Bromide oxidation by bromate in a frozen solution and reactive bromine species production

A sudden increase in tropospheric reactive bromine species (such as BrO radical) concentration has been observed in the Arctic springtime (bromine explosion). The reactive bromine radicals originate from the Br₂ oxidation; however, the role of ice chemistry in bromine activation, especially freezing-induced Br^- oxidation for Br₂ production, is not significantly considered yet. Notably, the freezing phenomenon is prevalent in polar region. In this study, we demonstrate that the Br^- containing water freezing can provide a potential Br^- oxidation pathway. The oxidation of Br^- by BrO₃^- was negligible under aqueous conditions, while it was highly accelerated (time scale of several years to minute) by freezing the solution. We proposed that the accelerated chemical reaction was due to the freezing concentration effect. The chemical transformation mechanism was suggested. The chemical transformation of Br^- was considered using the UV-visible absorbance spectrometer and the high-resolution mass spectroscopy (HRMS) measurement. The total Br content was measured using inductive coupled plasma mass spectroscopy (ICP-MS) and the dissolved Br species was monitored using ion chromatography. The spatial distribution of in-situ generated hypobromous acid in ice was analyzed using the Raman microscopy. The Br^- oxidation is increased as the decrease of initial pH and increase of reactant dose, but the influence of freezing temperature was not significant. This study provides experimental evidence for the freezing-induced bromide activation in the ice.