Spatiotemporal variability of ammonia as observed from space: global patterns and regional insights from hotspots

For decades, our understanding of the atmospheric distribution of ammonia has relied on a combination of in situ measurements, satellite remote sensing observations, and emission-driven atmospheric chemistry-transport model results.  However, the limitations of these approaches, including sparse spatial distribution of in-situ measurements and underutilization of satellite data due to challenges in validating column observations against surface measurements, motivate further investigation of the global spatiotemporal variability of atmospheric ammonia. In this study, we jointly analyze observations from two well-validated infrared satellite instruments, IASI and CrIS, which provide near-global coverage and whose different overpass times (IASI at around 09:30 and CrIS at around 13:30 local time) yield complementary information on diurnal variability.  We additionally examine the time-resolved ammonia concentrations from surface stations at selected locations within ammonia hotspot regions and assess how satellite observations compare over the same time frame. The results show that the magnitude of IASI-CrIS differences varies spatially. We investigate four factors that could impact the intercomparison -sensor sensitivity, seasonality, overpass time, and land-use characteristics- and find that their relative influence differs by location. At larger spatial scales, major global ammonia hotspots exhibit heterogeneous temporal behavior despite broadly consistent increasing decadal trends.  Beyond these long-term trends, we investigate the seasonal variability of ammonia at the regional scale and examine how climatological and regional characteristics shape the observed patterns.