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| Titel |
How well do different tracers constrain the firn diffusivity profile? |
| VerfasserIn |
C. M. Trudinger, I. G. Enting, P. J. Rayner, D. M. Etheridge, C. Buizert, M. Rubino, P. B. Krummel, T. Blunier |
| 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. 3 ; Nr. 13, no. 3 (2013-02-06), S.1485-1510 |
| Datensatznummer |
250017642
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| Publikation (Nr.) |
 copernicus.org/acp-13-1485-2013.pdf |
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| Zusammenfassung |
| Firn air transport models are used to interpret measurements of the
composition of air in firn and bubbles trapped in ice in order to reconstruct
past atmospheric composition. The diffusivity profile in the firn is usually
calibrated by comparing modelled and measured concentrations for tracers with
known atmospheric history. However, in most cases this is an under-determined
inverse problem, often with multiple solutions giving an adequate fit to the
data (this is known as equifinality). Here we describe a method to estimate
the firn diffusivity profile that allows multiple solutions to be identified,
in order to quantify the uncertainty in diffusivity due to equifinality. We
then look at how well different combinations of tracers constrain the firn
diffusivity profile. Tracers with rapid atmospheric variations like
CH3CCl3, HFCs and 14CO2 are most useful for constraining
molecular diffusivity, while &delta:15N2 is useful for constraining
parameters related to convective mixing near the surface. When errors in the
observations are small and Gaussian, three carefully selected tracers are
able to constrain the molecular diffusivity profile well with minimal
equifinality. However, with realistic data errors or additional processes to
constrain, there is benefit to including as many tracers as possible to
reduce the uncertainties. We calculate CO2 age distributions and their
spectral widths with uncertainties for five firn sites (NEEM, DE08-2,
DSSW20K, South Pole 1995 and South Pole 2001) with quite different
characteristics and tracers available for calibration. We recommend moving
away from the use of a firn model with one calibrated parameter set to infer
atmospheric histories, and instead suggest using multiple parameter sets,
preferably with multiple representations of uncertain processes, to assist in
quantification of the uncertainties. |
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