Sulfur isotope fractionation during oxidation of sulfur dioxide: gas-phase oxidation by OH radicals and aqueous oxidation by H2O2, O3 and iron catalysis

摘要

The oxidation of SO to sulfate is a key reaction in determining the roleof sulfate in the environment through its effect on aerosol size distributionand composition. Sulfur isotope analysis has been used to investigate sourcesand chemical processes of sulfur dioxide and sulfate in the atmosphere,however interpretation of measured sulfur isotope ratios is challenging dueto a lack of reliable information on the isotopic fractionation involved inmajor transformation pathways. This paper presents laboratory measurements ofthe fractionation factors for the major atmospheric oxidation reactions forSO: Gas-phase oxidation by OH radicals, and aqueous oxidation byHO, O and a radical chain reaction initiated by iron. Themeasured fractionation factor for S/S during the gas-phasereaction is α = (1.0089±0.0007)−((4±5)×10) (°C). Themeasured fractionation factor for S/S during aqueous oxidationby HO or O is α = (1.0167±0.0019)−((8.7±3.5) ×10)(°C). The observedfractionation during oxidation by HO and O appeared to becontrolled primarily by protonation and acid-base equilibria of S(IV) insolution, which is the reason that there is no significant differencebetween the fractionation produced by the two oxidants within theexperimental error. The isotopic fractionation factor from a radical chainreaction in solution catalysed by iron is α =(0.9894±0.0043) at 19 °C for S/S. Fractionationwas mass-dependent with regards to S/S for all the reactionsinvestigated. The radical chain reaction mechanism was the only measuredreaction that had a faster rate for the light isotopes. The results presentedin this study will be particularly useful to determine the importance of thetransition metal-catalysed oxidation pathway compared to other oxidationpathways, but other main oxidation pathways can not be distinguished based onstable sulfur isotope measurements alone.