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 tropos pheric aerosols using a gas-aqueous photochemical model. Over a range of li quid water contents (3x10(-4) g H2O m(-3) to 9 g H2O m(-3)) and pH values ( 0 to 8), we find that H2O2(aq) and O-3(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 pha se when pH is held constant between 5 and 6 and H2O2 is depleted in an air parcel. When pH is allowed to vary during the integration, H2O2(aq) is the most important oxidant in the aqueous phase. O-3(aq) is important primarily when the liquid water content is large (> 1 g m(-3)) and the solution pH i s above 4. O-3(aq) is also important when the pH is initially high (> 6) fo r quickly oxidizing SO2, and, thereby, reducing the pH into the pH region w here H2O2(aq) is the most important oxidant. OH(aq) may be important when H 2O2 is depleted and the liquid water content is large. When aerosols are pr esent during noncloudy days in summer, the aqueous-phase oxidation of SO2 i s insignificant compared with the gas-phase oxidation of SO2. We find, howe ver, that the SO2 oxidation in wet aerosols may be enhanced in winter or wh en the temperature is low (273 K) and the relative humidity is high. Uncert ainties in the reaction rate coefficients may significantly affect the conc entrations of oxidants and other compounds of photochemical origin. Using a relatively stringent criterion, a compressed gas-aqueous phase chemical me chanism for photochemical oxidation of SO2 is proposed for global troposphe ric modeling.