Atmos. Chem. Phys. Discuss., 11, 23029-23080, 2011
© Author(s) 2011. This work is distributed
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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.
A new multi-gas constrained model of trace gas non-homogeneous transport in firn: evaluation and behavior at eleven polar sites
E. Witrant1, P. Martinerie2, C. Hogan3, J. C. Laube3, K. Kawamura4, E. Capron5,6, S. A. Montzka7, E. J. Dlugokencky7, D. Etheridge8, T. Blunier9, and W. T. Sturges3
1Grenoble Image Parole Signal Automatique (GIPSA-lab), Université Joseph Fourier/CNRS, BP 46, 38 402 Saint Martin d'Hères, France
2Laboratoire de Glaciologie et Géophysique de l'Environnement (LGGE), CNRS/Université Joseph Fourier, BP 96, 38 402 Saint Martin d'Hères, France
3School of Environmental Sciences, University of East Anglia, Norwich NR4 7TJ, UK
4National Institute of Polar Research, 1-9-10 Kaga, Itabashi-ku, Tokyo 173-8515, Japan
5Laboratoire des Sciences du Climat et de L'Environnement, IPSL/CEA-CNRS-UVSQ, 91191 Gif-sur-Yvette, France
6British Antarctic Survey, Natural Environment Research Council, High Cross, Madingley Road, Cambridge CB3 0ET, UK
7NOAA Earth System Research Laboratory, Boulder, Colorado, USA
8Commonwealth Scientific and Industrial Research Organisation, Marine and Atmospheric Research, PMB 1, Aspendale, Vic. 3195, Australia
9Centre for Ice and Climate, Niels Bohr Institute, University of Copenhagen, Juliane Maries vej 30, 2100 Copenhagen Ø, Denmark

Abstract. Insoluble trace gases are trapped in polar ice at the firn-ice transition, at approximately 50 to 100 m below the surface, depending primarily on the site temperature and snow accumulation. Due to the different time scales for snow accumulation versus diffusion of gases through the snowpack, age differences between gases and the ice in which they are "trapped" can be large; e.g. several thousand years in central Antarctica (a low snow accumulation area). Models of trace gas diffusion in polar firn are used to relate firn air and ice core records of trace gases to their atmospheric history. We propose a new diffusion model based on the following contributions. First, the airflow transport model is revised in a poromechanics framework with specific emphasis on the non-homogeneous properties (convective layer, depth-dependent diffusivity and lock-in zone) and an almost-stagnant behavior described by Darcy's law (gravity effect). We then derive a non-linear least square multi-gas optimization scheme to calculate the effective firn diffusivity (automatic diffusivity tuning). The improvements associated with the additional constraints gained by the multi-gas approach are investigated (up to eleven gases for a single site are included in the optimization process). The model is applied to measured data from four Arctic (Devon Island, NEEM, North GRIP, Summit) and seven Antarctic (DE08, Berkner Island, Siple Dome, Dronning Maud Land, South Pole, Dome C, Vostok) sites and the depth-dependent diffusivity profiles are calculated. Among these different sites, a relationship between an increasing thickness of the lock-in zone defined from the isotopic composition of molecular nitrogen in firn air (denoted δ15N) and the snow accumulation rate is obtained, in accordance with observations. It is associated with reduced diffusivity depth-gradients in deep firn, which decreases gas density depth-gradients, at high accumulation rate sites. This has implications for the understanding of δ15N of N2 records in ice cores, in relation with past variations of the snow accumulation rate. Although the extent of layering is clearly a primary control on the thickness of the lock-in zone, our new approach that allows calculation of an estimated lock-in depth may lead to a better constraint on the age difference between the ice and entrapped gases.

Citation: Witrant, E., Martinerie, P., Hogan, C., Laube, J. C., Kawamura, K., Capron, E., Montzka, S. A., Dlugokencky, E. J., Etheridge, D., Blunier, T., and Sturges, W. T.: A new multi-gas constrained model of trace gas non-homogeneous transport in firn: evaluation and behavior at eleven polar sites, Atmos. Chem. Phys. Discuss., 11, 23029-23080, doi:10.5194/acpd-11-23029-2011, 2011.
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