Buizert, C and Martinerie, P and Petrenko, VV and Severinghaus, JP and Trudinger, CM and Witrant, E and Rosen, JL and Orsi, AJ and Rubino, M and Etheridge, DM and Steele, LP and Hogan, C and Laube, JC and Sturges, WT and Levchenko, VA and Smith, AM and Levin, I and Conway, TJ and Dlugokencky, EJ and Lang, PM and Kawamura, K and Jenk, TM and White, JWC and Sowers, T and Schwander, J and Blunier, T (2012) Gas transport in firn: multiple-tracer characterisation and model intercomparison for NEEM, Northern Greenland. Atmospheric Chemistry and Physics, 12 (9). 4259 -4277. ISSN 1680-7316

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Abstract

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 (1age) 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: Article
Additional Information: This is the final published version of the article (version of record). It first appeared online via Copernicus Publications at http://dx.doi.org/10.5194/acp-12-4259-2012 - please refer to any applicable terms of use of the publisher.
Subjects: G Geography. Anthropology. Recreation > GB Physical geography
Divisions: Faculty of Natural Sciences > School of Geography, Geology and the Environment
Related URLs:
Depositing User: Symplectic
Date Deposited: 03 May 2018 11:48
Last Modified: 03 May 2018 11:56
URI: http://eprints.keele.ac.uk/id/eprint/4839

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