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

Abstract

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, N20, and CH 4 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, N20, and CH 4 just above and at the base of the snowpack, several profiles of CO2, N20, and CH 4 mole fraction in the top 1 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 onedimensional 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 shortterm 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 threedimensional 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.

Analysis of Air Pressure Fluctuations and Topsoil Gas Concentrations within a Scots Pine Forest

“…In addition, an influence of air pressure fluctuations on soil gas transport was observed [2,[9][10][11]. This influence is referred to as turbulence-induced "pressure pumping" and has attracted great interest in recent years [8,[12][13][14][15][16][17][18][19][20]. Results from present research suggest that soil gas transport is enhanced up to 100% by air pressure fluctuations induced by airflow, dependent on the investigated soil [18,19,21].…”

A new mechanistic framework to predict OCS fluxes from soils

“…venting the soil surface (according to Bernouilli's equation) or turbulence above the soil surface. Typical air pressure fluctuations are of the order of 10 Pa (Maier et al, 2012;Massman et al, 1997). Pressure fluctuations can also result from nonhydrostatic density fluctuations caused by a change in the air composition with gas species of different molar mass as air or by temperature gradients, but the resulting flux is significant only in highly permeable (i.e.…”

“…When averaged over a long enough timescale (> 1 h) the advective flux starts to become negligible compared to the diffusive flux (e.g. Massman et al, 1997). Integration timescales of a few minutes were already assumed to allow liquid-vapour equilibration in Eq.…”

“…If the flow in the porous soil is laminar these velocity fields are given by Darcy's law (Massman et al, 1997;Scanlon et al, 2002):…”

Evaporation from porous surfaces into turbulent airflows: Coupling eddy characteristics with pore scale vapor diffusion

“…[21] The potential effects of the so-called ''turbulent pumping'' [Kimball and Lemon, 1971;Massman et al, 1997] on mass exchange were not considered in this study. Such turbulence-driven pressure pumping effects are minor in wet porous surfaces with small water-filled pores, as both wetness and small pore size act to dampen convection into the porous medium.…”