also contribute (Calvert and Stockwell, 1984). Ozone (O3) is a major source of the HO radical through its photodecomposition to the highly reactive, excited oxygen atom, O(1D), which reacts with water vapor:

The major elementary reactions that occur in the homogeneous atmospheric oxidation of SO2 are well established (Stockwell and Calvert, 1983; Calvert and Stockwell, 1984; Margitan, 1984; Calvert et al., 1985; Gleason et al., 1987). The first of those, Reaction A-3, involves the HO radical formed in Reaction A-2:

Reactions A-4 and A-5 occur rapidly following the rate-determining step (A-3). When coupled with Reaction A-6, Reactions A-3 and A-4 constitute the elements of a HO-HO2 radical chain propagation sequence; that is, although an HO radical is removed in Reaction A-3, another is regenerated in Reaction A-6:

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