Oxidation of S(IV) in rain-water collected in Japan

Rates of S(IV) oxidation in 48 rain-water and snowmelt samples collected at 15 sites in Japan were determined to assess the significance of the autoxidation in the sulfate formation channels of the aqueous phase. Sodium bisulfite aqueous solution was added to each of the samples, and the S(IV) concentration was followed by a modified West-Gaeke method. Most sample systems showed pseudo-first order kinetics, with rate constants ranging from 1 × 10−5 to 3 × 10−3 s−1. In several cases, however, the logarithmic concentration-time plot definitely deviated from a straight line. Some other kinetics such as rate = ka[S(IV)] + kb[S(IV)]12 could reproduce these non-linear plots of the reactions. Strong correlations were found between the first-order rate constant and the concentrations of some transition metals (Cr, Fe, Co, V and Mn, in order of decreasing correlation coefficients). The experimental rate constants agreed typically within a factor of three with calculations for the Mn concentration, but were much smaller than the predictions for the Fe levels. The experimental rates in the rain-water samples were applied to the atmospheric oxidation rate of SO2 and were compared with those of H2O2 and O3.