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Atmospheric nitrogen and sulfur deposition in Rocky Mountain National Park

Date

2008

Authors

Beem, Katherine B., author
Collett, Jeffrey L., advisor
Davis, Jessica G., committee member
Kreidenweis, Sonia M., committee member

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Rocky Mountain National Park (RMNP) is experiencing a number of adverse effects due to atmospheric nitrogen (N) and sulfur (S) compounds. Airborne nitrate and sulfate particles contribute to visibility degradation in the park while nitrogen deposition is producing changes in ecosystem function and surface water chemistry. Both sulfur and nitrogen compounds are essential nutrients for life; however, some environments have naturally limited supplies of sulfur and nitrogen which restrict biological activity. Increasing the amounts of these compounds can be toxic, even life threatening, to the ecosystem. Concerns about increasing deposition are especially important in national parks where excess nitrogen and sulfur can upset the delicate balance between species of flora and fauna in prized natural ecosystems. Measurements were made during the Rocky Mountain Airborne Nitrogen and Sulfur (RoMANS) study to quantify both N and S wet and dry deposition and to determine the most important species and pathways contributing to N deposition. Gas and particle concentrations were measured and precipitation samples were collected to gain a better understanding of nitrogen and sulfur transport to and deposition in RMNP. Samples were collected at 12 sites across the state of Colorado in March and April 2006 and at 13 sites in north central Colorado in July and August 2006. Historical data suggest that these are the seasons when N deposition in RMNP is greatest. The majority of wet deposition in the spring was from a single, large upslope snowstorm, while in the summer wet deposition inputs were spread across many more events. Total wet deposition of N in the summer was larger than during spring. Ammonium was the largest contributor to both spring and summer wet deposition in the park, followed by nitrate. Organic nitrogen, which is not routinely measured, contributed an average of 616.39 μg N/m2/event in the spring and 847.2 μg N/m2/event in the summer at the core sampling site. These deposition amounts were 22% and 16%, respectively, of total wet nitrogen deposition at this site. Dry deposition in RMNP was dominated by gaseous species which feature higher deposition velocities than accumulation mode aerosol particles. Ammonia, which is not routinely measured, was the largest contributor to dry N deposition followed by nitric acid. Dry deposition of fine particle nitrate and ammonium made only small contributions to total N deposition. Total N inputs were dominated by wet processes during both spring and summer. Wet deposition of organic nitrogen and dry deposition of gaseous ammonia comprised the 3rd and 4th largest contributions to the total N deposition budget. Together these pathways contributed nearly one-third of total measured N deposition, suggesting they should be examined more closely in assessing nitrogen impacts on national park ecosystems.

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Department Head: Richard Harlan Johnson.

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