The importance of the three major aqueous reactions thought to be responsible for the in-cloud conversion of SO2 to SO4(2-) was studied using the acidic deposition and oxidants model by suppressing each reaction individually and all reactions simultaneously. The reactions are the oxidation of SO2 by H2O2, or O3 and catalytic oxidation by O2 in the presence of Fe and Mn. The model simulations were for 19-24 April 1981. It was found that SO4(2-) precipitation concentrations were generally more sensitive to H2O2 oxidation than to O3 oxidation. The contribution of catalytic oxidation of O2 in the presence of Fe and Mn is insignificant everywhere and at all times. The contribution of H2O2 oxidation to SO4(2-) in precipitation is strongest in light precipitation areas while O3 oxidation can be greater than H2O2 oxidation in heavy precipitation areas. The effect of suppressing one reaction is mitigated by compensation through another mechanism. This is seen from the significant difference observed in the effects when individual suppressions were added together and when all reactions were suppressed simultaneously. From this, it is estimated that the contribution of aqueous oxidation of SO2 to SO4(2-) in precipitation is approximately 50-80 per cent. Further simulations show that the relationship between SO2 emissions and SO4(2-) production in the aqueous-phase through the oxidation reaction with O3 is always non-linear in view of the pH dependence of the reaction rates.
