Leveraging In Situ and Satellite Data to understand Changing Ammonia above an Agricultural Hotspot

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 agricultural 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, where excess reactive nitrogen (N) deposition 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) are used to assess temporal and spatial variability in NH₃ across three distinct land use categories in the region: agricultural, urban, and rural. The NH₃ mixing ratio from passive measurements was strongly correlated with the number of confined animal feedlot operations (CAFOs) 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 NH₃ trend of approximately 3% per year in agricultural and urban sub-regions. We attribute less than 0.2% of the increasing NH₃ trend to reductions in particle sulfate. The absolute trend follows the spatial distribution of CAFOs. In the agricultural sub-region, the absolute NH₃ trend is on average greater than 2 times larger than that observed in the urban sub-region. The ground-based observations do not have a trend. The lack of ground-based trend is attributed to increasing boundary layer height and dilution of concentrations, through analysis of ERA5 reanalysis data. Lofting NH₃ higher into the atmosphere can increase atmospheric lifetime, associated with transport and deposition further from source regions and increased particle formation. Elevated NH₃ from wildfire smoke was observed in August 2020, a period of active wildfire activity in northern Colorado, from IASI satellite retrievals. This elevation was less apparent in surface measurements, likely also due to the lofting of the smoke plume. Modeled smoke plumes from the Hazard Mapping System were used to assess the potential impacts of wildfires on observed NH₃ trends.