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| Titel |
Multi parameter tuning of a firn air transport model for the NEEM ice core site in Northern Greenland |
| VerfasserIn |
Christo Buizert, Vasilii Petrenko, Patricia Martinerie, Jeffrey Severinghaus, Mauro Rubino, David Etheridge, Chris Hogan, William Sturges, Ingeborg Levin, Thomas Blunier |
| Konferenz |
EGU General Assembly 2010
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 12 (2010) |
| Datensatznummer |
250038807
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| Zusammenfassung |
| The compacted snow (firn) found in the accumulation zone of major ice sheets acts as a
unique archive of old air. Contrary to ice cores, large sample volumes can be pumped from
the firn, making this archive especially suited for studying changes in the isotopic
composition of atmospheric trace gases. At the NEEM deep drilling site in northern
Greenland firn air has been sampled from 4 different bore holes during two field campaigns.
Through a collaboration of several laboratories NEEM firn air has been analyzed for an
unprecedented number of analytes, including isotopes of CO2, CH4, N2O, CO and
H2.
The atmospheric signal as recorded in the firn is affected by a number of processes such
as diffusion, advection and gravitational enrichment. Modeling of gas transport is therefore
essential for the interpretation of firn gas records. For the NEEM site there is a joint effort to
derive the firn transport properties by comparing the output of four different firn
models.
How the molecular diffusivity changes with depth is uncertain, and it is common practice
to tune the model by forcing it with a gas of relatively well-known atmospheric history
(usually CO2), and subsequently optimizing the fit to experimental data. By tuning to a single
atmospheric history, the problem is under-determined. Many gas age distributions can be
found that optimize the fit. To constrain the problem better the NEEM diffusivity profile is
tuned to an ensemble of analytes, including CO2, CH4, SF6, Î14CO2, and several
CFCs.
It is however not a priori clear how to combine constraints from different gases in the
tuning procedure. We introduce a method that can quantify how well a certain gas constrains
the diffusivity profile at each depth, based on 1) the particular shape of its atmospheric history
and uncertainties therein, 2) measurement uncertainties and 3) the possibility of in situ
alteration.
By taking these three factors into consideration, we can determine for each depth how to
weigh the individual contributions of the different gases in the ensemble. We let the gas that
places the most stringent constraint carry the most weight, thus exploiting the relative
strength of each gas to the fullest. |
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