Sulfur isotope fractionation during oxidation of sulfur dioxide: gas-phase oxidation by OH radicals and aqueous oxidation by H2O2, O3 and iron catalysis
1Abteilung Partikelchemie, Max-Planck-Institut für Chemie, Becherweg 27, 55128 Mainz, Germany
2Abteilung Luftchemie, Max-Planck-Institut für Chemie, Becherweg 27, 55128 Mainz, Germany
3Department of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139, USA
4Earth System Science Research Centre, Institute for Geosciences, University of Mainz, Becherweg 21, 55128 Mainz, Germany
Abstract. The oxidation of SO2 to sulfate is a key reaction in determining the role of sulfate in the environment through its effect on aerosol size distribution and composition. Sulfur isotope analysis has been used to investigate sources and chemistry of sulfur dioxide and sulfate in the atmosphere, however interpretation of measured sulfur isotope ratios is challenging due to a lack of reliable information on the isotopic fractionation involved in major transformation pathways. This paper presents measurements of the fractionation factors for the major atmospheric oxidation reactions for SO2: Gas-phase oxidation by OH radicals, and aqueous oxidation by H2O2, O3 and a radical chain reaction initiated by iron. The measured fractionation factor for 34S/32S during the gas-phase reaction is αOH = (1.0089±0.0007) − ((4±5)×10−5) T(°C). The measured fractionation factor for 34S/32S during aqueous oxidation by H2O2 or O3 is αaq=(1.0167±0.0019) − ((8.7±3.5) ×10−5) T(°C). The observed fractionation during oxidation by H2O2 and O3 appeared to be controlled primarily by protonation and acid-base equilbria of S(IV) in solution, and there was no significant difference between the fractionation produced by the two oxidants within the experimental error. The isotopic fractionation factor from a radical chain reaction in solution catalysed by iron is αFe = (0.989±0.0043) at 19 °C for 34S/32S. Fractionation was mass-dependent with regards to 33S for all the reactions investigated. The radical chain reaction mechanism was the only measured reaction that had a faster rate for the light isotopes, and will be particularly useful to determine the importance of the transition-metal catalysed oxidation pathway.