Geostationary observations of atmospheric ammonia over East Asia: spatio-temporal variations revealed by three years of FY-4B/GIIRS measurements

Abstract. Satellite observations play a crucial role in quantifying ammonia sources by capturing large-scale variations of atmospheric NH₃ concentrations. As the world's first geostationary hyperspectral infrared sounder, the Geostationary Interferometric Infrared Sounder (GIIRS) on board China's FengYun-4 satellite series provides a unique opportunity to monitor the diurnal cycle of NH₃. Using NH₃ retrievals between July 2022 and June 2025, this study investigates the spatio-temporal variability of NH₃ columns over East Asia, with a focus on daytime variations (07:00–19:00 local time) in major agricultural regions. Inter-comparison with polar-orbiting IASI and CrIS data shows that GIIRS NH₃ retrievals are consistent in capturing the spatial patterns and temporal dynamics. The NH₃ peaks occur between March and July, with the timing shifting from north to south, reflecting regional differences primarily driven by agriculture activities. Validation with ground-based FTIR measurements at Hefei in eastern China demonstrates the accuracy of GIIRS NH₃, with a correlation coefficient of 0.77 and an RMSE of 9.67 × 10¹⁵ molec/cm², while reproducing daytime variations observed by FTIR. For major agricultural areas, the NH₃ columns generally increase from early morning to late afternoon, reaching 1.10–1.56 times morning levels in summer and spring. Compared with GEOS-CF model simulations, the results reveal pronounced discrepancies in spatial distributions over the Sichuan Basin in southwestern China. These findings highlight the valuable capability of FY-4B/GIIRS in identifying and tracking daytime dynamics of NH₃ sources over East Asia, offering new insights beyond current low-Earth orbit (LEO) instruments.