An automated system for continuous measurements of trace gas fluxes through snow: an evaluation of the gas diffusion method at a subalpine forest site, Niwot Ridge, Colorado

摘要

An experimental system for sampling trace gas fluxes through seasonal snowpack was deployed at a subalpine site near treeline at Niwot Ridge, Colorado. The sampling manifold was in place throughout the entire snow-covered season for continuous air sampling with minimal disturbance to the snowpack. A series of gases (carbon dioxide, water vapor, nitrous oxide, nitric oxide, ozone, volatile organic compounds) was determined in interstitial air withdrawn at eight heights in and above the snowpack at ~hourly intervals. In this paper, carbon dioxide data from 2007 were used for evaluation of this technique. Ancillary data recorded inlcuded snow physical properties, i.e., temperature, pressure, and density. Various vertical concentration gradients were determined from the multiple height measurements, which allowed calculation of vertical gas fluxes through the snowpack using Fick's 1st law of diffusion. Comparison of flux results obtained from different height inlet combinations show that under most conditions fluxes derived from individual gradient intervals agree with the overall median of all data within a factor of 1.5. Winds were found to significantly influence gas concentration and gradients in the snowpack. Under the highest observed wind conditions, concentration gradients and calculated fluxes dropped to as low as 13% of non-wind conditions. Measured differential pressure amplitude exhibited a linear relationship with wind speed. This suggests that wind speed is a sound proxy for assessing advection transport in the snow. Neglecting the wind-pumping effect resulted in considerable underestimation of gas fluxes. An analysis of dependency of fluxes on wind speeds during a 3-week period in mid-winter determined that over this period actual gas fluxes were most likely 57% higher than fluxes calculated by the diffusion method, which omits the wind pumping dependency.