THE OXIDATION OF S(IV) DURING RIMING BY CLOUD DROPLETS

Experiments have been performed to investigate whether the process of freezing during riming in clouds may induce oxidation of dissolved SO2 to SO42-. The experiments were conducted in a cold room at varying te mperatures between -6 and -15 degrees C. Solutions containing dissolve d SO2 and NH4OH in various proportions, in the range of concentrations between 3 x 10(-5) and 10(-3) M, were sprayed. Rime was collected on a rotating cylinder and analyzed. Absorption of oxygen from laboratory air was prevented, except in the spray, to avoid spurious oxidation. Blank experiments were made at +3 to +6 degrees C. The results indicat e clearly that, as the dominant cation becomes NH4+ rather than H+, su bstantial oxidation of S(IV) occurs during riming. This is consistent with redox reactions taking place as a result of charge separation at the ice-water interface during freezing.