Determination of gas diffusivity in polar firn: Comparison between experimental measurements and inverse modeling

To study air diffusion in polar firns, and thus evaluate the mean age of ai r constituents as a function of depth, it is necessary to know the effectiv e diffusivity profile for the studied gas and firn. In this work we compare effective diffusivity profiles obtained by two methods: first, by experime ntal measurements on firn samples, second, using an inverse gas diffusion m odel. Results are obtained for three gases, CO2, CH4 and SF6, in firns expo sed to different climatic conditions. While measurements give similar resul ts for the different sites, the inverse model shows variations depending on the climatic conditions. We therefore conclude that diffusivities measured on small firn samples cannot be directly used to determine the tortuosity profile of real firn and that diffusion models have to be used.