Optimal estimation retrieval of tropospheric ammonia from the Geostationary Interferometric Infrared Sounder on board FengYun-4B

Abstract. Atmospheric ammonia (NH3) is a reactive nitrogen compoundthat pollutes our environment and threatens public health. Monitoring thespatial and temporal variations is important for quantifying its emissionsand depositions and evaluating the strategies for managing anthropogenicsources of NH3. In this study, we present an NH3 retrievalalgorithm based on the optimal estimation method for the GeostationaryInterferometric Infrared Sounder (GIIRS) on board China's FengYun-4Bsatellite (FY-4B/GIIRS). In particular, we examine the information contentbased on the degree of freedom for signal (DOFS) in retrieving the diurnalNH3 in East Asia, with a focus on two source regions including the NorthChina Plain and North India. Our retrieval is based on the FengYunGeostationary satellite Atmospheric Infrared Retrieval (FY-GeoAIR) algorithmand exploits the strong NH3 absorption window of 955–975 cm−1.Retrieval results using FY-4B/GIIRS spectra from July to December 2022 showthat the DOFS for the majority ranges from 0 to 1.0, mainly depending on the thermal contrast (TC) defined as the temperature difference between thesurface and the lowest atmospheric layer. Consistent with retrievals fromlow-Earth-orbit (LEO) infrared sounders, the detection sensitivity, asquantified by the averaging kernel (AK) matrix, peaks in the lowest 2 kmatmospheric layers. The DOFS and TC are highly correlated, resulting in atypical “butterfly” shape. That is, the DOFS increases when TC becomeseither more positive or more negative. The NH3 columns from FY-4B/GIIRSexhibit significant diurnal cycles that are consistent with the day–nightgradient from the collocated IASI retrievals in the North China Plain and NorthIndia for the averages in July–August, September–October, andNovember–December, respectively. A collocated point-by-point intercomparisonwith the IASI NH3 dataset shows generally good agreement with a smallsystematic difference in the summer months that may be attributed to theslight difference in a priori profiles. This study demonstrates thecapability of FY-4B/GIIRS in capturing the diurnal NH3 changes in EastAsia, which will have the potential to improve regional and global airquality and climate research.