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| Titel |
A new multi-gas constrained model of trace gas non-homogeneous transport in firn: evaluation and behaviour at eleven polar sites |
| VerfasserIn |
E. Witrant, P. Martinerie, C. Hogan, J. C. Laube, K. Kawamura, E. Capron, S. A. Montzka, E. J. Dlugokencky, D. Etheridge, T. Blunier, W. T. Sturges |
| 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 ; 12, no. 23 ; Nr. 12, no. 23 (2012-12-04), S.11465-11483 |
| Datensatznummer |
250011641
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| Publikation (Nr.) |
 copernicus.org/acp-12-11465-2012.pdf |
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| Zusammenfassung |
| Insoluble trace gases are trapped in polar ice at the firn-ice transition, at
approximately 50 to 100 m below the surface, depending primarily on the site
temperature and snow accumulation. Models of trace gas transport in polar
firn are used to relate firn air and ice core records of trace gases to their
atmospheric history. We propose a new model based on the following
contributions. First, the firn air transport model is revised in a
poromechanics framework with emphasis on the non-homogeneous properties and
the treatment of gravitational settling. We then derive a nonlinear least
square multi-gas optimisation scheme to calculate the effective firn
diffusivity (automatic diffusivity tuning). The improvements gained by the
multi-gas approach are investigated (up to ten gases for a single site are
included in the optimisation process). We apply the model to four Arctic
(Devon Island, NEEM, North GRIP, Summit) and seven Antarctic (DE08, Berkner
Island, Siple Dome, Dronning Maud Land, South Pole, Dome C, Vostok) sites and
calculate their respective depth-dependent diffusivity profiles. Among these
different sites, a relationship is inferred between the snow accumulation
rate and an increasing thickness of the lock-in zone defined from the
isotopic composition of molecular nitrogen in firn air (denoted
δ15N). It is associated with a reduced diffusivity value and an
increased ratio of advective to diffusive flux in deep firn, which is
particularly important at high accumulation rate sites. This has implications
for the understanding of δ15N of N2 records in ice cores, in
relation with past variations of the snow accumulation rate. As the snow
accumulation rate is clearly a primary control on the thickness of the
lock-in zone, our new approach that allows for the estimation of the lock-in
zone width as a function of accumulation may lead to a better constraint on
the age difference between the ice and entrapped gases. |
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