Leveraging In Situ and Satellite Data to Understand Ammonia in Northeast Colorado

The Colorado Front Range urban corridor and the plains to the east are important source regions of ammonia (NH₃), an unregulated pollutant primarily emitted from agriculture activities. Upslope flows driven by the mountain-plains circulation and synoptic scale storm circulations periodically transport these emissions into Rocky Mountain National Park located 50 km west of the urban corridor, where they can be deposited by wet and dry deposition processes. Excess reactive nitrogen (N) deposition in RMNP is a historical problem with well documented impacts on the ecosystem. A combination of low-cost Radiello passive sampler NH₃ measurements and NH₃ total column retrievals from the Infrared Atmospheric Sounding Interferometer (IASI) will be used to assess temporal and spatial variability in NH₃ across three distinct land use categories in the region: agricultural, urban, and rural. A strong seasonal cycle was observed at all measurement sites. The NH₃ mixing ratio from passive measurements was strongly correlated with the number of confined animal feedlot operations within a 12 km radius, confirming the importance of that emission source category. Ground-level passive NH₃ measurements have a strong correlation with monthly gridded IASI satellite retrievals. Using satellite retrievals, we find an increasing trend of approximately 3% per year in agricultural and urban sub-regions. The absolute trend observed in the agricultural sub-region is more than 2 times larger than that observed in the Denver metro urban sub-region. Based on trends of particle sulfate at the Chemical Speciation Network (CSN) site in Denver, only 0.1% per year increase in gaseous NH₃ could be attributed to reductions in particle sulfate associated with declining SO₂ emissions. Elevated NH₃ from wildfire smoke was observed in August 2020, a period of active wildfire activity in northern Colorado, from IASI satellite retrievals but was less apparent in surface measurements, likely due to lofting of the smoke plume.