Oxidation of So2 in Aqueous Droplets: Mass- Transport Limitation in Laboratory Studies and the Ambient Atmosphere*

Despite numerous laboratory investigations, the aqueous phase oxidation of SO1 remains illunderstood. A major reason for this is the failure to treat properly the combined problem of mass transport and chemical reaction. This problem has not been adequately addressed either by experiments with bulk solutions or by experiments with aqueous droplets that have been undertaken in order to circumvent the mass-transport problem associated with bulk solutions. The conclusions of these laboratory investigations conflict with each other and report reaction rates which are considerably different from those that would pertain to the same reactions in the ambient atmosphere. In this paper we apply the theoretical analysis that has been developed to describe mass transport and reaction of SO, in aqueous droplets (Schwartz and Freiberg, 1981) to an analysis of the data of two laboratory studies of this reaction system. It is shown that mass-transport limitation (both gas-phase and aqueous-phase) in the experiment of Barrie and Georgii (1976) may have reduced the measured rates by 20 % to as much as an order of magnitude from the intrinsic rate. In the experiment of van den Heuvel and Mason (1963) gas-phase mass-transport limitation is shown to have prevented the steady-state reagent concentration from having been achieved in the contact times employed. Consequently, the rate constants derived from these data by Scott and Hobbs (1967) are too low. Application of the analysis of Schwartz and Freiberg (1981) to an examination of SO, oxidation in clouds and fogs indicates that the mass-transport limitation of the oxidation rate is not significant except under conditions of very high oxidation rates, e.g. at high ozone or H,O1 concentration.