (4): Setting the travel time T = x/u and integrating along a known trajectory yields eq. (5): For area automobile emissions, we denote the aerosol sulfur to total sulfur ratio as f (W'ST -W'SO2) /WsT, where W'sT and W's0 are the area source terms for the automobile only. Then eq. (5) can be rewritten as eq. (6).   (6). Average values for each hour along a known trajectory were determined by using Los Angeles APCD monitoring data for H and SO.. The contribution of stationary sources to Ws02 along the trajectory path was calculated by using emission inventory data supplied by the LAAPCD (5). The automobile contribution to WS was calculated by updating (to summer 1973) the average miles driven as a function of time of day and position in the Los Angeles basin (6) and using the average sulfur content of Los Angeles area gasoline, 0.075 wt-% sulfur (7). Values of v 0.7 cm/sec (8) and va = 0.03 cm/sec (9) were used in eq. (7). location to determine the near-source SO2/ST ratio and its variability. This ratio was found to range from 0.88 to 0.98, depending on the type of source in the downwind direction.
The aerosol sulfur analyses were performed by using an aerosol vaporization technique developed in this laboratory. The method, a modification of gas evolution analysis, measures the total sulfur mass in the aerosol of all sulfur compounds which either vaporize (e.g., H2SO4) or decompose [e.g., (NH4) 2SOJ below approximately 1100°C.
The aerosol is vaporized in an all glass-Teflon system into a stream of clean air flowing directly into a flame photometric detector (Meloy SA-160) for total sulfur (see Fig. 1). Electronic integration of the sulfur peak results in the mass of sulfur vaporized from the sample. Standardization is accomplished by using known volumes of doubly distilled water solutions containing 5-500 ng sulfur as H.SO4, (NH4) 2SO, and Na SO4. Such compounds are included among the major sulfur compounds existing in urban aerosols (10).
The method depends upon the resistance heating of a thin stainless steel strip (0.001 in. X 0.25 in. X 0.80 in.) to about 11000C by a capacitor discharge through the strip. The analysis of impactor samples collected directly on such strips is accomplished by mounting the strip in the glass system and Experimental Total filter and cascade impactor aerosol samples of 1 hr duration were taken at Pasadena in July 1973 and at California State Upiversity at Dominguez Hills in October 1973 and analyzed for sulfur. Sulfur gas and other parameters were also measured at both sites. Because of the many possible trajectory starting locations, it is difficult to sample at both ends of the same trajectory. Therefore, 5-days were spent at the Dominguez Hills April 1975 (7) subsequent capacitor discharge. More than 95%o of the sulfur placed on such a strip for standardization is recovered after a single heating. In order to analyze filter samples, a small disk cut from a glass fiber filter is placed between two stainless steel strips and heated by a capactor discharge of approximately twice the energy as that used for a single strip. It is necessary to heat such samples twice in order to recover more than 95% of the measurable sulfur. Replicate analysis is possible for filter samples. Any punch size less than 1/4 in. diameter can be used, depending on the quantity of sulfur aerosol on the filter. Total filter analysis is also possible by water extraction of a filter medium, placement of an aliquot of the extract on a clean single strip and heating. The stainless steel strips are prepared for use by heating at 900°C for about 2 hr. The glass fiber filters (Gelman type A) are prepared by heating at 450°C for about 8 hr. These preparation methods reduce background sulfur to less than 1 ng per single strip and 4 ng per disk. Table 1 shows the calculated k for 3 days in July and the general starting location of the trajectories. The Alamitos Bay sources consist of two large power plants, and the El Segundo sources of two power plants plus a refinery. An initial value of 0.97 was used for the SO'-/ST ratios from such sources.

Results
The calculated rate constants vary significantly, indicating dependence on parameters (such as O, free radicals, olefins, relative humidity) involved in the complex photochemistry occurring in the Los Angeles atmosphere.
An estimate of the contribution of automobile emitted sulfur to the aerosol sulfur at Pasadena can be made by substituting the rate constants shown in Table 1 into eq. (6) and eliminating the area automobile source (W for industrial sources only) to determine the resulting SO1./ST ratio at Pasadena. Comparison of this ratio with that actually found results in the aerosol sulfur presently due to the automobile. Such calculations for the samples shown in Table 1 indicate 2-11 % of the aerosol sulfur measured at Pasadena resulted from conversion of auto-emitted SO,. However, 20-70% of the total sulfur at Pasadena resulted from SO2 emissions from the automobile (calculated for the samples shown in Table 1 by using the CO concentration at Pasadena as a tracer). This results because the automobile emits only about 15%o of the SO2 emitted in the Los Angeles basin, but that emission can occur anywhere along the trajectory. Any SO. emitted late in the trajectory is given heavier weight than earlier emissions in determination of the total sulfur at a receptor site, but smaller weight in determination of the aerosol sulfur. Figures 2 and 3 show the estimated increase in aerosol sulfur at Pasadena due to the conversion of SO2 to H2SO in automobile oxidation catalytic converters. It is assumed that such vehicles use an unleaded fuel of sulfur content 0.075 wt-%o, a quantity typical of present Los Angeles gasoline.  Table 1.  Table 1.
shown in Table 1. The other two curves indicate the range of response. The average sulfur aerosol concentration of 20 1±g/m3 as sulfate results in a 20%o increase in aerosol sulfur at Pasadena, assuming a 50% conversion before emission (11). Figure 3 shows the dynamic effect of replacement of older automobiles without catalytic converters by those with such devices for the average and higher curves in Figure 2 at two levels of the converted fraction (f = 0.50 and f 0.75). This assumes the birth-death schedule and average mileage as a function of vehicle age for Los Angeles County as given by Lees et al. (12). These estimates indicate that the introduction of oxidation catalytic converters will have little effect on the aerosol sulfur at downwind receptor sites such as Pasadena during the first few years; however, a significant effect is estimated to occur as the system approaches that of all automobiles having such catalytic converters. Local receptor effects of the oxidation catalytic converters must be considered along with the downwind receptor effects estimated above. Table 2 shows sulfate concentrations due to the conversion of SO2 to H.SO in automobile reactors estimated by using measured CO concentrations as a tracer for such emissions and on assuming all automobiles to have such reactors. These concentrations are in addition to other aerosol sulfate already existing in the air. Although they show some variance in the estimated sulfate concentrations, both the 7-mode and CVS emission factors result in a substantial addition to the aerosol sulfur burden close to high density automobile traffic.

Conclusions
Estimates have been made of the pseudo first-order rate term for S02 to aerosol sulfur conversion in the Los Angeles atmosphere. This rate appears to depend upon parameters such as O0, free radicals, olefins, and relative humidity which are involved in the complex photochemistry of the Los Angeles atmos- phere. The calculation procedure outlined here has been used to estimate the dynamic and steady-state effect of the introduction of automobile oxidation catalytic converters on aerosol sulfur. This shows a significant increase in aerosol sulfur at a receptor site such as Pasadena and a much larger increase in aerosol sulfur at receptor sites near highdensity automobile traffic.