CO2 flux through a Wyoming seasonal snowpack: Diffusional and pressure pumping effects

The movement of trace gases through porous media results from a combination of molecular diffusion and natural convection forced by turbulent atmospheric pressure pumping. This study presents observational and modeling results of an experiment to estimate the C02 flux through a seasonal snowpack in the Rocky Mountains of southern Wyoming, USA. Profiles of C02 mole fraction in the top 1 m of the snowpack, C02 mole fraction just above and at the base of the snowpack, and snowpack density profiles were obtained at a meadow site during February and March 1994, when the snowpack was about 2 m deep. Turbulent atmospheric pressure fluctuations, sampled at 15 Hz at the snow surface, were obtained using a fast response differential pressure sensor. A one-dimensional steady state diffusion model and a one-dimensional time-dependent pressure pumping model are used to estimate C02 fluxes through the snowpack. Boundary conditions are provided by the C02 mole fraction grab samples just above the snowpack and at the snow/soil interface. The pressure pumping model is driven by the observed pressure fluctuations. Both models include the observed layering of the snowpack. The C02 flux predicted by the diffusion model varied between 0.026 mg-C02 m-2 s-1 (on 3 March) and 0.032 mg-C02 m-2 s-1 (on 23 February). The pressure pumping model fluxes were greater than the steady state diffusional C02 fluxes by 19 to 31 %.