Air oxidation of S(IV) in cloud-water samples

Aerobic oxidation of S(IV) was investigated in cloud-water samples collected at Great Dun Fell,U.K., as part of a wider project into cloud-water chemistry. The rate was found to be first-order in S(IV) concentration, and the reaction rate constant. k_S(IV), was found to vary from 10^–5 to 10^–3 s^–1. The rate constant was highly correlated with H⁺ concentration (pH 3.5 to 6.5) and Fe concentration (<0.02 to=">^–6 mol dm^–3). The aerobic oxidation of S(IV) does not contribute significantly to SO₂ oxidation in clouds at Great Dun Fell, however, the reaction may be of consequence in clouds and fogs at polluted or urban sites with elevated trace metal concentrations. Also, this reaction may be responsible for the oxidation of S(IV) in cloud-water samples during storage.