Collaborative Research: The impact of bromine chemistry on the isotopic composition of nitrate at Summit, Greenland

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Nitrate is one of the major ions found in alpine and polar snow, yet it continues to be one of the least understood chemical components in ice core records. Recent work ahs shown nitrate isotope ratios to be a powerful tool for the study of nitrate in snow and ice cores. The isotopic composition of nitrate has been shown to contain information about the source of the nitrate (nitrogen oxides) and the oxidation processes that convert nitrogen oxides to nitrate in the atmosphere prior to deposition. Because hydroxyl and peroxy radicals have very different isotopic compositions than ozone, one can now distinguish the impact of the different oxidation processes that produce nitrate in the atmosphere. Seasonal observations of the oxygen isotopic composition of nitrate in snow at Summit, Greenland, cannot be understood in terms of standard, local photochemistry. The most likely causes of the model and observation discrepancy are that the box model lacks transport of nitrate from regions outside of Summit and the influence of halogen chemistry (i.e., BrO) on nitrate. Recent measurements suggest a higher than expected presence of BrO in the boundary layer above Summit. The aim of this project is to quantify the influence of bromine chemistry on nitrate production in the spring and summer. The approach includes field and laboratory measurements as well as modeling. During spring and summer field seasons, BrO and multiple gas-phase measurements will be conducted on-site, while isotopic analyses of snow and atmospheric samples will be completed in the laboratory. The oxygen isotopic composition of nitrate in snow and glacial ice holds potential for quantitatively reconstructing paleoatmospheric oxidant concentrations, but it is important to constrain how much this tracer reflects local versus regional or hemispheric scale chemistry. Furthermore, since BrO at Summit most likely originates from a natural source, and this source is affected by changes in climate, the influence of halogen chemistry on nitrogen oxides has important implications for the interpretation of recent and deep ice core records of nitrate.