Ammonia dry deposition in an alpine ecosystem traced to agricultural emission hotpots

Elevated reactive nitrogen (N_r) deposition is a concern for alpine ecosystems, and dry NH₃ deposition is a key contributor. Understanding how emission hotspots impact downwind ecosystems through dry NH₃ deposition provides opportunities for effective mitigation. However, direct NH₃ flux measurements with sufficient temporal resolution to quantify such events are rare. Here, we measured NH₃ 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 NH₃ sensors on a mobile laboratory and an eddy covariance tower to measure NH₃ concentrations and fluxes. Our spatial sampling illustrated an upslope event that transported NH₃ emissions from the hotspot to RMNP. Observed NH₃ deposition was significantly higher when backtrajectories passed through only the agricultural region (7.9 ng m⁻² s⁻¹) versus only the urban area (1.0 ng m⁻² s⁻¹) and both urban and agricultural areas (2.7 ng m⁻² s⁻¹). Cumulative NH₃ fluxes were calculated using observed, bidirectional modeled, and gap-filled fluxes. More than 40% of the total dry NH₃ 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 NH₃ hotspot, and more observations are needed to quantify the impacts of these hotspots on dry NH₃ deposition in these regions.