Spatiotemporal Variability of NH3 across the Contiguous United States

Ammonia (NH₃) is a critical aerosol precursor species with limited knowledge of its spatiotemporal variability and emissions magnitude. Satellite NH₃ measurements can be used to help improve air quality forecasts of fine particulate matter (PM_2.5). IASI Level 2 NH₃ (v2.1) data were oversampled at high-resolution (0.02°×0.02°), yielding monthly NH₃ maps covering the contiguous U.S. (CONUS) averaged over 2008 – 2016. A preliminary comparison has been done with the GFDL AM3 modeling results based on a bottom-up emission inventory, Magnitude And Seasonality of Agricultural Emissions model for NH₃ (MASAGE_NH₃). The maps were able to capture the hotspots and showed general agreement with the modeling results, mostly in the intensive agriculture regions, such as the San Joaquin Valley (SJV), Central and Southern High Plains Aquifer, Platte River Valley, and Northwest Iowa Plains. Strong differences of peak emissions in different regions were found across the contiguous U.S., indicating the influence of different agricultural land uses. Land use data from the United States Department of Agriculture (USDA) and emissions data from the MASAGE_NH₃ emission inventory were used to confirm the hotspots. For example, high NH₃ in the San Joaquin Valley corresponds with the dairy NH₃ emissions, while the peak in North Carolina corresponds with NH₃ emissions from swine/hogs. A case study was performed in selected 1°×1° regions to further study the seasonality and compare with the Ammonia Monitoring Network (AMoN) derived seasonality where available. The results showed a good NH₃ seasonality agreement between AMoN and IASI. In the feedlot-dominated regions, the NH₃ abundance peaks in summer and correlates with the surface air temperature profile, indicating the temperature-driven NH₃ emissions. In the selected croplands-dominated regions, a peak in spring suggests the influence of fertilizer application. These monthly variations will be compared with model outputs with MASAGE_NH₃. Further comparison will also be done with the Community Multiscale Air Quality Modeling System (CMAQ) modeling results. The results demonstrate that utilizing oversampled IASI NH₃ data can help improve NH₃ inventories deduced from top-down inversions and contribute to understanding the extremely high spatiotemporal heterogeneity of NH₃.