| 16 Feb 2026
Global and diurnal variations in tropospheric ammonia observed from a constellation of hyperspectral infrared sounders in three different LEO orbits
Abstract. As a reactive nitrogen compound, atmospheric ammonia (NH3) plays a key role in the global nitrogen cycle. Tracking the spatiotemporal dynamics of NH3 is crucial to quantify its emissions and depositions, as well as offering insights to inform 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 equatorial overpassing time at 05:30 am/pm, while FY-3F operates in a mid-morning orbit with overpassing time at 10:00 am/pm. Both are equipped with the second-generation High Spectral Infrared Atmospheric Sounder (HIRAS-II). CrIS, operating with overpassing time at 01:30 am/pm, 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 maps of global NH3 for every 4-hour in each day are retrieved. The retrieval results in four weeks of different seasons in 2024, as a demonstration, show elevated columns in global major source regions, including Western Europe, North America, North China Plain and North India. In addition, the diurnal and seasonal cycles of NH3 over these regions using all observations in 2024 are also investigated. The constellation reasonably captures the diurnal (every 4-hour) and seasonal cycles of NH3 columns, effectively mitigating the constraints in regions without geostationary observations. Consistency of the retrievals among different satellites is demonstrated by comparing with geostationary observations from the Geostationary Interferometric Infrared Sounder (GIIRS). The sensitivity of NH3 detection in the lower atmosphere as quantified by the column averaging kernel (AVK) from the retrieval shows diurnal variations that dependent on thermal contrast, defined as the temperature difference between the surface and the lower atmospheric layer. This study demonstrates the capability of the integrated constellation, comprising FY-3E/HIRAS-II (dawn-dusk), FY-3F/HIRAS-II (mid-morning), and CrIS (afternoon), to monitor global and diurnal NH3 variations at unprecedentedly six distinct times of a day, and has the potential to enhance the global climate-monitoring capacity of polar-orbiting meteorological satellites.
Competing interests: The co-author Z.-C. Zeng is a member of the editorial board of journal AMT.
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