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

Abstract

Elevated reactive nitrogen (Nr) 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.

Review of methods for assessing deposition of reactive nitrogen pollutants across complex terrain with focus on the UK

“…Few studies [e.g. 32,33] have been undertaken at high elevation, mostly in North America (~3000 m elevation). Precipitation (and fog) sampling at exposed and windy high elevation locations remains a significant challenge in terms of collecting precipitation that is representative of the amount reaching the ground [34,35,36].…”

“…There is an extensive literature on N r emission uxes of NO and N 2 O, but relatively few geographical locations have been characterized for total annual N r deposition, particularly at background sites. Few studies 32,33 have been undertaken at high elevation, mostly in North America ($3000 m elevation). Precipitation (and fog) sampling at exposed and windy high elevation locations remains a signicant challenge in terms of collecting precipitation that is representative of the amount reaching the ground.…”

Long-term Atmospheric Deposition of Nitrogen and Sulfur and Assessment of Critical Loads Exceedances at Canadian Rural Locations

“…This was because the increase in NH3 dry deposition from 2002 to 2010 was largely offset by the decrease in NO2 dry deposition. The importance of agricultural NH3 to local and downwind total N deposition is evident (Walker et al, 2019;Hu et al, 2021;Pan et al, 2021); however, quantifying its role remains elusive in natural lands in Canada because of limited measurements.…”

Elevated Nitrogen Deposition to Fire‐Prone Forests Adjacent to Urban and Agricultural Areas, Colorado Front Range, USA

“…Agricultural emissions from livestock operations and crop production are likely partially responsible for the shift toward chemically reduced forms of Nr deposition. Concentrated animal feeding operations are known hotspots of ammonia emissions in northeastern Colorado, located directly east of the Front Range (Li et al., 2017; Pan et al., 2021; Van Damme et al., 2018), and the use of nitrification inhibitors on fields may also be responsible for increased ammonia emissions (Lam et al., 2017; Sun et al., 2015). In addition to the agricultural emissions, urban ammonia emissions from the rapidly growing Boulder‐Denver metro area have also likely contributed to the shift toward reduced forms of Nr deposition (Wetherbee et al., 2019).…”