Oxidation of sulphur dioxide in aerosol droplets, catalysed by manganous sulphate

Abstract

Experiments are described in which the changes in size distribution of water droplets in an aerosol were measured when they were exposed to humid air dosed with sulphur dioxide, such that manganese-catalysed oxidation occurred in the droplets. The size changes have been compared with calculations (a) of the growth induced by reaction and subsequent water condensation, and (b) of the changes produced by coagulation. The results are in good agreement when the most recently revised values of the rate constants of controlling steps are used. The initial rate of oxidation is related to the overall concentrations of SO₂ and manganese in the aerosol. The conditions in the experiments were chosen to be similar to those a few metres from the chimney mouth in a typical industrial stack plume. Both coagulation and growth contributed significantly, and sulphuric acid production was substantially complete in 10–20 min. Light scattering size analysis was successfully applied to these rather polydisperse aerosols (σ₀∼ 0.5) of small modal diameter (a_M⩽ 0.3 µm).