ACP Atmospheric Chemistry and Physics ACP 1680-7324 Copernicus GmbH Göttingen, Germany 10.5194/acp-12-4259-2012 Gas transport in firn: multiple-tracer characterisation and model intercomparison for NEEM, Northern Greenland Buizert C. 1 Martinerie P. 2 Petrenko V. V. 3 15 Severinghaus J. P. 4 Trudinger C. M. 5 Witrant E. 6 Rosen J. L. 7 Orsi A. J. 4 Rubino M. 1 5 Etheridge D. M. 1 5 Steele L. P. 5 Hogan C. 8 Laube J. C. 8 Sturges W. T. 8 Levchenko V. A. 9 Smith A. M. 9 Levin I. 10 Conway T. J. 11 Dlugokencky E. J. 11 Lang P. M. 11 Kawamura K. 12 Jenk T. M. 1 White J. W. C. 3 Sowers T. 13 Schwander J. 14 Blunier T. 1 Centre for Ice and Climate, Niels Bohr Institute, University of Copenhagen, Juliane Maries vej 30, 2100 Copenhagen Ø, Denmark Laboratoire de Glaciologie et Géophysique de l'Environnement, CNRS, Université Joseph Fourier-Grenoble, BP 96, 38 402 Saint Martin d'Hères, France Institute of Arctic and Alpine Research, University of Colorado, Boulder, CO 80309, USA Scripps Institution of Oceanography, Univ. of California, San Diego, La Jolla, CA 92093, USA Centre for Australian Weather and Climate Research/ CSIRO Marine and Atmospheric Research, Aspendale, Victoria, Australia Grenoble Image Parole Signal Automatique, Université Joseph Fourier/CNRS, Grenoble, France Department of Geosciences, Oregon State University, Corvallis, OR 97331-5506, USA School of Environmental Sciences, University of East Anglia, Norwich, NR4 7TJ, UK Australian Nuclear Science and Technology Organisation, Locked Bag 2001, Kirrawee DC NSW 2232, Australia Institut für Umweltphysik, University of Heidelberg, INF 229, 69120 Heidelberg, Germany NOAA Earth System Research Laboratory, Boulder, Colorado, USA National Institute of Polar Research, 10-3 Midorichou, Tachikawa, Tokyo 190-8518, Japan The Earth and Environmental Systems Institute, Penn State University, 317B EESB Building, University Park, PA 16802, USA Climate and Environmental Physics, Physics Institute, University of Bern, Sidlerstrasse 5, 3012 Bern, Switzerland current address: Earth and Environmental Sciences, University of Rochester, Rochester, NY 14627, USA 14 05 2012 12 9 4259 4277 This is an open-access article ditributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. This article is available from http://www.atmos-chem-phys.net/12/4259/2012/acp-12-4259-2012.html The full text article is available as a PDF file from http://www.atmos-chem-phys.net/12/4259/2012/acp-12-4259-2012.pdf

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 (Δage) at the firn-ice transition is calculated to be 182<sup>+3</sup><sub>&minus;9</sub> 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.

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