| 13 May 2026
Assessment of the impact of tropical anthropogenic and biomass-burning emissions on tropospheric ozone (2007–2021) using the GEOS-Chem model constrained by satellite observations
Abstract. Ozone (O3) is a secondary species formed and is considered a hazardous pollutant in the troposphere. Over the recent decades, tropospheric O3 has undergone significant regional variations. This paper focuses on the impact of the evolution of anthropogenic and biomass-burning emissions in the tropics upon tropospheric and the chemical regimes of O3 using numerical simulations performed of the last 15-years with the GEOS-Chem model. Satellite datasets derived from OMI (HCHO and NO2) and IASI-SOFRID (CO and O3) are used as observational constraints to ensure the most reliable representation of BB and ANT emissions. The results show that the simulation (REF) combining GFAS for BB and CAMS for ANT emission provides the best match with the selected satellite observations.
The analysis of REF-simulation shows that positive trends in tropospheric ozone column (TOC) are observed mainly in the Northern Hemisphere, in Asia regions, Temperate North America, Europe, and some tropical regions (Equatorial Asia and Southern Hemisphere South America), driven by increased of ANT and BB emissions and favourable photochemical conditions.
Sensitivity tests with fixing emissions in the tropical band, reveal that the increase in tropical TOC is mainly due to tropical ANT emissions, which occur in chemical regimes nd: transparent by VOC-limited. Further results highlight that effective mitigation of future increases in TOC will depend mostly on control of ANT NOx emissions in tropical regions, where chemical conditions favour high O3 production, while underlying the marginal role of BB in moderating regional O3 levels.
Competing interests: At least one of the (co-)authors is a member of the editorial board of Atmospheric Chemistry and Physics.
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This paper has clear deficiencies regarding writing style, grammar, explanation of the goals of the paper, accurate referencing, and awareness of previously published work. The deficiencies regarding writing style, grammar, and explanation of the goals, are a nuisance because they confuse the readers and waste their valuable time. The deficiencies regarding accurate referencing and awareness of previously published work are much more problematic, because the reader has diminished confidence in the analysis and its relevance to previously published work. When considered together, these deficiencies add up to a paper that does not meet the standards of the journal. While this work could be improved to meet the standards of the journal, my recommendation to the editors is to reject the paper so that the authors can start over, and then submit a paper that would be more easily reviewed.
1) A major deficiency of this analysis is its avoidance of the impacts of the COVID-19 pandemic on emissions and global ozone production, which is puzzling given the growing body of evidence on this phenomenon and its relevance to this study’s focus on 2007-2021 ozone trends (Bouarar et al., 2021; Miyazaki et al., 2021; Ziemke et al., 2022, 2025; Putero et al. 2023; Steinbrecht et al., 2021; Clark et al., 2021; Chang et al., 2022; Van Malderen et al., 2025). Given that the research community is fully aware of the impacts of COVID-19, any reader would want to know about the impacts of COVID-19 on the present study.
Not only do the authors avoid any discussion of COVID-19, they also avoid mentioning the impact of COVID-19 on the TOAR-II combined satellite product (Hubert et al., 2026). As one of the co-authors on the submitted paper is also a co-author on Hubert et al. (2026), this author team has access to the manuscript by Hubert et al. (2026). I read the version of Hubert et al. (2026) that was made available for the TOAR-II open comment period at the end of 2025. Assuming that there have been no major changes to this paper since it was submitted for peer-review, this author team is aware of the following major finding from Hubert et al. (2026): for the period 2005-2019 the combined satellite product shows a general increase of the tropospheric ozone burden, but for the period 2020-2022 there was a sharp decrease in ozone around the globe that is at least partially associated with the COVID-19 pandemic. Therefore, when the authors state on line 665 that their continuous ozone increase for 2007-2021 is consistent with the findings of Hubert et al. (2026), I am puzzled as to how they can make this conclusion; and I am even more puzzled as to why they don’t mention the ozone decrease in 2020 that was observed by Hubert et al. (2026). This is a major discrepancy that cannot be ignored.
2) The standard of English is very poor. While I understand that the authors may not speak English as a first language, and that some degree of grammatical errors should be expected, this is no longer an excuse in the modern age of large language models (LLM), such as ChatGPT. The authors can easily run their text through a LLM to flag errors associated with grammar, spelling and awkward phrasing. I expect this would correct >95% of the errors and the paper would be much more understandable. Following are some examples of poor writing style.
Here is the very first sentence of the Abstract:
“Ozone (O3) is a secondary species formed and is considered a hazardous pollutant in the troposphere.”
“secondary species formed” is extremely awkward and also very vague. Do you mean to say that it is a secondary pollutant? Furthermore, the statement that ozone is “a hazardous pollutant in the troposphere” lacks specificity because ozone only acts as a pollutant at the surface where it comes into contact with humans and vegetation.
The topic of this sentence is then repeated in the first sentence of the Conclusions:
“Tropospheric O3 is a secondary pollutant produced by the photochemical oxidation of VOCs in the presence of NOx.” This sentence is accurate and well written, so it is strange that the first sentence of the Abstract is so poorly written.
Another example appears on line 48: “The overall increase in O3 in the troposphere therefore seems mainly coming from the tropics”.
Line 22, poor sentence structure: “Gaudel et al. (2018) highlighted that tropospheric O3 is the source of important discrepancies between observations and models, particularly in the tropics.”
Ozone is not the source of the discrepancies, which would be due to shortcomings of either the observations or the models, or both.
3) The Introduction lacks organization, and comes across as a collection of disjointed sentences that drift from one topic to another. For example, the fifth paragraph in the Introduction (line 48) has only 4 sentences, but jumps around between three different topics. It starts off saying that increasing ozone is driven by the tropics. It then mentions a TOAR-II working group, which is not mentioned again in the paper, so it’s not clear why this statement is made. It then vaguely discusses the use of satellites for monitoring ozone and precursors globally.
4) I found it difficult to grasp the purpose of this study. The paragraph on line 69 starts off talking about IASI SOFRID satellite products, and then abruptly shifts to stating, “The main purpose of this study is to assess the impact of the evolution of two surface emissions in the tropics upon tropospheric and chemical regimes of O3”, but this sentence is so poorly written that I could not understand the meaning of “two surface emissions”. I had to read the Methods section to finally figure out that “two surface emissions” refers to anthropogenic and biomass burning emissions inventories.
5) I found several instances of inaccurate referencing, which erodes my confidence in this analysis.
For example:
In the second sentence of the Conclusions the authors state:
“Zhang et al. (2016) highlighted that, over the last decade, emissions of O3 precursors have shifted from the mid-latitudes toward equatorial regions, causing to an increase in global tropospheric O3 burden, as tropical regions provide circumstances that favour O3 photochemistry.”
This statement claims that Zhang et al. addressed the “last decade”, which would be 2015-2025, but the paper was published in 2016, so obviously this can’t be true. Furthermore, a quick look at the title of Zhang et al. (2016) shows that it focused on the period 1980-2010, which is not the last decade (not even close).
Another example:
Lines 26-27, “In tropics, strong solar radiation, high temperatures, and abundant biogenic VOC emissions enhance photochemical activity, resulting in O3 formation features that are different from those observed in mid-latitude regions (Jacob and Winner, 2009).”
This sentence claims that Jacob and Winner (2009) discuss the differences in ozone production between the tropics and mid-latitudes, but this is not true. Jacob and Winner (2009) do not discuss this topic and a quick word search shows that the words “tropics” or “tropical” don’t even appear in Jacob and Winner (2009).
6) Many references are out of date, which indicates a lack of knowledge of current research. For example, when discussing the impacts of ozone on human health (line 17) the authors cite 3 studies, all more than 20 years old. There are plenty of recent studies with more robust results than these older studies, and one wonders why they were not cited. In particular, the reference to WHO (2003) is odd because this an old and relatively obscure document (the citation in the list of references is also very odd as it repeats the same information twice). In contrast, the recent WHO report from 2021 is up-to-date and very well known across the atmospheric sciences community (according to Google Scholar it has been cited over 4600 times):
WHO global air quality guidelines. Particulate matter (PM2.5 and PM10), ozone, nitrogen dioxide, sulfur dioxide and carbon monoxide. Geneva: World Health Organization; 2021. License:
CC BY-NC-SA 3.0 IGO.
References:
Bouarar, I, et al. (2021), Ozone Anomalies in the Free Troposphere During the COVID-19 Pandemic, GRL, 48, e2021GL094204, https://doi.org/10.1029/2021GL094204
Chang, K, et al. (2022), Impact of the COVID-19 economic downturn on tropospheric ozone trends: an uncertainty weighted data synthesis for quantifying regional anomalies above western North America and Europe, AGU Advances, https://doi.org/10.1029/2021AV000542
Clark, H, et al. (2021), The effects of the COVID-19 lockdowns on the composition of the troposphere as seen by In-service Aircraft for a Global Observing System (IAGOS) at Frankfurt, ACP, 21, 16237–16256, https://doi.org/10.5194/acp-21-16237-2021
Miyazaki, K, et al. (2021), Global tropospheric ozone responses to reduced NOx emissions linked to the COVID-19 worldwide lockdowns, Sci. Adv., 7, eabf7460, https://doi.org/10.1126/sciadv.abf7460
Putero, D, et al. (2023), Fingerprints of the COVID-19 economic downturn and recovery on ozone anomalies at high-elevation sites in North America and western Europe, Atmos. Chem. Phys., 23, 15693–15709, https://doi.org/10.5194/acp-23-15693-2023
Steinbrecht, W, et al. (2021), COVID‐19 Crisis Reduces Free Tropospheric Ozone Across the Northern Hemisphere, GRL, 48, https://doi.org/10.1029/2020GL091987
Van Malderen, R, et al (2025), Ground-based tropospheric ozone measurements: regional tropospheric ozone column trends from the TOAR-II/HEGIFTOM homogenized datasets, ACP, 25, 9905–9935, https://doi.org/10.5194/acp-25-9905-2025
Ziemke, J, et al (2022), NASA satellite measurements show global‐scale reductions in free tropospheric ozone in 2020 and again in 2021 during COVID‐19, GRL 49, e2022GL098712.
Ziemke, J, et al. (2025), Ten years of tropospheric ozone from DSCOVR EPIC: science and applications, Frontiers in Remote Sensing, 6, pp1634922