Bromine Monoxide Radical Likely Dominates Dimethylsulfide Oxidation before the Springtime Onset of Arctic Ice Melt

To understand the atmospheric fate of dimethylsulfide (DMS) during the Arctic spring-to-summer transition, we measured volatile methylated sulfur (VMS) species, including DMS and DMS-derived oxidation products, dimethyl sulfoxide (DMSO), and dimethylsulfone (DMSO2), in the Fram Strait during May and June of 2023. Elevated DMSO contributions to VMS accompany depleted O3, high surface albedo, and air temperatures < 0 °C in air masses influenced by snow-covered sea-ice regions, suggesting that BrO•-initiated oxidation dominated DMS loss prior to the onset of sea-ice melt. In GEOS-Chem simulations, BrO• is the dominant oxidant for DMS, but modeled BrO• concentrations from sea-salt debromination alone are too low to replicate observed O3, DMSO, and DMSO2, suggesting a missing source of reactive bromine from sunlit saline snow. BrO•-initiated oxidation reduces the DMS lifetime and oxidizes DMS through methanesulfinic acid (MSIA) more quickly in the gas phase and in higher yields compared to •OH-initiated oxidation alone. Once melt onset begins and snowpack-driven halogen activation shuts off, •OH oxidation likely dominates Arctic DMS oxidation. Our observations highlight the seasonal variability of halogen oxidation impacting sulfur species in the Arctic marine region, which are likely to change as the sea-ice extent decreases and the melt season in the Arctic lengthens.