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.