Global and diurnal variations in tropospheric ammonia observed from a constellation of hyperspectral infrared sounders in three different LEO orbits

As a reactive nitrogen compound, atmospheric ammonia (NH3) plays a key role in the global nitrogen cycle. Tracking the spatiotemporal dynamics of NH3 is essential for quantifying its emissions and deposition and for informing the regulation of anthropogenic emission sources. Currently, the diurnal cycle of NH3 remains under-constrained, particularly in regions lacking geostationary satellite observations, which poses a challenge to accurate emission quantification. To address this gap, we construct an integrated constellation to achieve quasi-geostationary-like global monitoring coverage, comprising China's FengYun-3 (FY-3) series satellites and the Cross-track Infrared Sounder (CrIS). FY-3E operates in a dawn-dusk orbit with an equatorial overpass time of 05:30/17:30 (Local Solar Time, LST), while FY-3F operates in a mid-morning orbit with an overpass time of 10:00/22:00 LST. Both satellites are equipped with the second-generation High Spectral Infrared Atmospheric Sounder (HIRAS-II). CrIS, with an overpass time of 01:30/13:30 LST, provides supplementary observations in an afternoon orbit. In this study, hyperspectral infrared observations from the constellation are utilized to retrieve global NH3 columns based on the optimal estimation method. Six global NH3 maps are retrieved per day at approximately 4 h intervals. The retrieval results for four representative weeks in different seasons of 2024, as a demonstration, show elevated columns in major source regions, including the North China Plain, the Indo–Gangetic Plain, North America and Western Europe. In addition, the diurnal and seasonal cycles of NH3 over these regions using all observations in 2024 are also investigated. The constellation captures the diurnal (every 4 h) and seasonal cycles of NH3 columns, effectively mitigating the constraints in regions without geostationary observations. Further cross-comparisons with the Geostationary Interferometric Infrared Sounder (GIIRS) retrievals and ground-based Fourier transform infrared spectroscopy (FTIR) measurements demonstrate that the polar-orbiting constellation retrievals are internally consistent and broadly consistent with independent observations, supporting their capability to capture major NH3 spatial patterns and temporal variability. The sensitivity of NH3 detection in the lower atmosphere, quantified by the column averaging kernel (AVK), exhibits diurnal variability linked to thermal contrast, defined as the temperature difference between the surface and the lowest atmospheric layer. This study demonstrates the capability of an integrated constellation comprising FY-3E/HIRAS-II in a dawn-dusk orbit, FY-3F/HIRAS-II in a mid-morning orbit, and CrIS in an afternoon orbit to monitor global NH3 distributions and diurnal variability at an unprecedented six local times per day. This approach highlights the potential of polar-orbiting meteorological satellites to enhance the global climate monitoring.