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

Abstract

We examine factors controlling the photochemical oxidation of SO 2 in tropospheric aerosols using a gas‐aqueous photochemical model. Over a range of liquid water contents (3×10 −4 g H 2 O m −3 to 9 g H 2 O m −3 ) and pH values (0 to 8), we find that H 2 O 2 (aq) and O 3 (aq) provide the major sinks for SO 2 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 SO 2 in the aqueous phase when pH is held constant between 5 and 6 and H 2 O 2 is depleted in an air parcel. When pH is allowed to vary during the integration, H 2 O 2 (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 is above 4. O 3 (aq) is also important when the pH is initially high (>6) for quickly oxidizing SO 2 and, thereby, reducing the pH into the pH region where H 2 O 2 (aq) is the most important oxidant. OH(aq) may be important when H 2 O 2 is depleted and the liquid water content is large. When aerosols are present during noncloudy days in summer, the aqueous‐phase oxidation of SO 2 is insignificant compared with the gas‐phase oxidation of SO 2 . We find, however, that the SO 2 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 SO 2 is proposed for global tropospheric modeling.