An isotopic analysis of ionising radiation as a source of sulphuric acid

M B Enghoff,J O P Pedersen,Y Ueno,M S Johnson,C Meusinger,N Bork,H Svensmark,N Yoshida,S Hattori,M Nakagawa,S Danielache

doi:10.5194/acp-12-5319-2012

Abstract

Abstract. Sulphuric acid is an important factor in aerosol nucleation and growth. It has been shown that ions enhance the formation of sulphuric acid aerosols, but the exact mechanism has remained undetermined. Furthermore some studies have found a deficiency in the sulphuric acid budget, suggesting a missing source. In this study the production of sulphuric acid from SO2 through a number of different pathways is investigated. The production methods are standard gas phase oxidation by OH radicals produced by ozone photolysis with UV light, liquid phase oxidation by ozone, and gas phase oxidation initiated by gamma rays. The distributions of stable sulphur isotopes in the products and substrate were measured using isotope ratio mass spectrometry. All methods produced sulphate enriched in 34S and we find an enrichment factor (δ34S) of 8.7 ± 0.4‰ (1 standard deviation) for the UV-initiated OH reaction. Only UV light (Hg emission at 253.65 nm) produced a clear non-mass-dependent excess of 33S. The pattern of isotopic enrichment produced by gamma rays is similar, but not equal, to that produced by aqueous oxidation of SO2 by ozone. This, combined with the relative yields of the experiments, suggests a mechanism in which ionising radiation may lead to hydrated ion clusters that serve as nanoreactors for S(IV) to S(VI) conversion.

Highlights

Aerosols – small particles suspended in air – are frequently formed by condensation of gas molecules in Earth’s atmosphere
The production methods are standard gas phase oxidation by OH radicals produced by ozone photolysis with UV light, liquid phase oxidation by ozone, and gas phase oxidation initiated by gamma rays
Sulphate is generated in the setup in all different operation modes and yields vary widely – for instance more sulphuric acid is formed with UV light present than for the gamma source but the sources combined yield less than each individual source

Summary

Introduction

Aerosols – small particles suspended in air – are frequently formed by condensation of gas molecules in Earth’s atmosphere. The effect of these particles on the radiative balance of our climate system, through direct and indirect mechanisms, is the greatest uncertainty in the radiative forcing budget (Forster et al, 2007). One issue is that the mechanism behind the formation of aerosols remains unknown, even though several mechanisms have been proposed (Kulmala, 2003; Curtius, 2006; Kurten et al, 2008) and major advances have recently been made (Kirkby et al, 2011). Ionic cluster formation is estimated to account for 1–30 % of total particle formation, based on 7 European

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Conclusion