| 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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The manuscript 'Global and diurnal variations in tropospheric ammonia observed from a constellation of hyperspectral infrared sounders in three different LEO orbits' by Hua et al. presents an evaluation of the global spatial and diurnal variability of NH3 based on observations from the HIRAS-II instruments aboard the Chinese FY-3E and FY-3F satellites along with observations from CrIS on JPSS-1.
The authors utilize a harmonized retrieval approach and the differing overpasses of these three satellites to provide unique insights into the diurnal variability of NH3 which remains broadly uncertain and was previously challenging to capture globally from the existing polar-orbiting instruments. The NH3 observations from these three satellites are combined to yield a quasi-geostationary dataset with observations made roughly every 4 hours throughout the day. Furthermore, a comparison is performed of each polar-orbiting dataset against true geostationary NH3 observations from GIIRS, which serves to provide a consistent intercomparison point. The authors demonstrate that their quasi-geostationary constellation is capable of resolving the seasonal and diurnal variability globally and in several high-emission regions of interest.
Overall, the manuscript is well written and the results are presented in a clear manner. I believe that the manuscript fits well within the scope of AMT. There are some sections of the manuscript that can be improved both in terms of clarity and level of detail, but after these revisions are made, I recommend it for publication.
Major comments:
For example, on L127 - L128, the authors state "A profile scaling approach is used to retrieve a single scale factor applied to a fixed a priori NH3 profile." and later that "A fixed a priori profile instead of a variable one is adopted, so that any spatiotemporal changes in retrieved
NH3 column concentrations reflect only information from satellite observations, rather than variations in the prior". However the authors do not describe how the initial prior profile was selected. Additionally, using the same fixed profile shape for both daytime and nighttime retrievals would likely bias the retrieved ammonia total column high or low.
Since this study being considered for AMT and is also partly a presentation of these harmonized retrievals and with a modified approach from previous studies, I think more details should be provided on, for example, the choice of a priori profile shape and how much that impacts the resulting total columns (and potentially also the diurnal cycles). This could be included in the appendix, but this is important to support the robustness of the remainder of the results presented in the manuscript.
Furthermore, a comparison of the retrievals with a ground-based dataset like an FTIR such as the one at Hefei (https://doi.org/10.1016/j.atmosenv.2022.119256) could strengthen the case for the satellite retrievals even more. This could be included as part of Section 3.3 and leads me to my next comment.
The paper places a significant focus on seasonal variability, but the comparisons presented between FY-3E, FY-3F and CrIS versus FY-4B/GIIRS are only shown on the yearly scale. It would be valuable to also show the comparisons monthly and/or seasonally to help identify whether there are any seasonal biases or inconsistencies across the satellite products, as these may be covered up or averaged out when looking at the correlations at the yearly scale.
Additionally, there is a pretty consistent slope on the order of 0.86-0.89 in almost all of the comparisons of FY-3E, FY-3F and CrIS with GIIRS but this is not discussed. Can the authors provide some potential explanations? Is this due to some systematic bias in the GIIRS product and/or the products retrieved in this study?
Providing more information on the impact of the filters is important, particularly since the paper presents night-time retrievals which typically are far more challenging due to poorer observational conditions, and these filters likely impact the data density. Additionally, it seems a different filtering criterion is applied in Section 3.2, but it is not immediately clear to me why a different criterion is necessitated and also if it is applied on top of the earlier criterion or not.
By limiting it to single weeks, the results are more subject to things such as transient events (e.g., biomass burning, manure application etc) that could skew or bias the seasonal results. This is already apparent in the results, for example the large wildifire over North America in July. I suggest revising the seasonal analysis to include averages over more months. Later on in Section 3.2 full months are seemingly used, so this would make it more consistent with that Section and the results there as well.
Minor comments:
differences in emission intensity and atmospheric chemistry." but anthropogenic activities and differences in emission intensity are quite directly linked here for NH3 in most cases no?