The relative importance of various pathways for the oxidation of sulfur dioxide and nitrogen dioxide in sunlit continental fair weather clouds

Abstract

A simple box model of a sunlit small cumulus cloud has been used to explore the efficiency of various chemical reactions contributing to the oxidation of sulfur dioxide and nitrogen dioxide in clouds. The principal aqueous-phase processes of sulfur(IV) oxidation are reactions with ozone, with hydroperoxides, with OH radicals, and catalytic reactions involving transition metals. The last two oxidants initiate chain oxidation processes, which were analyzed in detail. The results indicate that chain reactions are not very effective, partly because the chain carriers are scavenged, and partly because chain termination overrides chain propagation. Hydrogen peroxide is the most effective oxidant in S(IV) oxidation, contributing about 80% to the total rate. Peroxynitric acid also contributes appreciably, in addition to ozone. The oxidation of nitrogen dioxide to nitric acid occurs to 60% in the gas phase by reaction with OH radicals. In the aqueous phase, the reaction of peroxynitric acid with hydrogensulfite is most important, contributing 20–30% to the total rate.