Ammonia Dry Deposition in an Alpine Ecosystem Traced to Agricultural Emission Hotpots.

Elevated reactive nitrogen (N) deposition is a concern for alpine ecosystems, and dry NH deposition is a key contributor. Understanding how emission hotspots impact downwind ecosystems through dry NH deposition provides opportunities for effective mitigation. However, direct NH flux measurements with sufficient temporal resolution to quantify such events are rare. Here, we measured NH fluxes at Rocky Mountain National Park (RMNP) during two summers and analyzed transport events from upwind agricultural and urban sources in northeastern Colorado. We deployed open-path NH sensors on a mobile laboratory and an eddy covariance tower to measure NH concentrations and fluxes. Our spatial sampling illustrated an upslope event that transported NH emissions from the hotspot to RMNP. Observed NH deposition was significantly higher when backtrajectories passed through only the agricultural region (7.9 ng m s) versus only the urban area (1.0 ng m s) and both urban and agricultural areas (2.7 ng m s). Cumulative NH fluxes were calculated using observed, bidirectional modeled, and gap-filled fluxes. More than 40% of the total dry NH deposition occurred when air masses were traced back to agricultural source regions. More generally, we identified that 10 (25) more national parks in the U.S. are within 100 (200) km of an NH hotspot, and more observations are needed to quantify the impacts of these hotspots on dry NH deposition in these regions.