From Source to Receptor, Unpacking Emissions and Deposition of Reactive Nitrogen
September 08, 2025
Lilly Naimie
Committee: Jeffrey Collett (Advisor); Russ Schumacher; Emily Fischer; Jill Baron (Ecosystem Science and Sustainability); Megan Willis (Chemistry)
Abstract
Deposition of excess nutrients can have negative impacts on ecosystem health. This work will focus on frontiers in reactive nitrogen (Nr) deposition and emissions. Nr deposition in the US has experienced a regime shift, due to effective nitrous oxide (NOx) emissions regulations, and is now dominated by reduced N (gaseous ammonia (NH3) and particulate ammonium (NH4+)). Nr deposition is of particular interest in sensitive ecosystems, including Rocky Mountain National Park (RMNP).First, bidirectional exchange fluxes of NH3, a rarely quantified and highly uncertain process, will be simulated in RMNP. Bidirectional NH3 fluxes can change rapidly in magnitude and sign, under changing NH3 concentrations and micrometeorology conditions. NH3 concentration and micrometeorology data are both technically challenging, costly, and generally unavailable. Here, we test whether more commonly available biweekly NH3 concentration data and meteorological reanalysis data can be substituted with acceptable results. NH3 net deposition simulated to a forest ecosystem in RMNP was underestimated by 45% using biweekly NH3 concentration data; the bias was removed by applying a measured average diel concentration profile. NH3 deposition was overestimated by a factor of two using ERA5 reanalysis data.
Second, we assess changing emissions of NH3 from the Colorado Front Range, an important source region to RMNP during upslope flows. Spatial patterns of NH3 in this NE Colorado source region, from both satellite (IASI, Infrared Atmospheric Sounding Interferometer) and in situ observations, were strongly correlated with the number of animals in nearby confined animal feeding operations (CAFOs). Satellite observations reveal large increases over NE Colorado during the period 2013-2023, with increases over an agricultural region more than three times greater than over the Denver metro region. Decreases in particulate NH4+ formation, following emissions reductions in sulfur and nitrogen oxides, and increases in wildfire smoke are estimated to account for a small portion of the increase, which appears spatially to be dominated by increased emissions from agricultural sources.
Last, we will consider the western US, which is home to additional agricultural regions, ecosystems that are sensitive to excess Nr deposition, and increasing wildfire frequency. The effects of changing agricultural and smoke emissions are quantified using oversampled data from the Cross-track Infrared Sounder (CrIS), at 2 km resolution, for the warm-season, when peak agricultural emissions and wildfire frequency occur. The largest total column NH3 increases were associated with agricultural regions across the western US, ranging from a 1.5 to 4.8% increase per year. In the Colorado Front Range, the NH3 concentration trend above the agricultural subregion averaged 2.7% per year, decreasing to 2.5% per year when periods of wildfire smoke were removed. Over Idaho’s Snake River Valley, the NH3 concentration trend of 1.5% per year did not change when smoke periods were removed. The spatial footprint of agricultural hotspots is increasing by 7% per year across the western US, a trend that may indicate expansion of agricultural activities, increasing lifetime of emitted NH3, or both, with important implications for increased NH3 deposition to nearby sensitive ecosystems.