Distinguishing summertime atmospheric production of nitrate across the East Antarctic Ice Sheet

Surface snow and atmospheric samples collected along a traverse from the coast to the ice sheet summit (Dome A) are used to investigate summertime atmospheric production of nitrate (NO₃ ^–) across East Antarctica. The strong relationship observed between δ¹⁵N and δ¹⁸O of nitrate in the surface snow suggests a large (lesser) extent of nitrate photolysis in the interior (coastal) region. A linear correlation between the oxygen isotopes of nitrate (δ¹⁸O and Δ¹⁷O) indicates mixing of various oxidants that react with NO_x (NO_x = NO + NO₂) to produce atmospheric nitrate. On the plateau, the isotopes of snow nitrate are best explained by local reoxidation chemistry of NO_x, possibly occurring in both condensed and gas phases. Nitrate photolysis results in redistribution of snow nitrate, and the plateau snow is a net exporter of nitrate and its precursors. Our results suggest that while snow-sourced NO_x from the plateau due to photolysis is a significant input to the nitrate budget in coastal snow (up to ∼35%), tropospheric transport from mid-low latitudes dominates (∼65%) coastal snow nitrate. The linear relationship of δ¹⁸O vs. Δ¹⁷O of the snow nitrate suggests a predominant role of hydroxyl radical (OH) and ozone (O₃) in nitrate production, although a high Δ¹⁷O(O₃) is required to explain the observations. Across Antarctica the oxygen isotope composition of OH appears to be dominated by exchange with water vapor, despite the very dry environment. One of the largest uncertainties in quantifying nitrate production pathways is the limited knowledge of atmospheric oxidant isotopic compositions.