OXIDATION OF S(IV) IN ATMOSPHERIC WATER BY PHOTOOXIDANTS AND IRON IN THE PRESENCE OF COPPER

We have simulated some of the reactions that result in oxidation of S( IV) and production of secondary photooxidants in cloud droplets, using gamma-radiation to generate hydroperoxyl (HO2), superoxide (O-2(-)), and hydroxyl (OH.) radicals in oxygenated, formate-containing solution s. In the absence of transition metals, HO2/O-2(-) undergoes dispropor tionation to form hydrogen peroxide (H2O2). S(IV) oxidation can be acc ounted for entirely by reactions with H2O2 and OH. in such systems, an d contrary to earlier reports, the oxidation of S (IV) by HO2/O-2(-) i s unimportant. In solutions containing dissolved Fe, oxidation of S(IV ) also occurs through Fe(III)-catalyzed reactions with O-2. However, a t the low concentrations of Cu (5-100 nM) typically measured in cloud droplets, Fe-catalyzed oxidation of S(IV) by O-2 is inhibited through the reduction of FeOH2+ by Cu(I), which is produced by the reduction o f Cu2+ by HO2/O-2(-).