Contributions of Ammonia Dry Deposition to Excess Nitrogen Deposition in Rocky Mountain National Park

Rocky Mountain National Park (RMNP) was established to preserve the natural landscape, including montane, subalpine, and alpine ecosystems. Excess reactive nitrogen (N) deposition in RMNP is a historical problem with well documented impacts including changes in soil biogeochemistry, enhanced microbial activity, and increased N in freshwater bodies. The Front Range and the plains to the east are marked by both agricultural and urban emissions of ammonia (NH₃) and NO_x. Upslope flows driven by the mountain-plains circulation periodically transport these emissions into RMNP. Dry and wet deposition of reduced N are estimated to account for more than half of the N deposited into RMNP, with substantial contributions from Front Range and NE Colorado sources. However, NH₃ dry deposition is poorly quantified due to the bidirectional nature of NH₃ atmosphere-surface exchange. To better quantify NH₃ dry deposition, measurements of NH₃ were made at two heights above a RMNP forest canopy and used to parameterize a bidirectional flux model with a suite of additional measurements including atmospheric turbulence, relative humidity, solar radiation, precipitation, surface wetness, soil temperature, soil moisture, and soil and vegetation compositions. Biweekly passive NH₃ and meteorological measurements are used, along with the model, to evaluate the importance of NH₃ in the annual RMNP nitrogen deposition budget from August 2021 through August 2022. Wet deposition from the National Trends Network and additional gas measurements are used to calculate the importance of other nitrogen deposition pathways and to estimate the NH₃ diel pattern. Local meteorological and back trajectory analysis are used to investigate reactive nitrogen transport pathways throughout the study period, including during periods of elevated deposition. A significant fraction of NH₃ dry deposition occurred during upslope transport events, indicating impacts on the park from Front Range sources. The importance of upslope transport is consistent with previous work and indicates that mitigation strategies in the Front Range have the potential to effectively reduce N deposition in RMNP. Measurements from our intensive campaign indicate that NH₃ dry deposits 10 times faster to forests than grasslands in RMNP. Dry deposition during the summer in RMNP (1.4 kg N ha^-1) is approximately equivalent to the 5-year annual average goal of 1.5 kg N ha^-1 for wet inorganic nitrogen deposition set in the Nitrogen Deposition Reduction Plan to avoid adverse ecosystem effects.