Transport and deposition of reactive nitrogen in Rocky Mountain National Park

Nitrogen deposition is a concern in many protected ecosystems and there are often few observations in these areas to understand the transport of key species. Measurements in and around Rocky Mountain National Park (RMNP) during several field campaigns provide insights into transport of pollutants in the region and important deposition pathways. The state of Colorado has several distinct areas of reactive N emissions. The eastern plains of Colorado are home to large agricultural sources including confined animal feeding operations while the Colorado Front Range is a densely populated urban corridor that forms a boundary between the mountains and plains. Sampling targeted concentrations in these regions of different emission sources and in RMNP. Measurements of reactive nitrogen included ammonia and nitric acid and ammonium, nitrate, and organic nitrogen in particles and precipitation. These data were examined for spatial gradients in concentration, the influence of wind direction on reactive nitrogen transported to the park, and their contributions to nitrogen deposition.

High concentrations of reduced nitrogen species occur on the eastern plains of Colorado while oxidized nitrogen concentrations are highest along the Front Range. While winds are usually from the west, the highest 10% of concentrations measured in RMNP were associated with upslope (easterly) transport pattern. During the measurement period sites were also located west of the park to observe background conditions. Concentrations at RMNP were more than 50% higher than these background sites, again indicating large contributions from sources east of the park. Nitrogen deposition was also influenced by upslope flow. Based on local wind direction at the measurement site in RMNP, 59% of the wet nitrogen deposition for the year occurred during upslope (easterly) wind events. Nitrogen deposition in RMNP is dominated by wet deposition of ammonium and nitrate, followed closely by dry deposition of ammonia and wet deposition of organic nitrogen. Both wet deposition of organic nitrogen and dry deposition of ammonia are not routinely included in traditional calculations of nitrogen deposition or the critical load. However, for this yearly study these pathways contributed 1.37 kg N•ha ^-1yr ^-1 or 40% of the total nitrogen deposition budget. Nitrogen deposition for the year not including organic nitrogen or ammonia was 2.09 kg N•ha ^-1yr ^-1, well above the critical load set for the region of 1.5 kg N•ha ^-1yr ^-1.