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A study of sulfur dioxide oxidation pathways over a range of liquid water contents, pH values, and temperatures

We examine factors controlling the photochemical oxidation of SO2 in tropospheric aerosols using a gas-aqueous photochemical model. Over a range of liquid water contents (3x10 -4g H20 m -3 to 9 g H20 m -3) and pH values (0 to 8), we find that H202(aq) and O3(aq) provide the major sinks for SO2 in the aqueous phase when pH is held constant at below 5 and larger than 6, respectively. OH(aq) may be an important oxidant of SO2 in the aqueous phase when pH is held constant between 5 and 6 and H2 02 is depleted in an air parcel. When pH is allowed to vary during the integration, H202(aq) is the most important oxidant in the aqueous phase. O3(aq) is important primarily when the liquid water content is large (> 1 g m -3) and the solution pH is above 4.0 3 (aq) is also important when the pH is initially high (> 6) for quickly oxidizing SO2 and, thereby, reducing the pH into the pH region where H202(aq) is the most important oxidant. OH(aq) may be important when H20 2 is depleted and the liquid water content is large. When aerosols are present during noncloudy. days in summer, the aqueous-phase oxidation of SO2 is insignificant compared with the gas-phase oxidation of SO2. We find, however, that the SO2 oxidation in wet aerosols may be enhanced in winter or when the temperature is low (273 K) and the relative humidity is high. Uncertainties in the reaction rate coefficients may significantly affect the concentrations of oxidants and other compounds of photochemical origin. Using a relatively stringent criterion, a compressed gas-aqueous phase chemical mechanism for photochemical oxidation of SO2 is proposed for global tropospheric modeling. Atmospheric sulfate is environmentally important. It is a com- ponent of acid rain, which is harmful to crops, plants, and fresh- water reservoirs. It is also a component of tropospheric aerosols, which affect Earth's radiation budget. In the stratosphere, sulfate aerosols often serve as nuclei for polar stratospheric cloud parti- cles. Oxidation of sulfur dioxide is the most important anthropo- genic source of sulfate in the free troposphere. This oxidation is relatively slow in the gas phase (-1 day lifetime), but more rapid in the aqueous phase. The speed of aqueous oxidation depends on pH, liquid water content, and the availability of oxidants (Pandis and Seinfeld, 1989; Gurciullo and Pandis, 1997). In a cloud, oxidation of sulfur dioxide occurs predominantly in the aqueous phase (Chameides and Davis, 1982; Wang and Chang, 1993) owing to the presence of a sufficiently high liquid water content. The contribution of aqueous-phase formation of sulfate in clouds to the total sulfate production in the troposphere is, however, limited by the relatively small total volume of cloud water in the troposphere (Liang and Jacob, 1997). Unlike clouds, aerosols are ubiquitous in the troposphere. The liquid water con- tent of an aerosol plume, even in polluted air, is about 1000 times smaller than that of a cloud. As a result, processing of sulfur