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Snowpits along a traverse from coastal East Antarctica to the summit of theice sheet (Dome Argus) are used to investigate the post-depositionalprocessing of nitrate (NO3−) in snow. Seven snowpits from sites with accumulationrates between 24 and 172 kg m−2 a−1 were sampled to depths of150 to 300 cm. At sites from the continental interior (low accumulation,< 55 kg m−2 a−1), nitrate mass fraction is generally> 200 ng g−1 in surface snow and decreases quickly withdepth to < 50 ng g−1. Considerably increasing values ofδ15N of nitrate are also observed (16–461 ‰vs. air N2), particularly in the top 20 cm, which is consistent withpredicted fractionation constants for the photolysis of nitrate. Theδ18O of nitrate (17–84 ‰ vs. VSMOW (Vienna Standard Mean OceanWater)), on theother hand, decreases with increasing δ15N, suggestive ofsecondary formation of nitrate in situ (following photolysis) with a lowδ18O source. Previous studies have suggested that δ15N and δ18O of nitrate at deeper snow depths should bepredictable based upon an exponential change derived near the surface. Atdeeper depths sampled in this study, however, the relationship betweennitrate mass fraction and δ18O changes, with increasingδ18O of nitrate observed between 100 and 200 cm. Predicting theimpact of post-depositional loss, and therefore changes in the isotopes withdepth, is highly sensitive to the depth interval over which an exponentialchange is assumed. In the snowpits collected closer to the coast(accumulation > 91 kg m−2 a−1), there are no obvioustrends detected with depth and instead seasonality in nitrate mass fractionand isotopic composition is found. In comparison to the interior sites,the coastal pits are lower in δ15N (−15–71 ‰ vs. air N2) and higher in δ18O ofnitrate (53–111 ‰ vs. VSMOW). The relationships foundamongst mass fraction, δ15N, δ18O and Δ17O (Δ17O = δ17O–0.52 × δ18O) of nitrate cannot be explained by local post-depositionalprocesses alone, and are instead interpreted in the context of a primaryatmospheric signal. Consistent with other Antarctic observational andmodeling studies, the isotopic results are suggestive of an importantinfluence of stratospheric ozone chemistry on nitrate formation during thecold season and a mix of tropospheric sources and chemistry during the warmseason. Overall, the findings in this study speak to the sensitivity ofnitrate isotopic composition to post-depositional processing and highlightthe strength of combined use of the nitrogen and oxygen isotopes for amechanistic understanding of this processing

atmospheric chemistry and physics

Investigation of post-depositional processing of nitrate in East Antarctic snow: isotopic constraints on photolytic loss, re-oxidation, and source inputs