Deactivating Effect of Hydroxyl Radicals Reactivity by Sulfate and Sulfite Functional Groups in Aqueous Phase─Atmospheric Implications for Small Organosulfur Compounds

Aqueous-phase oxidation of organosulfur compounds initiated by •OH radicals could be an important transformation pathway and can play a significant role in the atmospheric abundance and fate of these compounds. Here, we measured the temperature-dependent kinetics of •OH reactions with five C1 and C2 organosulfur compounds (two alkyl sulfates, methyl sulfate (MS) and ethyl sulfate (ES), and three sulfonates, methanesulfonate (MSA), hydroxymethanesulfonate (HMS), and 2-hydroxyethylsulfonate (HES)) in aqueous phase using laser photolysis-long path absorption (LP-LPA) and competition kinetics methods. The rate constants ranged from 106 to 108 L mol–1 s–1 and exhibited a clear positive temperature (T) dependence over a temperature range of 278 to 318 K. Furthermore, the two sulfur-containing groups, sulfate (−OSO3–) group for alkyl sulfates and sulfite (−SO3−) group for sulfonates, exhibit strong deactivating effect on the kinetics. This could be explained by the strong electron-withdrawing nature of the −OSO3– and −SO3– groups, which lowers the hydrogen abstraction rates during •OH oxidation. Overall, this study demonstrates that the oxidation of small alkyl sulfates and sulfonates by •OH radical in aqueous phase is significantly modulated by the presence of the sulfur functional groups. The deactivating effect of sulfur functional groups sheds light on predicting the lifetimes of other organosulfur compounds in the atmosphere.