Chemical Pathways for Sulfate and Nitrate Production

Atmospheric reactions modify the physical and chemical properties of emitted materials, changing removal rates and exerting a major influence on acid deposition rates. Sulfur dioxide can be converted to sulfate by reactions in gas, aerosol, and aqueous phases. As we noted in Chapter 17, the aqueous-phase pathway is estimated to be responsible for more than half of the ambient atmospheric sulfate concentrations, with the remainder produced by the gas-phase oxidation of S02 by OH (Walcek et al. 1990 Karamachandani and Venkatram 1992 Dennis et al. 1993 McHenry and Dennis 1994). These results are in agreement with box model calculations suggesting that gas-phase daytime S02 oxidation rates are l-5% per hour, while a representative in-cloud oxidation rate is 10% per minute for 1 ppb of H202. 

Fogs in polluted environments have the potential to increase aerosol concentrations by droplet-phase reactions but, at the same time, to cause reductions because of the rapid deposition of larger fog droplets compared to smaller particles (Pandis et al. 1990). Pandis et al. (1992) estimated that more than half of the sulfate in a typical aerosol air pollution episode was produced inside a fog layer the previous night. 

The low amount of liquid water associated with particles (volume fraction 10-10, compared to clouds, for which the volume fraction is on the order of 10-7) precludes significant aqueous-phase conversion of S02 in such droplets. These particles can contribute to sulfate formation only for very high relative humidities (90% or higher) and in areas close to emissions of NH3 or alkaline dust. Seasalt particles can also serve as the sites of limited sulfate production (Sievering et al. 1992), as they are buffered by the alkalinity of seawater. The rate of such a reaction as a result of the high pH of fresh seasalt particles is quite rapid, 60 pM min-1, corresponding to 8% h 1 for the remote oceans (S02 = 0.05 ppb). Despite this initial high rate of the reaction, the extent of such production may be quite limited. For a seasalt concentration of 100 nmol m 3, the alkalinity of seasalt 

Snider and Vali (1994) reported studies of S02 oxidation in winter orographic clouds in which S02 was released and the increased concentrations of sulfate in cloudwater relative to the unperturbed cloud were compared to decreased concentrations of H2O2. Despite considerable scatter, the data fell fairly close to the one-to-one line, indicative of the expected stoichiometry of the reaction. 

By analogy to the sulfur system, atmospheric nitrate sources can be distinguished into primary, gas phase, aqueous phase, and aerosol phase. Primary nitric acid emissions are considered to be small (U.S. EPA 1996) and can be neglected. 

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