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

Abstract

The oxidation of dissolved SO 2 (as S(IV)) by dissolved O 3 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 O 3 oxidation and exceeds that of O 3 at pH of about 7.5 to 8.5, depending on the ambient SO 2 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 O 3 -S(IV) rate is faster than the aerobic mechanism, aqueous-phase transport limitations on ozone can slow the constrained O 3 -S(IV) rate to the degree that it is slower than the aerobic reaction. In general, the O 3 -S(IV) rate is favored over the aerobic mechanism at low SO 2 and high O 3 concentrations, whereas the aerobic mechanism is favored at high SO 2 concentrations. The analysis indicates that the O 3 -S(IV) mechanism is more important in the remote regions where the SO 2 concentration can be extremely low. When the SO 2 concentration approaches 1 ppb or greater the aerobic mechanism is likely to be more important. Copyright 2005 by the American Geophysical Union.