Water-soluble atmospheric species incorporated in cloud droplets and aqueous aerosols may dissociate into ions, and the resulting aqueous-phase redox chemistry provides yet another pathway for oxidation of species in the atmosphere. The importance of this pathway has been established for S02, which dissociates in water to HSO3- and S032~ (pKaX = 1.9, pKa2 = 7.2). Rapid oxidation of S02 by H202 in cloud was first suggested by Penkett et al. (1979):
Aircraft measurements by Daum et al. (1984) demonstrated that the reaction is sufficiently fast to titrate either S02 or H202 in cloud (whichever is limiting). It is now well accepted that this mechanism dominates over gas-phase oxidation by OH as a sink for S02 in the atmosphere (Chin et al, 1996).
Additional nonradical oxidants may also be important for oxidation of S02 in clouds and aqueous aerosols, but their importance is not as well verified as for H202. At high pH values (pH > 5), O3(aq) reacts rapidly with S032-:
This mechanism, taking place in alkaline sea salt aerosols, could represent a major sink for S02 in the marine boundary layer (Chameides and Stelson, 1992). Additional S02 oxidants in sea salt aerosol may include HOC1 and HOBr produced by halogen radical chemistry (Vogt et al, 1996). In polluted clouds, aqueous-phase autoxidation catalyzed by Fe(III) could provide the dominant S02 sink (Jacob and Hoffmann, 1983).
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