Atmos. Chem. Phys. Discuss., 11, 15975-16021, 2011
www.atmos-chem-phys-discuss.net/11/15975/2011/
doi:10.5194/acpd-11-15975-2011
© Author(s) 2011. This work is distributed
under the Creative Commons Attribution 3.0 License.
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This discussion paper has been under review for the journal Atmospheric Chemistry and Physics (ACP). Please refer to the corresponding final paper in ACP.
Gas transport in firn: multiple-tracer characterisation and model intercomparison for NEEM, Northern Greenland
C. Buizert1, P. Martinerie2, V. V. Petrenko3, J. P. Severinghaus4, C. M. Trudinger5, E. Witrant6, J. L. Rosen7, A. J. Orsi4, M. Rubino1,5, D. M. Etheridge1,5, L. P. Steele5, C. Hogan8, J. C. Laube8, W. T. Sturges8, V. A. Levchenko9, A. M. Smith9, I. Levin10, T. J. Conway11, E. J. Dlugokencky11, P. M. Lang11, K. Kawamura12, T. M. Jenk1, J. W. C. White3, T. Sowers13, J. Schwander14, and T. Blunier1
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

Abstract. Compacted snow (firn) preserves a continuous record of atmospheric composition up to a century back in time. Firn air transport modeling is essential for interpretation of firn gas records. Each site needs to be characterised individually through a tuning procedure, in which the effective diffusivity at each depth is adjusted to optimise the agreement between modeled and measured mixing ratios of a selected reference gas (usually CO2). We present the characterisation of the NEEM site, Northern Greenland (77.45° N 51.06° W), where an ensemble of ten reference tracers is used to constrain the diffusivity reconstruction. 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 model tuning, and define a root mean square criterion that is minimised in the tuning. 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. The modern day Δage, i.e. the difference between gas age and ice age, is calculated to be 182 ± 8 yr. We find evidence that diffusivity does not vanish completely in the firn lock-in zone, as is commonly assumed. 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.

Citation: 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. Discuss., 11, 15975-16021, doi:10.5194/acpd-11-15975-2011, 2011.
 
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