Ammonia emissions and depositions over the contiguous United States derived from IASI and CrIS using the directional derivative approach

Abstract. Atmosphere ammonia (NH₃), primarily emitted from agriculture, poses significant threats to ecosystems, climate, and human health through nitrogen deposition and secondary aerosol formation. NH₃ flux estimates remain highly uncertain due to limited direct observations and complex emission–deposition processes. Here, we estimated NH₃ fluxes over the contiguous United States using satellite observations from the Infrared Atmospheric Sounding Interferometer (IASI, 2008–2022) and Cross-track Infrared Sounder (CrIS, 2012–2022). By applying a directional derivative approach, we minimized the impact of offsets in satellite-derived vertical column densities. Our results highlight major agricultural emission hotspots, including the San Joaquin Valley in California, the Snake River Valley in Idaho, the Texas panhandle, the Great Plains, Southeastern Pennsylvania, and Eastern North Carolina. NH₃ removal was predominantly driven by deposition near sources rather than chemical transformation, with strong sinks in vegetation-dense regions such as forests, grasslands, shrublands, and wetlands. Seasonal flux variations showed peaks in warm months and lower values in winter, driven by temperature-dependent volatilization from livestock production and fertilizer application. Satellite-based estimates aligned well with bottom-up inventories, effectively capturing spatial and seasonal patterns while revealing additional insights into key flux hotspots and peak seasons. CrIS consistently reported higher fluxes than IASI, especially in spring, reflecting differences in their overpass times. Combining IASI (morning overpass) and CrIS (midday overpass) observations enables a better understanding of diurnal NH₃ flux dynamics. These findings provide critical insights into NH₃ spatiotemporal variabilities, complementing inventory-based approaches and informing nitrogen management and environmental policy, particularly in regions with limited ground-based monitoring.