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| Titel |
Investigation of post-depositional processing of nitrate in East Antarctic snow: isotopic constraints on photolytic loss, re-oxidation, and source inputs |
| VerfasserIn |
G. Shi, A. M. Buffen, M. G. Hastings, C. Li, H. Ma, Y. Li, B. Sun, C. An, S. Jiang |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1680-7316
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| Digitales Dokument |
URL |
| Erschienen |
In: Atmospheric Chemistry and Physics ; 15, no. 16 ; Nr. 15, no. 16 (2015-08-24), S.9435-9453 |
| Datensatznummer |
250119989
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| Publikation (Nr.) |
 copernicus.org/acp-15-9435-2015.pdf |
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| Zusammenfassung |
| Snowpits along a traverse from coastal East Antarctica to the summit of the
ice sheet (Dome Argus) are used to investigate the post-depositional
processing of nitrate (NO3−) in snow. Seven snowpits from sites with accumulation
rates between 24 and 172 kg m−2 a−1 were sampled to depths of
150 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 with
depth to < 50 ng g−1. Considerably increasing values of
δ15N of nitrate are also observed (16–461 ‰
vs. air N2), particularly in the top 20 cm, which is consistent with
predicted fractionation constants for the photolysis of nitrate. The
δ18O of nitrate (17–84 ‰ vs. VSMOW (Vienna Standard Mean Ocean
Water)), on the
other hand, decreases with increasing δ15N, suggestive of
secondary formation of nitrate in situ (following photolysis) with a low
δ18O source. Previous studies have suggested that δ15N and δ18O of nitrate at deeper snow depths should be
predictable based upon an exponential change derived near the surface. At
deeper depths sampled in this study, however, the relationship between
nitrate mass fraction and δ18O changes, with increasing
δ18O of nitrate observed between 100 and 200 cm. Predicting the
impact of post-depositional loss, and therefore changes in the isotopes with
depth, is highly sensitive to the depth interval over which an exponential
change is assumed. In the snowpits collected closer to the coast
(accumulation > 91 kg m−2 a−1), there are no obvious
trends detected with depth and instead seasonality in nitrate mass fraction
and 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 of
nitrate (53–111 ‰ vs. VSMOW). The relationships found
amongst mass fraction, δ15N, δ18O and Δ17O (Δ17O = δ17O–0.52 × δ18O) of nitrate cannot be explained by local post-depositional
processes alone, and are instead interpreted in the context of a primary
atmospheric signal. Consistent with other Antarctic observational and
modeling studies, the isotopic results are suggestive of an important
influence of stratospheric ozone chemistry on nitrate formation during the
cold season and a mix of tropospheric sources and chemistry during the warm
season. Overall, the findings in this study speak to the sensitivity of
nitrate isotopic composition to post-depositional processing and highlight
the strength of combined use of the nitrogen and oxygen isotopes for a
mechanistic understanding of this processing. |
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