Sulfur isotope fractionation during oxidation of sulfur dioxide: gas-phase oxidation by OH radicals and aqueous oxidation by H<sub>2</sub>O<sub>2</sub>, O<sub>3</sub> and iron catalysis

Abstract

Abstract. The oxidation of SO 2 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 chemical processes 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 laboratory measurements of the fractionation factors for the major atmospheric oxidation reactions for SO 2 : Gas-phase oxidation by OH radicals, and aqueous oxidation by H 2 O 2 , O 3 and a radical chain reaction initiated by iron. The measured fractionation factor for 34 S/ 32 S during the gas-phase reaction is α OH = (1.0089±0.0007) − ((4±5)×10 −5 )T ( • C). The measured fractionation factor for 34 S/ 32 S during aqueous oxidation by H 2 O 2 or O 3 is α aq = (1.0167±0.0019) − ((8.7±3.5)×10 −5 )T ( • C). The observed fractionation during oxidation by H 2 O 2 and O 3 appeared to be controlled primarily by protonation and acid-base equilibria of S(IV) in solution, which is the reason that there is 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 α Fe = (0.9894±0.0043) at 19 • C for 34 S/ 32 S. Fractionation was mass-dependent with regards to 33 S/ 32 S for all the reactions investigated. The radical chain reaction mechanism was the only measured reaction that had a faster rate for the light isotopes. The results presented in this study will be particularly useful to determine the importance of the transition metal-catalysed oxidation pathway compared to other oxidation pathways, but other main oxidation pathways can not be distinguished based on stable sulfur isotope measurements alone.

SO<sub>2</sub> photolysis as a source for sulfur mass-independent isotope signatures in stratospehric aerosols

“…Experimental studies of OH oxidation (R1) showed an inverse isotope effect, but with a smaller magnitude, with 34 SO 2 reacting about 1 % faster than 32 SO 2 (Harris et al, 2012). Although the experimentally measured isotope effect might be sufficient to explain the roughly 2 % enrichment in H 34 2 SO 4 relative to H 32 2 SO 4 following the major Mt.…”

“…For several years following large injections of SO 2 into the stratosphere, stratosphere-derived sulfate can dominate sulfate deposition in ice cores and, when corrected for background levels, can preserve the sulfur isotopic composition of stratospheric sulfate aerosols. Experimental studies demonstrate that OH oxidation of SO 2 (R1) does not produce mass-independent sulfur isotope anomalies (Harris et al, 2012(Harris et al, , 2013, so an additional oxidation mechanism is required to produce the mass-independent sulfur isotope signatures. Three reactions have been proposed to explain these isotope anomalies: excited-state photochemistry of SO 2 in the 250 to 350 nm absorption region (Savarino et al, 2003;Hattori et al, 2013), SO 2 photolysis in the 190 to 220 nm absorption region , and SO 3 photolysis (Pavlov et al, 2005).…”

“…OH and O oxidation of SO 2 (Reactions R1 and R7) are massdependent (Harris et al, 2012;Whitehill and Ono, 2012;Ono et al, 2013). However, oxidation of SO via Reaction (R6) will trap the isotopic composition of SO as SO 3 and carry the mass-independent isotope signature from SO 2 photolysis (R4).…”

“…Although attempts were made to flush the system with nitrogen (< 3 ppb H 2 O) prior to each experiment, water could be absorbed onto the surfaces of the system. The presence of water will cause HO x chemistry to occur and open up an additional (massdependent, see Harris et al, 2012) channel for sulfate formation. The amount of water in the system also affects the amount of SO 3 that ends up as sulfate aerosols.…”

Effect of the pollution control measures on PM2.5 during the 2015 China Victory Day Parade: Implication from water-soluble ions and sulfur isotope

“…Hence, paired sulfur and oxygen isotopes provide a powerful tool for constraining the source of sulfate (Schiff et al, 2005;Proemse et al, 2012). In addition, the stable isotopes have also been used to investigate the oxidation processes of SO 2 and transport pathways of sulfur in the atmosphere (Norman et al, 2006;Harris et al, 2012).…”