Factors influencing nitrogen isotopes of snow nitrate: implications for interpretations of ice core nitrate records

Abstract:

The records of nitrate concentration and its isotopic composition (δ¹⁵N, δ¹⁸O and Δ¹⁷O) in ice cores are sought to reconstruct past levels of atmospheric NO_x, the natural variability in NO_x sources, and variations in tropospheric oxidants. However, these practices have been hampered by post-depositional processing of snow nitrate, which alters snow nitrate concentrations as well as its isotopic composition. Snow accumulation rates influence the degree of post-depositional processing. At sites with high snow accumulation rates, such as Summit, Greenland, the degree of post-depositional processing is thought to be minimal. Thus, variations in δ¹⁵N(NO₃^-) in Summit ice cores have been linked to NO_x source changes, assuming the conservation of nitrogen isotope signatures during the conversion of NO_x to nitrate. However, the marked decrease in δ¹⁵N(NO₃^-) from ~1850 to 1970 observed in Summit ice cores is difficult to explain by the addition of anthropogenic NO_x to the natural background, as higher atmospheric δ¹⁵N(NO₃^-) values are frequently observed in polluted regions compared to pristine regions. Alternatively, we hypothesized that this decrease can be explained by changes in atmospheric acidity. Atmospheric acidity influences the partitioning of nitrate in gas- and aerosol-phases, inducing isotopic effects. Increased atmospheric acidity beginning ~ 1850 arising from anthropogenic SO₂ emissions leads to elevated gas-phase HNO₃ which is depleted in δ¹⁵N relative to aerosol nitrate. The preferential transport of HNO₃ to the Arctic then leads to a decrease in ice core δ¹⁵N(NO₃^-). This hypothesis is supported by the significant correlation between δ¹⁵N(NO₃^-) and acidity records, and is supported by a model simulation. The result of this study indicates the importance of atmospheric processes to ice core δ¹⁵N(NO₃^-), and suggests that the link between ice core δ¹⁵N(NO₃^-) and NO_x sources could be problematic even at high snow accumulation sites. In addition, our δ¹⁵N(NO₃^-) measurements in Summit ice cores expanding to the glacial-interglacial timescale indicate clear evidence of the effects of post-depositional processing, making it even more difficult to link ice core δ¹⁵N(NO₃^-) variations to NO_x source changes.