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 SO/sub 2/ in air are reanalyzed on the basis that the principal photochemical reaction of SO/sub 2/ is SO/sub 2/ + OH + M ..-->.. HSO/sub 3/ + M. The ideas that gaseous H/sub 2/SO/sub 4/ is the end product of the reaction and that nuclei were formed from clusters of H/sub 2/SO/sub 4/ and H/sub 2/O molecules are shown to be probably incorrect on the basis of (1) comparison to nucleation rates expected from the theory of heteromolecular homogeneous nucleation and (2) 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 SO/sub 3/ . H/sub 2/O are nuclei precursors. We suggest a mechanism involving the formation and recombination of hydrated forms of HSO/sub 3/ and HSO/sub 5/ radicals to explain nucleation for conditions of relative humidity greater than about 5%. For lower relative humidities the reaction SO/sub 2/ + O + M ..-->.. SO/sub 3/ + M followed by the formation of a hydrated complex of SO/sub 3/ ismore » suggested as controlling nucleation. A model based on these mechanisms yields the results that nuclei consist of single molecules of H/sub 2/S/sub 2/O/sub 6/ or possibly H/sub 2/S/sub 2/O/sub 8/, plus their associated H/sub 2/O molecules, at high relative humidities (>5%) and that at low relative humidities the nuclei consist of single molecules of H/sub 2/SO/sub 4/, plus associated H/sub 2/O molecules. These mechanisms are used as a basis for suggesting a general explanation for the phenomenon of photoinduced nucleation. 50 references, 4 figures, 3 tables.« less