Ozone in interstitial air was studied in the seasonal, mid-latitude snowpack at a subalpine forest site at Niwot Ridge, Colorado, from January to June 2004. Sampling techniques were developed for continuous, vertical gradient measurements of ozone and temperature at four depths in the snowpack. During this time period, ozone in ambient air ranged from 15 to 80 ppbv, while in the snowpack ozone mixing ratios generally were below 5 ppbv, showed little variability, and decreased to less than 10% of ambient air levels within the first 10–20 cm below the surface. This ozone gradient (ambient air-snowpack air) appeared to be independent of solar radiation cycles. These findings are in contrast to similar studies in the polar snowpack, where a much deeper penetration of ozone into the snowpack and strong dependencies of the ozone gradient on incoming solar radiation levels has been reported. These observations imply that ozone levels in the seasonal, mid-latitude snowpack are determined by different processes, and overall are lower than in the year-round snowpack. A new question that needs to be addressed is to what degree these contrasting findings are caused by differences in the physical properties of these snowpacks (which will affect gas exchange processes), their chemical composition, and by the influence of the substrate below the snow and soil-snowpack-atmosphere gas exchange processes.
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Vol. 39 • No. 3