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| Titel |
Gas diffusivity and permeability through the firn column at Summit, Greenland: measurements and comparison to microstructural properties |
| VerfasserIn |
A. C. Adolph, M. R. Albert |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1994-0416
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| Digitales Dokument |
URL |
| Erschienen |
In: The Cryosphere ; 8, no. 1 ; Nr. 8, no. 1 (2014-02-28), S.319-328 |
| Datensatznummer |
250116025
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| Publikation (Nr.) |
 copernicus.org/tc-8-319-2014.pdf |
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| Zusammenfassung |
| The physical structure of polar firn plays a key role in the mechanisms by
which glaciers and ice sheets preserve a natural archive of past atmospheric
composition. This study presents the first measurements of gas diffusivity
and permeability along with microstructural information measured from the
near-surface firn through the firn column to pore close-off. Both fine- and
coarse-grained firn from Summit, Greenland are included in this study to
investigate the variability in firn caused by seasonal and storm-event
layering. Our measurements reveal that the porosity of firn (derived from
density) is insufficient to describe the full profiles of diffusivity and
permeability, particularly at porosity values above 0.5. Thus, even a model
that could perfectly predict the density profile would be insufficient for
application to issues involving gas transport. The measured diffusivity
profile presented here is compared to two diffusivity profiles modeled from
firn air measurements from Summit. Because of differences in scale and in
firn processes between the true field situation, firn modeling, and
laboratory measurements, the results follow a similar overall pattern but do
not align; our results constitute a lower bound on diffusive transport. In
comparing our measurements of both diffusivity and permeability to previous
parameterizations from numerical 3-D lattice-Boltzmann modeling, it is
evident that the previous relationships to porosity are likely site-specific.
We present parameterizations relating diffusivity and permeability to
porosity as a possible tool, though use of direct measurements would be far
more accurate when feasible. The relationships between gas transport
properties and microstructural properties are characterized and compared to
existing relationships for general porous media, specifically the
Katz–Thompson (KT), Kozeny–Carman (KC), and Archie's law approximations.
While those approximations can capture the general trend of gas transport
relationships, they result in high errors for individual samples and fail to
fully describe firn variability, particularly the differences between coarse-
and fine-grained firn. We present a direct power law relationship between
permeability and gas diffusivity based on our co-located measurements;
further research will indicate if this type of relationship is site-specific.
This set of measurements and relationships contributes a unique starting
point for future investigations in developing more physically based models of
firn gas transport. |
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