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| Titel |
Air–snow transfer of nitrate on the East Antarctic Plateau – Part 1: Isotopic evidence for a photolytically driven dynamic equilibrium in summer |
| VerfasserIn |
J. Erbland, W. C. Vicars, J. Savarino, S. Morin, M. M. Frey, D. Frosini, E. Vince, J. M. F. Martins |
| 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 ; 13, no. 13 ; Nr. 13, no. 13 (2013-07-08), S.6403-6419 |
| Datensatznummer |
250018741
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| Publikation (Nr.) |
 copernicus.org/acp-13-6403-2013.pdf |
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| Zusammenfassung |
| Here we report the measurement of the comprehensive isotopic composition
(δ15N, Δ17O and δ18O) of nitrate
at the air–snow interface at Dome C, Antarctica (DC,
75°06' S, 123°19' E), and in snow
pits along a transect across the East Antarctic Ice Sheet (EAIS) between
66° S and 78° S. In most of the snow pits, nitrate loss
(either by physical release or UV photolysis of nitrate) is observed and
fractionation constants associated are calculated. Nitrate collected from
snow pits on the plateau (snow accumulation rate below
50 kg m−2 a−1) displays average fractionation constants of
(−59±10) ‰, (+2.0±1.0) ‰ and
(+8.7±2.4)‰ for δ15N, Δ17O and
δ18O, respectively. In contrast, snow pits sampled on the
coast show distinct isotopic signatures with average fractionation constants
of (−16±14) ‰, (−0.2±1.5) ‰ and
(+3.1±5.8) ‰, for δ15N, Δ17O and
δ18O, respectively. Our observations corroborate that
photolysis (associated with a 15N / 14N fractionation
constant of the order of –48 ‰ according to Frey et al. (2009) is the
dominant nitrate loss process on the East Antarctic Plateau, while on the
coast the loss is less pronounced and could involve both physical release and
photochemical processes. Year-round isotopic measurements at DC show a~close
relationship between the Δ17O of atmospheric nitrate and
Δ17O of nitrate in skin layer snow, suggesting
a photolytically driven isotopic equilibrium imposed by nitrate recycling at
this interface. Atmospheric nitrate deposition may lead to fractionation of
the nitrogen isotopes and explain the almost constant shift of the order of
25 ‰ between the δ15N values in the atmospheric and
skin layer nitrate at DC. Asymptotic δ15N(NO3−) values
calculated for each snow pit are found to be correlated with the inverse of
the snow accumulation rate (ln(δ15N as. + 1) =
(5.76±0.47) ċ (kg m−2 a−1/ A) + (0.01±0.02)),
confirming the strong relationship between the snow accumulation rate and the degree of isotopic
fractionation, consistent with previous observations by
Freyer et al. (1996). Asymptotic Δ17O(NO3−) values on the
plateau are smaller than the values found in the skin layer most likely due
to oxygen isotope exchange between the nitrate photoproducts and water
molecules from the surrounding ice. However, the apparent fractionation in
Δ17O is small, thus allowing the preservation of a portion of
the atmospheric signal. |
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