Temporal Dynamics and Controls of Carbon Dioxide, Ozone, and Nitrogen Oxides in Seasonally Snow-Covered Forest Ecosystems

Seok, Brian

Graduate Thesis Or Dissertation

Temporal Dynamics and Controls of Carbon Dioxide, Ozone, and Nitrogen Oxides in Seasonally Snow-Covered Forest Ecosystems Public Deposited

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Citeable URL: https://scholar.colorado.edu/concern/graduate_thesis_or_dissertations/4f16c2920

Abstract

Understanding how reactive gases interact with the snowpack is important as these gases influence the oxidation capacity of the atmosphere and ozone formation, which is important in understanding the impact of climate change on the Earth system. Trace gas exchange studies conducted in the Polar Regions showed that snow is a highly reactive medium for photochemical reactions, heterogeneous reactions, and physical exchange processes. The impact that snowpack can have on the overlying atmosphere can be significant depending on its location, but the importance of these effects are not yet well quantified. Thus, there is a need to investigate how reactive gases interact with the snowpack at various locations. Yet, the majority of studies investigating the snow-air exchange of reactive gases are done in the Polar Regions and only a handful are done in the mid-latitudes. If we want to be able to accurately predict how the atmospheric oxidation capacity and formation of ozone will respond to climate change, more measurements in the mid-latitude, seasonal snowpack needs to be done. To address this issue, I conducted trace gas measurements of carbon dioxide, ozone, and nitrogen oxides in two different seasonally snow-covered forest ecosystems. One study site was high elevation, subalpine forest, and the other study site was a low elevation, deciduous forest. I used a novel, automated measurement system based on the gradient diffusion method (GDM) to determine trace gas fluxes though snow at those sites. This system enabled me to evaluate the assumptions and semi-empirically quantify the errors in GDM. I demonstrated how the uncertainties in calculating average seasonal flux values using GDM can be reduced by statistically combing data from multiple gradient heights. I also found that wind or pressure induced variations at the snow surface can result in a large (> 30%) underestimation in inferred flux. Then by analyzing the differences in the dynamics of the trace gases in snow between the high and low elevation sites, I was able to demonstrate and re-emphasize that snow is a complex medium, and that the influence of snow on chemical and biological exchange is highly variable and dependent on climatic conditions and geographical location. For instance, subnivial soil was the main source of nitrogen oxides at the high elevation site, while atmospheric deposition and photochemistry in the snowpack were the sources of nitrogen oxides at the low elevation site. I also investigated the dynamics of ozone and nitrogen oxides during snow-free conditions at the low elevation site to better understand observations during snow-covered conditions. I discovered that transport of pollutants can strongly influence the dynamics of nitrogen oxides in-snow during snow-covered conditions and in-canopy during snow-free conditions. Finally, preliminary results from model study of trace gas exchange in Polar snowpack suggest that gas-phase chemistry in the snowpack interstitial air is insufficient for explaining the observed dynamics of ozone and nitrogen oxides in snow and that other not yet well understood chemical and physical processes in the snowpack drive these observed variations. These results collectively demonstrate the importance of understanding the biosphere-atmosphere interaction in snow-covered environments.

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