Persistent wind-induced enhancement of diffusive CO2 transport in a mountain forest snowpack

Diffusion dominates the transport of trace gases between soil and the atmosphere. Pressure gradients induced by atmospheric flow and wind interacting with topographical features cause a small but persistent bulk flow of air within soil or snow. This forcing, called pressure pumping or wind pumping, leads to a poorly quantified enhancement of gas transport beyond the rate of molecular diffusion. This study was conducted to quantify the role of pressure pumping in enhancement of CO2 transport through a mountain forest seasonal snowpack. Observations of 12CO2 and 13CO2 within the snowpack, soil, and air of a subalpine forest were made over three winters in the Rocky Mountains, USA. These molecules differ in their rates of diffusion, providing a means to quantify the relative importance of diffusion and advection. An empirical model was developed to describe the transport of these gases through the snowpack, assuming that isotopic variability was caused solely by wind. We found that advection was a persistent phenomenon within the snowpack. Under calm conditions, isotopic patterns followed those associated with diffusion. In the presence of wind, the 4.4‰ isotopic effect of diffusion was diminished, and transport was enhanced beyond the diffusive rate for a given mole fraction gradient. Pressure pumping in our forest snowpack enhanced transport of CO2 beyond molecular diffusion by up to 40% in the short term (hours) but by at most 8%–11% when integrated over a winter. These results should be applicable to trace gas transport in a variety of biogeochemical applications. Key Points Pressure pumping causes ventilation of trace gases from a seasonal snowpack Pressure pumping enhances trace gas transport beyond the rate of molecular diffusion Enhanced transport is 40% higher in the short term but 8%–11% higher over a winter