1Centre for Ice and Climate, Niels Bohr Institute, University of Copenhagen, Juliane Maries vej 30, 2100 Copenhagen Ø, Denmark
2Laboratoire de Glaciologie et Géophysique de l'Environnement, CNRS, Université Joseph Fourier-Grenoble, BP 96, 38 402 Saint Martin d'Hères, France
3Institute of Arctic and Alpine Research, University of Colorado, Boulder, CO 80309, USA
4Scripps Institution of Oceanography, Univ. of California, San Diego, La Jolla, CA 92093, USA
5Centre for Australian Weather and Climate Research/ CSIRO Marine and Atmospheric Research, Aspendale, Victoria, Australia
6Grenoble Image Parole Signal Automatique, Université Joseph Fourier/CNRS, Grenoble, France
7Department of Geosciences, Oregon State University, Corvallis, OR 97331-5506, USA
8School of Environmental Sciences, University of East Anglia, Norwich, NR4 7TJ, UK
9Australian Nuclear Science and Technology Organisation, Locked Bag 2001, Kirrawee DC NSW 2232, Australia
10Institut für Umweltphysik, University of Heidelberg, INF 229, 69120 Heidelberg, Germany
11NOAA Earth System Research Laboratory, Boulder, Colorado, USA
12National Institute of Polar Research, 10-3 Midorichou, Tachikawa, Tokyo 190-8518, Japan
13The Earth and Environmental Systems Institute, Penn State University, 317B EESB Building, University Park, PA 16802, USA
14Climate 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
Received: 02 May 2011 – Discussion started: 26 May 2011
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 (Δage) 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.
Revised: 10 Apr 2012 – Accepted: 23 Apr 2012 – Published: 14 May 2012
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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., Kawamura, K., Jenk, T. M., White, J. W. C., Sowers, T., Schwander, J., and Blunier, T.: Gas transport in firn: multiple-tracer characterisation and model intercomparison for NEEM, Northern Greenland, Atmos. Chem. Phys., 12, 4259-4277, doi:10.5194/acp-12-4259-2012, 2012.