An isotopic analysis of ionising radiation as a source of sulphuric acid
Sulphuric acid is an important factor in aerosol nucleation and growth.\nIt has been shown that ions enhance the formation of sulphuric acid\naerosols, but the exact mechanism has remained undetermined. Furthermore\nsome studies have found a deficiency in the sulphuric acid budget,\nsuggesting a missing source. In this study the production of sulphuric\nacid from SO2 through a number of different pathways is investigated.\nThe production methods are standard gas phase oxidation by OH radicals\nproduced by ozone photolysis with UV light, liquid phase oxidation by\nozone, and gas phase oxidation initiated by gamma rays. The\ndistributions of stable sulphur isotopes in the products and substrate\nwere measured using isotope ratio mass spectrometry. All methods\nproduced sulphate enriched in S-34 and we find an enrichment factor\n(delta S-34) of 8.7 +/- 0.4 parts per thousand (1 standard deviation)\nfor the UV-initiated OH reaction. Only UV light (Hg emission at 253.65\nnm) produced a clear non-mass-dependent excess of S-33. The pattern of\nisotopic enrichment produced by gamma rays is similar, but not equal, to\nthat produced by aqueous oxidation of SO2 by ozone. This, combined with\nthe relative yields of the experiments, suggests a mechanism in which\nionising radiation may lead to hydrated ion clusters that serve as\nnanoreactors for S(IV) to S(VI) conversion.