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Insoluble trace gases are trapped in polar ice at the firn-ice transition, atapproximately 50 to 100 m below the surface, depending primarily on the sitetemperature and snow accumulation. Models of trace gas transport in polarfirn are used to relate firn air and ice core records of trace gases to theiratmospheric history. We propose a new model based on the followingcontributions. First, the firn air transport model is revised in aporomechanics framework with emphasis on the non-homogeneous properties andthe treatment of gravitational settling. We then derive a nonlinear leastsquare multi-gas optimisation scheme to calculate the effective firndiffusivity (automatic diffusivity tuning). The improvements gained by themulti-gas approach are investigated (up to ten gases for a single site areincluded in the optimisation process). We apply the model to four Arctic(Devon Island, NEEM, North GRIP, Summit) and seven Antarctic (DE08, BerknerIsland, Siple Dome, Dronning Maud Land, South Pole, Dome C, Vostok) sites andcalculate their respective depth-dependent diffusivity profiles. Among thesedifferent sites, a relationship is inferred between the snow accumulationrate and an increasing thickness of the lock-in zone defined from theisotopic composition of molecular nitrogen in firn air (denotedδ15N). It is associated with a reduced diffusivity value and anincreased ratio of advective to diffusive flux in deep firn, which isparticularly important at high accumulation rate sites. This has implicationsfor the understanding of δ15N of N2 records in ice cores, inrelation with past variations of the snow accumulation rate. As the snowaccumulation rate is clearly a primary control on the thickness of thelock-in zone, our new approach that allows for the estimation of the lock-inzone width as a function of accumulation may lead to a better constraint onthe age difference between the ice and entrapped gases

A new multi-gas constrained model of trace gas non-homogeneous transport in firn: evaluation and behaviour at eleven polar sites