the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 25 Nov 2025
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.
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- RC1: 'Comment on egusphere-2025-5699', Anonymous Referee #1, 28 Jan 2026 reply
Data sets
FengYun-4B/GIIRS FYGeoAIR NH3 retrievals from July 2022 to June 2025 Zeng, Z.-C https://doi.org/10.5281/zenodo.17193848
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- 1
This study introduces the first multi-year dataset of geostationary ammonia observations from the FengYun-4B satellite. A comprehensive evaluation of the dataset is presented through comparison with polar orbiting satellites, as well as ground-based FTIR observations. The peak months of ammonia over East Asia are also examined, revealing an interesting pattern related to agricultural emissions. The uniqueness of the data is highlighted by investigating diurnal ammonia variations from early morning to afternoon. Large discrepancies between observations and modelling, as revealed by the data, show that modelling still needs to improve its ability to capture diurnal ammonia variations. Overall, the paper is well structured and clearly written. The retrieval data are unique, and the findings are novel. Given that the IRS, which overlooks Europe and Africa, has recently been launched, and that more geostationary hyperspectral sounders may be in the planning stage, this study is timely. The paper fits the scope of ACP and can be published once the following minor issues have been addressed.
(1) In the introduction section, atmospheric chemistry modelling, such as GEOSchem, has been widely used to study ammonia at global and regional scales. Several studies related to improving NH₃ modelling are suggested for addition.
(2) Figure 1. Why do the observation domains have curved edges? Perhaps the domain has been filtered by viewing zenith angle. Please clarify.
(3) L166. Using the TC threshold may affect the results. Here, 5K is used. Please describe how much data can be retained after this filtering. Perhaps a histogram of the data numbers for different TCs could be made.
(4) Figure 2: The spatial maps for 7–9 h and 17–19 h show some unrealistic edges that look odd. Are these due to the different observation hours? Please explain and see if this can be mitigated.
(5) The agreement in Figure 5 looks very good. In Southeast Asia, there are strong biomass burning emissions every spring. How might this affect the peak? Currently, I see that the majority of the region peaks in March and April.
(6) The MIX inventory data in L315-316 can differ greatly from the observations. Could you provide one or two more references that use the MIX inventory? How reliable is this inventory?
(7) The number of data points in Figure 8 varies considerably between satellites, especially for CrIS, which has many fewer data points. Please explain.
(8) In Section 3.3, the Sichuan Basin is an interesting case, as the topographic effect can be significant. L454-458 does not provide much background information on this region. I suggest adding some references to studies of NH₃ in this region. What is the current understanding of ammonia emissions over this region from polar orbiting satellites?
(9) L530: FY-4C/GIIRS was launched in late 2025. This can be reflected in the rephrases. Could you also provide more details on FY-4C/GIIRS and explain what improvements it offers over 4A and 4B GIIRS?
(10) Please also provide a description of the IRS onboard the MTG, which was launched in 2025.