Gas transport in firn: multiple-tracer characterisation and model intercomparison for NEEM, Northern Greenland

Buizert, C.; Martinerie, P.; Petrenko, V. V.; Severinghaus, J. P.; Trudinger, C. M.; Witrant, E.; Rosen, J. L.; Orsi, A. J.; Rubino, M.; Etheridge, D. M.; Steele, L. P.; Hogan, C.; Laube, J. C.; Sturges, W. T.; Levchenko, V. A.; Smith, A. M.; Levin, I.; Conway, T. J.; Dlugokencky, E. J.; Lang, P. M.; ... (2012). Gas transport in firn: multiple-tracer characterisation and model intercomparison for NEEM, Northern Greenland. Atmospheric chemistry and physics, 12(9), pp. 4259-4277. European Geosciences Union 10.5194/acp-12-4259-2012

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Air was sampled from the porous firn layer at the NEEM site in Northern Greenland. We use an ensemble of ten reference tracers of known atmospheric history to characterise the transport properties of the site. By analysing uncertainties in both data and the reference gas atmospheric histories, we can objectively assign weights to each of the gases used for the depth-diffusivity reconstruction. We define an objective root mean square criterion that is minimised in the model tuning procedure. Each tracer constrains the firn profile differently through its unique atmospheric history and free air diffusivity, making our multiple-tracer characterisation method a clear improvement over the commonly used single-tracer tuning. Six firn air transport models are tuned to the NEEM site; all models successfully reproduce the data within a 1σ Gaussian distribution. A comparison between two replicate boreholes drilled 64 m apart shows differences in measured mixing ratio profiles that exceed the experimental error. We find evidence that diffusivity does not vanish completely in the lock-in zone, as is commonly assumed. The ice age- gas age difference (1 age) at the firn-ice transition is calculated to be 182+3−9 yr. We further present the first intercomparison study of firn air models, where we introduce diagnostic scenarios designed to probe specific aspects of the model physics. Our results show that there are major differences in the way the models handle advective transport. Furthermore, diffusive fractionation of isotopes in the firn is poorly constrained by the models, which has consequences for attempts to reconstruct the isotopic composition of trace gases back in time using firn air and ice core records.

Item Type:

Journal Article (Original Article)

Division/Institute:

08 Faculty of Science > Other Institutions > Teaching Staff, Faculty of Science
08 Faculty of Science > Physics Institute > Climate and Environmental Physics

UniBE Contributor:

Schwander, Jakob and Blunier, Thomas

Subjects:

500 Science > 530 Physics

ISSN:

1680-7316

Publisher:

European Geosciences Union

Language:

English

Submitter:

Doris Rätz

Date Deposited:

04 Sep 2015 09:30

Last Modified:

04 Sep 2015 09:30

Publisher DOI:

10.5194/acp-12-4259-2012

BORIS DOI:

10.7892/boris.71431

URI:

https://boris.unibe.ch/id/eprint/71431

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