Nucleation by free radicals from the photooxidation of sulfur dioxide in air

Abstract

Previously reported experimental results of the production of condensation nuclei by the photooxidation of SO2 in air are reanalyzed on the basis that the principal photochemical reaction of SO2 is SO2 + OH + M → HSO3 + M. The ideas that gaseous H2SO4 is the end product of the reaction and that nuclei were formed from clusters of H2SO4 and H2O molecules are shown to be probably incorrect on the basis of (a) comparison to nucleation rates expected from the theory of heteromolecular homogeneous nucleation and (b) calculations indicating that nucleation rates were kinetically controlled such that the nuclei formed contained only one or two sulfur-bearing entities. The nucleation phenomena are compatible with the idea that free radicals and the hydrated complex SO3·H2O are nuclei precursors. We suggest a mechanism involving the formation and recombination of hydrated forms of HSO3 and HSO5 radicals to explain nucleation for conditions of relative humidity greater than about 5%. For lower relative humidities the reaction SO2 + O + M → SO3 + M followed by the formation of a hydrated complex of SO3 is suggested as controlling nucleation. A model based on these mechanisms yields the results that nuclei consist of single molecules of H2S2O6 or possibly H2S2O8, plus their associated H2O molecules, at high relative humidities (>5%) and that at low relative humidities the nuclei consist of single molecules of H2SO4, plus associated H2O molecules. These mechanisms are used as a basis for suggesting a general explanation for the phenomenon of photoinduced nucleation. © 1980 American Chemical Society.