Seasonal bromate formation in the Arctic snowpack: Implications for the bromine biogeochemical cycle

Bromine exists in multiple chemical forms in the atmosphere, with bromide (Br^-) being the predominant species in snow. Here, we detect and track the formation of bromate (BrO₃^-) in Arctic snow and propose a mechanism for its production. Our observations reveal relevant BrO₃^- concentrations, reaching up to 5% of total bromine, during springtime. The evidence for the persistence of BrO₃^- in the snowpack and its production as a function of solar radiation suggests a snow-driven photochemical process, with negligible contributions from direct aerosol deposition, and emphasizes the role of the snowpack as a reactive matrix for photooxidation, providing insights into the bromine cycle in the Arctic. Our findings are supported by quantum chemical calculations, which explore both radical and ionic reaction mechanisms and the matrix effect, thereby supporting the role of snowpack in promoting BrO₃^- formation. The presence of BrO₃^- in snowpack represents a reservoir of nonreactive bromine, with potential implications for understanding halogen chemistry in polar environments.