Observing Tropospheric Carbon Monoxide and Ammonia from the Geostationary Interferometric Infrared Sounder (GIIRS) onboard FengYun-4B

The Geostationary Interferometric Infrared Sounder (GIIRS) onboard FengYun-4 series satellites is the world’s first geostationary hyperspectral infrared sounder. With hyperspectral measurement covering the carbon monoxide (CO) and ammonia (NH₃) absorption windows around 2150 cm^-1 and 9150 cm^-1, respectively, GIIRS provides a unique opportunity for monitoring the diurnal variabilities of atmospheric CO and NH₃ over East Asia. In this study, we develop the FengYun Geostationary satellite Atmospheric Infrared Retrieval (FY-GeoAIR) algorithm to retrieve the CO and NH₃ profiles from FY-4B/GIIRS data and provide CO and NH₃ maps at a spatial resolution of 12 km and a temporal resolution of 2 hours. The performance of the algorithm is first evaluated by conducting retrieval experiments using simulated synthetic spectra. The result shows that the GIIRS data provide significant information for constraining CO and NH₃ profiles. The degree of freedom for signal (DOFS) and retrieval error are both significantly correlated with thermal contrast (TC), the temperature difference between the surface and the lower atmosphere. Retrieval results from one month of GIIRS spectra in July 2022 show that the DOFS for the majority is between 0.6 and 1.2 for the CO total column and between 0 and 1.0 for the NH₃ total column. Consistent with retrievals from low-earth-orbit (LEO) infrared sounders, the largest observation sensitivity, as quantified by the averaging kernel (AK), is in the free troposphere for CO and in the lower troposphere for NH₃. The diurnal changes in DOFS and vertical sensitivity of observation are primarily driven by the diurnal TC variabilities. This study demonstrates the capability of GIIRS in observing the diurnal CO and NH₃ changes in East Asia, which will have great potential in improving local and global air quality and climate research.