The relationship between upslope wind events and wet deposition of nitrogen in rocky mountain national park

Nitrogen deposition is an issue across the United States, but especially in Rocky Mountain National Park because an increased amount of deposition results in negative effects on the sensitive ecosystem. Possible effects include the alteration of the nitrogen cycle, soil mineralization, and stream acidification. Nitrogen-containing pollutants also contribute to decreases in air quality and visibility. The plains of eastern Colorado and the urban centers at the base of the Rocky Mountains emit reduced and oxidized reactive nitrogen compounds that are periodically transported into Rocky Mountain National Park. Sources of these pollutants include vehicle emissions, nitrogen fertilizers, confined animal feeding, and the burning of fossil fuels. Winds in the region are typically from the west; however, upslope winds (easterly) may be a more important factor affecting the amount of nitrogen deposition RMNP receives. This research focuses on how nitrogen-containing pollutants are transported into Rocky Mountain National Park.

The National Atmospheric Deposition Program (NADP) monitors nitrogen deposition at many sites across the United States. There are two sites in RMNP, which have been operating since 1978. To understand the importance of upslope flow to nitrogen deposition in the region the nitrogen deposition records were analyzed in conjunction with air mass back-trajectories generated using the Hybrid Single Particle Lagrangian Integrated Trajectory Model (HYSPLIT). Daily precipitation events between 2000 and 2010 were paired with trajectories to determine if the deposition events were associated with upslope (easterly) flow. Winds were classified as upslope winds if the backward trajectory was from the southeast, east, or northeast direction. The backward trajectories were compared to vector maps as well as surface maps for consistency. The vector maps were used to identify the speed and direction of the wind during precipitation events. The surface maps were used to distinguish synoptic scale winds from mountain valley wind patterns.

Based on this analysis, it was found that 44% of the total wet nitrogen deposition that occurred between 2000 and 2010 was associated with upslope winds. There was some variation from month to month because upslope events are more common during the summer and less common during the winter. Also, this research suggests that there is a direct relationship between precipitation and deposition because more precipitation often resulted in more deposition. Ammonium was the slightly larger deposition pathway when compared to nitrate during upslope wet deposition events (ammonium 52 %, nitrate 48%). However, during individual deposition events the relative importance of ammonium and nitrate varied. The summer months had the most wet upslope deposition with 34% of the events occurring in July and August. Very few wet upslope events occurred during the winter months. Deposition during the spring/summer of 2004 and 2005 was high compared to the other years included in this analysis, this highlighting the variability of deposition from year to year.