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

Recent studies have shown that many snow covered ecosystems can lose a significant amount of carbon in the winter by respiration under the seasonal snowpack. Transport of trace gases through a snowpack is dominated by diffusion, but also influenced by advection through the process of pressure pumping. To quantify the importance of pressure pumping, CO2 fluxes were examined over 3 winters within a snowpack at the Niwot Ridge AmeriFlux site in the Rocky Mountains of Colorado (3050 m elevation). CO2 and its carbon isotope composition (δ13C) were monitored in vertical profiles within the snowpack, at the soil-snow interface, and within the soil, using tunable diode laser spectroscopy. Ancillary measurements included wind and other weather measurements, snow and soil temperature, soil moisture, and snowpack physical parameters (density profiles, snow water equivalence or SWE). Fluxes were determined based on Fick’s law of diffusion. Since pressure pumping alters isotopic mixing relationships away from fully diffusive, theory was developed to use variability in isotopic mixing relationships to quantify how pressure pumping influences fluxes. This was accomplished using an enhancement to the molecular diffusivity parameter. CO2 fluxes were small (<0.5 μmol m-2 s-1) but persistent through the cold winter period, increasing to as large as 1.5 μmol m-2 s-1 during snowmelt. Pressure pumping led to a winter-long enhancement of CO2 fluxes of roughly 10% over purely molecular diffusion. Flux measurement techniques which rely on Fick’s law must account for the influence of pressure pumping.