ACPD Atmospheric Chemistry and Physics Discussions ACPD 1680-7375 Copernicus GmbH Göttingen, Germany 10.5194/acpd-11-23029-2011 A new multi-gas constrained model of trace gas non-homogeneous transport in firn: evaluation and behavior at eleven polar sites Witrant E. 1 Martinerie P. 2 Hogan C. 3 Laube J. C. 3 Kawamura K. 4 Capron E. 5 6 Montzka S. A. 7 Dlugokencky E. J. 7 Etheridge D. 8 Blunier T. 9 Sturges W. T. 3 Grenoble Image Parole Signal Automatique (GIPSA-lab), Université Joseph Fourier/CNRS, BP 46, 38 402 Saint Martin d'Hères, France Laboratoire de Glaciologie et Géophysique de l'Environnement (LGGE), CNRS/Université Joseph Fourier, BP 96, 38 402 Saint Martin d'Hères, France School of Environmental Sciences, University of East Anglia, Norwich NR4 7TJ, UK National Institute of Polar Research, 1-9-10 Kaga, Itabashi-ku, Tokyo 173-8515, Japan Laboratoire des Sciences du Climat et de L'Environnement, IPSL/CEA-CNRS-UVSQ, 91191 Gif-sur-Yvette, France British Antarctic Survey, Natural Environment Research Council, High Cross, Madingley Road, Cambridge CB3 0ET, UK NOAA Earth System Research Laboratory, Boulder, Colorado, USA Commonwealth Scientific and Industrial Research Organisation, Marine and Atmospheric Research, PMB 1, Aspendale, Vic. 3195, Australia Centre for Ice and Climate, Niels Bohr Institute, University of Copenhagen, Juliane Maries vej 30, 2100 Copenhagen Ø, Denmark 16 08 2011 11 8 23029 23080 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-discuss.net/11/23029/2011/acpd-11-23029-2011.html The full text article is available as a PDF file from http://www.atmos-chem-phys-discuss.net/11/23029/2011/acpd-11-23029-2011.pdf

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 &delta;<sup>15</sup>N) 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 &delta;<sup>15</sup>N of N<sub>2</sub> 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.

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