Oxidation of S(IV) in sea-salt aerosol at high pH: Ozone versus aerobic reaction

摘要

The oxidation of dissolved SO2 (as S(IV)) by dissolved O3 is known to increase rapidly with pH and is generally thought to be the mechanism responsible for the oxidation of S(IV) in sea-salt aerosol at high pH. Recently, Hoppel et al. (2001) have shown that an aerobic reaction studied by Zhang and Millero (1991) increases even more rapidly with pH than O3 oxidation and exceeds that of O3 at pH of about 7.5 to 8.5, depending on the ambient SO2 concentrations. At high pH, both of these oxidation mechanisms proceed so rapidly that the actual rate is constrained by both gas-phase and aqueous-phase transport processes. A method of analysis is developed to calculate the transport-limited reaction rates for both of these mechanisms in sea-salt aerosol. Even when the unconstrained O3-S(IV) rate is faster than the aerobic mechanism, aqueous-phase transport limitations on ozone can slow the constrained O3-S(IV) rate to the degree that it is slower than the aerobic reaction. In general, the O3-S(IV) rate is favored over the aerobic mechanism at low SO2 and high O3 concentrations, whereas the aerobic mechanism is favored at high SO2 concentrations. The analysis indicates that the O3-S(IV) mechanism is more important in the remote regions where the SO2 concentration can be extremely low. When the SO2 concentration approaches 1 ppb or greater the aerobic mechanism is likely to be more important.