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An evaluation has been made of the existing kinetic data related to the elementary, homogeneous reactions of SO2 within the troposphere. A set of recommended values of the rate constants for these reactions is presented. The results show that the direct photooxidation of S02 by way of the electronically excited states of SO2 is relatively unimportant for most conditions which occur within the troposphere. The oxidation of SO2 within the natural troposphere is expected to occur largely by way of reactions 39, 31, and 33, with reaction 39 being the dominant path: HO + SO2 (+M) → HOSO2 (+M) (39); HO2 + SO2 → HO + SO3 (31); CH302 + S02 CH30 + SO3 (33). For certain special conditions within the troposphere the oxidation of SO2 by way of the products of the ozone-olefin reaction may be significant. Also the reaction of 0(3P) with SO2 may contribute somewhat to the S02 removal in NO2-rich, 02-deficient stack gases in sunlight during the early stages of dilution of the plume.

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The complete paper upon which most of the talk was based is: "Mechanism of the Homogeneous Oxidation of Sulfur Dioxide in the Troposphere", by Jack G. Calvert, Fu, Su, Jan W. Bottenheim, and Otto P. Strausz which appeared in the Preprint Volume I, Plenary Papers, International Symposium on Sulfur in the Atmosphere, July, 1977, and which was presented at the Symposium on September 7-14, 1977, Dubrovnik, Yugoslavia. The paper has been published in Atmospheric Environment, 12, 197 (1978).

Key words: Kinetics; photochemistry; review; sulfur dioxide; troposphere.

Summary of Session

The discussion was concerned with two basic problems - what is the mechanism of conversion of SO2 in the atmosphere, and what do we know about aerosol formation arising from SO and NO reactions. There appeared to be a consensus that the reaction of SO2 with OH is the most important homogeneous mechanism, but there is still great interest in quantifying the role of HO2, RO2, and the Criegee intermediate in this process. There is considerable experimental work underway on aerosol formation, the incorporation of NO in aerosols, and the role of specific radicals in

aerosol formation.

Whitten opened the discussion with a descriptions of his modeling results for the Los Angeles basin. He pointed out that while OH levels could be reduced by reducing hydrocarbon or NO, levels, they were relatively constant for a given HC/NO ratio. Thus if the reaction of OH with 502 controls the SO2 level, control of the HC or NO levels will not necessarily have any affect on the OH levels if the HC/NOx ratio remains

fixed. A similar conclusion was reached in the
case of HO2 radical levels, although the new
value for the rate of NOx+ HO2 means that HO2
levels are reduced to the point where the
HO2+ SO2 reaction is probably unimportant.

Whitten also reported modeling studies which suggest that a fairly rapid conversion of S02 to sulfate takes place in fog droplets. The observations used in this study were based on one days sulfate collections from 14 monitor stations (24 hour averages). A photochemical model was not sufficient to account for the SO2 conversion rate.

Miller reported on smog chamber results which supported Whittens observations. He also discussed measurements which show that the rate constant for HO2+ SO2 is no greater than 1 ppm-1 min-1 and for CH3O2 + SO2 is no greater than 2 ppm-1 min-1. The role of NOx in aerosol formation was also discussed.

Heicklen suggested that excited molecules such as NO2, formed by irradiation at wavelengths above 400 nm, might react with S02. Also photo excited aldehydes and ketones might be important. Cox reported on the photolysis of HONO in the presence of SO2. The end product was an aerosol. With

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Jeffries reported smog chamber results on the effect of added CO on SO2 conversion rates in natural background air, which indicate that OH is the only important oxidizing species for S02. Ravishankara noted that aerosols are readily formed in uv flashed H20-S02 mixtures, probably by photolysis of H20. Huie raised the question of the reactivity of the Criegee intermediate with respect to SO2, pointing out that if the Criegee intermediate isomerized to a dioxirane intermediate it probably decomposes through a "hot" acid or ester before it has time to react with S02. However, Calvert noted, one would expect for larger olefins an increasing chance of stabilizing the Criegee intermediate and observing some direct chemistry.

Niki noted that generating HO2 by reacting Cl with H2 in the presence of 02 and S02 led to H202 but no conversion of the S02. If NO was added (to drive the HO2 to OH), then sulfuric acid aerosol is formed. In the case of CH 302 some conversion of SO2 is observed and there is evidence to suggest that sulfones are possibly formed. Jeffries asked if Niki has any evidence that water intercepted the Criegee intermediate to make acetic acid. Niki has not yet carried out an experiment with added H20. Jeffries noted that in earlier work they had not seen acetic but had seen formic acid in the reaction of ozone with propylene.

Comments

David F. Miller, Battelle-Columbus Laboratories, Columbus, Ohio 43201

Gary Whitten presented modeling results relating OH concentrations to initial concentrations of NMHC (nonmethane hydrocarbons) and NO. According to his model, OH concentrations are predicted to be nearly constant for any NMHC/NO ratio. I'd like to add that our smog chemaber results, reposted last year in Dubrovnik, led to the same conclusion. This finding has a very important implication regarding precursor controls designed for limiting ozone. Because S02 competes with NMHC and NO for OH, proportional control of NMHC and NO could result in an increase in the conversion of SO2 to sulfate.

Secondly, I'd like to comment on some smog chamber experiments in which we've irradiated mixtures of nitrous acid with S02 and either CO ur CH to estimate the SO2 oxidation rates attributable to HO2 and CH302. Although our analyses of the data are less than satisfactory, primarily because of so much uncertainty about the nitrogen oxides chemistry, we estimate that the rate for HO2+ SO2 is not greater than 1 ppm1 min1 and the

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When we irradiate just SO2 with nitrous acid we observe an NO, loss in excess of that for the experiment without SO2. The amount of the NO loss corresponds to the amount of H2SO4 formed. We suspected that either NO or NO2 might be incorporat ed in the aerosol phase. It has been suggested, for example, that HSO4 might react rapidly with NO2 to give aerosol mixtures of sulfuric and nitric acids. However recent chemical analyses of filter collections from such reactions show very low nitrate levels relative to sulfate. Thus, if such reactions occur, the nitric acid apparently ends up in the gas phase.

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In a series of experiments performed in UNC's outdoor aerosol chamber, SO2 (at 0.3 ppm) oxidation in natural background air (< 20 ppb NC, < 50 ppb C organics) was followed by observing aerosol number (by CN), aerosol volume (by EAA), and aerosol sulfur content (by XRF analysis of filters). Runs were repeated with various additional amounts of CO added (5, 10, 15, 25 ppm). The additional CO resulted in delays in time of CN peak, small increases in 03 produced, and reduced aerosol volume (by both EAA + XRF).

In one run, no CO was added initially, but when a steady rate of increase in aerosol volume had been established, 25 ppm of CO was injected; aerosol volume production (i.e. growth) was totally stopped within 4 minutes. CO's role in this otherwise low concentration system is to convert OH to HO2. It seems clear that OH was by far the major oxidizing species. It is expected that under urban conditions, however, (i.e. higher NO concentrations) the effects of CO would not be observed because the higher NO converts HO2 to OH.

A. R. Ravishankara, Applied Science Laboratories, EES, Georgia Institute of Technology, Atlanta, Georgia 30332

We have noticed formation of aerosols directly in our system when~ 300 mTorr of H2O and SO2 are photolyzed. This mechanism could be important where water concentrations are high i.e., very quick oxidation of sulfur dioxide leading directly to aerosol. (Even a mixture of S02, 03 and H20 gives aerosols). The water concentration needed to get this aerosol formation seemed rather magical

aerosols formed only after a critical amount of water was present.

Recommendations

It is recommended that kinetic and chemical data regarding S02 chemistry in the troposphere be obtained. The classes of reactions are of six types, with the first four of these being almost equally important.

1. Of most importance is obtaining both product and rate information of HO SO2 and RO SO2 with H2O, NO, O2, hydrocarbons, N3, and combinations of these gases.

2. Of essentially equal importance is obtaining information on the fate of SO2 in 03-olefin-02 reactions. There are three subsections of this

problem which should be attacked in the following order:

a) characterize the intermediates which which react with S02

b) obtain products and rate coefficients for the reactions of these intermediates with S02

c) study the effect of adducts such as H2O, NO, hydrocarbons, NH3, and combinations of these gases.

3. More data is needed on the rate coefficients (and products) for the reactions of HO2, HO, and 0(3P) with S02. These data should include pressure, temperature, and humidity studies. In the case of HO2, there is a large uncertainty in the rate constant. With regard to HO and 0(3P), fairly reliable values exist. However because of the importance of the HO radical, which appears to be the most important species for S02 removal, it is important to have as accurate a rate coefficient as possible.

4. The reactions of RO2 radicals with SO2 should be investigated to determine products and rate coefficients at a variety of pressures, temperatures, and humidities, and in the presence of NO, O2, NH3 and hydrocarbons. The reactions of RO radicals with S02 appear to be unimportant in the troposphere, and we do not give a high priority to their study. However it would be useful to actually have rate coefficients for RO reactions with S02 to know exactly what role these reactions do play.

5. A low priority recommendation is the study of the possible reaction of electronically excited NO2 with SO2. There is no evidence that a reaction occurs, but this should be confirmed.

6. The direct photoexcitation of SO2 is not important in the removal of SO2 in the troposphere, and we do not recommend studies in this area. However we do point out that such reactions may be important in the formation of sulfur-containing organic aerosols. If so then such reactions could be of significance in aerosol chemistry.

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