Spatial and Temporal Trends in Ammonia in Northeast Colorado
Jeffrey L. Collett, Jr.1, *, Lillian E. Naimie1, Da Pan1, Amy P. Sullivan1, Katherine Benedict2, and Lena Low1
Upslope flow from northeastern Colorado is an important mechanism for transporting air pollutants from active urban and agricultural regions into Rocky Mountain National Park (RMNP). This includes both oxidized and reduced forms of nitrogen, including nitric acid and ammonia (NH3). NH3 emissions in the region come primarily from animal feeding operations and fertilized cropland; however, urban/traffic emissions and wildfires can also be contributors at certain times and locations. Excess reactive nitrogen (Nr) deposition in RMNP is a historical problem with well documented impacts on the ecosystem. While increased regulation of NOx emissions has resulted in decreasing deposition of nitrate in RMNP, NH3 remains an unregulated pollutant with only voluntary efforts to reduce its emissions. The fraction of RMNP Nr wet deposition comprised by NH4+ has, consequently, been increasing.
In order to better understand NH3 sources, spatial patterns and long-term trends in NE Colorado, a combination of surface passive sampler NH3 measurements and NH3 total column retrievals from the Infrared Atmospheric Sounding Interferometer (IASI) will be used. We focus on urban, rural, and agricultural regions. A strong seasonal cycle was observed at all measurement sites. The NH3 mixing ratio from passive measurements was strongly correlated with the number of nearby confined animal feedlot operations, further documenting the importance of that emission source category. Ground-level passive NH3 measurements exhibit a strong correlation with monthly gridded IASI satellite retrievals. Using satellite retrievals, we find an increasing NH3 trend of approximately 3% per year in agricultural and urban sub-regions. The absolute trend observed in the agricultural areas is more than double that observed in the Denver metro region, suggesting a larger increase in agricultural than urban emissions. In the Denver metro area, only a small fraction of the increase in gaseous NH3 could be attributed to reductions in particle sulfate. Elevated NH3 from wildfire smoke was observed in the satellite record in August 2020, a period of active wildfire activity in northern Colorado, but was less apparent in surface measurements, likely due to lofting of the smoke plume.
1 Colorado State University, Fort Collins, CO
2 Los Alamos National Laboratory, Los Alamos, NM
*Corresponding Author: collett@colostate.edu