Catalytic sulfate formation mechanism influenced by important constituents of cloud water via the reaction of SO2 oxidized by hypobromic acid in marine areas

Abstract

Comprehensive investigations of the possible formation pathways of sulfate, the main composition of atmospheric aerosol in marine areas, continue to challenge atmospheric chemists. As one of the most important oxidation routes of S(IV) contributing to sulfate formation, the reaction process of S(IV) oxidized by hypobromic acid, which is ubiquitous with the gas-phase mixing ratios of ∼310 ppt and has a well-known oxidative capacity, has attracted wide attention. However, little information is available about the detailed reaction mechanism. Especially, due to the abundant species in cloud water, the potential effect of these compositions on these reaction processes and the corresponding effect mechanism are also uncertain. Using high-level quantum chemical calculations, we theoretically elucidate the two-step mechanism of Br⁺ transfer proposed by experiment through the verification of the key BrSO₃⁻ intermediate formation and subsequent hydrolysis reaction or the uncovered reaction of BrSO₃⁻ intermediate with OH⁻. Further, the novel and more competitive mechanisms (OH⁺ or O atom transfer pathways) that have not been considered in previous studies, leading to sulfate formation directly, have been found. Furthermore, it should be mentioned that we revealed the effect mechanism of constituents catalyzed in cloud water, especially the important H₂O-catalyzed mechanism. In addition, all the above pathways follow this catalytic mechanism. This finding indicates a linkage between the complex nature of the atmospheric constituents and related atmospheric reaction, as well as the enhanced occurrence of atmospheric secondary sulfate formation in the atmosphere. Hence, this exploration of sulfate formation related to hypobromic acid could provide a better understanding about the sources of sulfate in marine areas.