A model investigation of turbulence‐driven pressure‐pumping effects on the rate of diffusion of CO2, N2O, and CH4 through layered snowpacks

Abstract

Pressure pumping at the Earth's surface is caused by short‐period atmospheric turbulence, longer‐period barometric changes, and quasi‐static pressure fields induced by wind blowing across irregular topography. These naturally occurring atmospheric pressure variations induce periodic fluctuations in airflow through snowpacks, soils, and any other porous media at the Earth's surface. Consequently, the uptake or release of trace gases from soils and snowpacks is a combination of molecular diffusion and advection forced by pressure pumping. Using model‐estimated fluxes, this study attempts to quantify the influence that turbulent pressure fluctuations with periods between 0.1 and 1000 s can have on the rate of exchange of CO2, N2O, and CH4 through a seasonal snowpack. Data for this study were collected at a forested subalpine meadow site in the Rocky Mountains of southern Wyoming, during February 1995 when the snowpack was distinctly layered and approximately 1.4 m deep. The data include mole fraction of CO2, N2O, and CH4 just above and at the base of the snowpack, several profiles of CO2, N2O, and CH4 mole fraction in the top l m of the snowpack, and a profile of snowpack density and tortuosity. Turbulent atmospheric pressure‐pumping fluctuations, sampled at approximately 11 Hz for several hours, were obtained with a fast response differential pressure sensor. A one‐dimensional steady state diffusion model and one‐ and three‐dimensional time‐dependent pressure‐pumping models are used to estimate the gas fluxes through the snowpack. Boundary conditions are provided by grab samples just above the snowpack and at the soil/snow interface. The pressure‐pumping models are driven by the observed pressure fluctuations, and all models include the observed layering of the snowpack. As with previous studies the present results indicate that the effects of pressure pumping are diminished with increasingly strong gradients. Furthermore, we conclude that unless pressure pumping influences the gas concentrations at the boundaries of the snowpack, it appears unlikely that it can significantly impact the rate of gaseous diffusion through the snowpack. Even two‐ and three‐dimensional effects, which can have a significant short‐term impact on the fluxes and concentration profiles, are nearly eliminated when averaged over a period of hours. It is also suggested that vertical layering is important for three‐dimensional pressure‐pumping studies and that the time‐dependent temperature term, which is traditionally ignored when modeling dynamic pressure variations, may in fact be dominant in some situations and probably should be incorporated in future modeling studies of pressure pumping.