the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 07 May 2025
Large and increasing stratospheric contribution to tropospheric ozone over East Asia
Abstract. Severe surface ozone pollution in South Korea and China in May–June is due in part to an elevated background subsiding from the free troposphere (750–350 hPa). Using IAGOS commercial aircraft observations and the GEOS-Chem model, we show that free tropospheric ozone over East Asia in May–June is the highest in the world and has increased from 68±3 ppb (mean and interannual standard deviation) in 2000–2004 to 78±4 ppb in 2015–2019. Free tropospheric ozone over East Asia is highest when carbon monoxide (CO) is low, both in the observations and GEOS-Chem, implying a large stratospheric influence on ozone. We find from GEOS-Chem that East Asia is a global hotspot for stratospheric downwelling of ozone and that this makes a major contribution to the free tropospheric ozone over the region in May–June. Stratospheric downwelling of ozone over East Asia in GEOS-Chem increased by 40 % from 2000–2004 to 2015–2019, which can explain the observed free tropospheric ozone increase over this period. Increased stratospheric downwelling over East Asia appears to be driven by a strengthening of the jet stream. The large and increasing stratospheric contribution to the surface ozone background over East Asia is a major impediment to meeting ozone air quality standards.
Competing interests: Kelvin Bates is a member of the ACP editorial board.
Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims made in the text, published maps, institutional affiliations, or any other geographical representation in this paper. While Copernicus Publications makes every effort to include appropriate place names, the final responsibility lies with the authors. Views expressed in the text are those of the authors and do not necessarily reflect the views of the publisher.- Preprint
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RC1: 'Comment on egusphere-2025-1799', Anonymous Referee #1, 02 Jun 2025
“Large and increasing stratospheric contribution to tropospheric ozone over East Asia” by Colombi et al.
This study explores the origin of elevated free tropospheric ozone over East Asia in May-June. Using a combination of aircraft observations and GEOS-Chem model simulations, the authors attribute the elevated free tropospheric ozone largely to increased stratospheric downwelling, which has intensified over the past two decades. Tagged ozone simulations are performed to further support the substantial stratospheric contribution to both free tropospheric and boundary layer ozone.
This study addresses a scientifically important topic. However, the current manuscript suffers from lack of clarity. In particular, the methodology is not always clearly described, making it difficult to assess the robustness of the modeling and attribution approaches. Additionally, some of the key results are not sufficiently analyzed or discussed in the context of existing literature. Greater emphasis on quantitative evaluation and clearer explanation of modeling assumptions would significantly strengthen the manuscript.
Therefore, I recommend a major revision before the manuscript can be reconsidered for publication. The authors should improve the clarity of the methodological descriptions, provide a more thorough analysis of the results, and better contextualize their findings within the broader body of research.
Specific comments are provided below.
Major comments:
Line 76: The statement “Free tropospheric ozone in East Asia is higher than in Europe or the United States for all seasons” appears to be inconsistent with the data shown in Figure 1. Specifically, free tropospheric ozone over East Asia is lower than that over the United States during July-August for both 2000-2004 and 2015-2019, and also lower than over Europe for July-August 2000-2004. Additionally, both Europe and the United States show a decreasing trend in free tropospheric ozone during July-August between these two time periods. Can you clarify these?
Line 93: The increase in ozone under low CO conditions in 2015–2019 is attributed to enhanced stratospheric influence. Could alternative explanations such as long-range transport from relatively cleaner region, also be consistent with the observed pattern? Have the authors considered using additional diagnostics (e.g., water vapour, tropopause height) to more definitively identify stratospheric intrusions? While the authors include annual mean vertical advective flux of ozone at 100 hPa, this does not isolate the specific period of interest. Given that both stratosphere-troposphere exchange (STE) and tropospheric chemistry exhibit strong seasonal variability, a more targeted analysis may strengthen the conclusions.
Line 94: It is mentioned that the ozone drop at CO < 70 ppb likely reflects tropical air. Are there supporting evidences, e.g., trajectory analysis or meteorological conditions to support this interpretation?
Line 95: Here the lack of ozone enhancement under high CO is attributed to elevated background free tropospheric ozone. This is a plausible interpretation, but it would benefit from further support. Could the authors strengthen this explanation by including additional tracer analyses (e.g., NOx) or by using the chemical transport model better differentiate the influence of biomass burning versus other anthropogenic sources? This would enhance confidence in the attribution and clarify the role of different emission sectors.
Do the findings agree or contrast with other recent studies of East Asian outflow and free tropospheric ozone? It will be helpful if the authors contextualize their observations within broader literature trends.
Line 135: The current text states that the model "is consistent" with IAGOS but does not specify the degree of agreement or uncertainty. This weakens the validation claim. Could the authors provide quantitative metrics, e.g., NME, RMSE, correlation coefficient?
Line 138: The sentence "we find in sensitivity simulations that it does not contribute significantly to the maximum over East Asia" is vague. It lacks detail on methodology and assumptions. Please clarify what emissions or processes were perturbed in these simulations to isolate the contribution?
Line 169: Can the authors clarify how the tagged ozone simulation replicates full-chemistry ozone? Specifically, what are the assumptions behind using archived odd oxygen production and loss frequencies, and what limitations does this impose? Since tagged ozone simulations neglect non-linear interactions in ozone chemistry, how do the authors ensure that important non-linear processes are not introducing biases in attribution?
How sensitive are the results to the definition of the odd oxygen family? Since Bates et al. (2020) show that expanding odd oxygen to include HOx species significantly increases stratospheric attribution, why was the expanded odd oxygen approach not used in this study?
Minor comments:
Line 33: Can you provide details on how surface ozone concentrations in the two countries compare to those in other Asian countries and the rest of the world?
Line 37: “Understanding the role of both domestic……”is it domestic or anthropogenic?
Line 51: Which months do you refer as late spring? It’s a bit unclear whether the late spring “maximum” itself is increasing, or if free tropospheric ozone level in general is increasing over time. Consider splitting or rephrasing the sentence to improve readability.
Line 56: I think it's better to use "decadal change" or "long-term change" instead of multi-decadal change, unless you're explicitly covering more than 20 years.
Line 65: Could you clarify how representative the IAGOS aircraft data are for the selected regions, e.g., sampling frequency? Were any data quality filters or uncertainty thresholds applied to the raw IAGOS data before averaging and trend analysis?
I feel it would be better to provide a table showing the number of IAGOS profiles of ozone and CO for each month during the study period over East Asia, Europe, and the United States.
Line 77: You mention an increase of up to 15 ppb in East Asia. How does the change compare to interannual variability of free tropospheric ozone?
Figure 1, line 83: “…..with numbers of profiles in italics.” Are those the average number of flights per month for the entire period (March-April, May-June etc.) or the total number of flights for that period?
Line 97: In this study, high ozone with low CO is not seen over Europe or the U.S. Could regional meteorology or emissions trends explain the differences?
Line 139: Could you add more detail about the KORUS-AQ campaign and expand on its relevance to the analysis?
Line 140: The term "PBL" is not clearly defined in the text. Based on the caption of Figure 4, the PBL appears to be considered as the layer from the surface to 750 hPa. This should also be included in the text.
Line 141: Have you considered validating the simulated CO mixing ratio against available observations?
Technical corrections:
Line 51: "Chen at al." → "Chen et al."
Figure 7, Line 211: “….interannual standard deviations, The right…” → “….interannual standard deviations. The right…”
Citation: https://doi.org/10.5194/egusphere-2025-1799-RC1 -
CC1: 'Comment on egusphere-2025-1799', Jun Meng, 12 Jun 2025
Hi authors, great manuscript! I noticed a potential citation error on line 115. If you did not implement the coarse dust particle size distribution in GEOS-Chem yourselves, the correct reference should be Meng et al. (2021), GMD (https://gmd.copernicus.org/articles/14/4249/2021/). The one currently cited—Meng et al. (2021), GRL (https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2021GL097287) - may not be the appropriate source in this context.
Citation: https://doi.org/10.5194/egusphere-2025-1799-CC1 -
RC2: 'Comment on egusphere-2025-1799', Anonymous Referee #2, 16 Jun 2025
Colombi et el. have studied the alarming rise in the pre-monsoon free tropospheric ozone over east Asia based on aircraft observations and estimated the stratospheric contribution using GEOS-Chem modelling. The problem is important and the work has been carried out in a well-defined manner. The submitted manuscript, hence, deserves publication in Atmospheric Chemistry and Physics. I have the following queries that the authors need to address in their revised version of the manuscript.
- All the vertical profiles of ozone mixing ratios show a visible sharp increment above the 350 hPa pressure level, except one. The profile in East Asia during July-August shows minimal change in mixing ratio during both periods (2000-2004 and 2015-2019). This distinct behaviour deserves some discussion.
- The authors show, in Figure 2, that concentration of ozone is highest under low CO conditions during 2015-2019 period, which was absent during 2000-2004 period. They describe this hike resulting due to increasing stratospheric contribution. However, this phenomenon has been shown in the figure only for May-June. The authors need to include the March-April and July-August data in the figure along with related discussion and explanation to show if the increased stratospheric contribution is observed there or not.
- In addition, the statement “The ozone drop for the lowest CO values (< 70 ppb) likely reflects tropical air.” in Line 94 is a very tentative attempt to describe a critical behaviour that the authors are trying to address. They need to provide a more concrete explanation for the sudden drop in ozone concentration with proper reasoning and support.
- The authors state “From the expanded odd oxygen family perspective, much of the free tropospheric production over East Asia in Figure 6 could be of stratospheric origin.” in Line 186-7. Can they provide a brief discussion in this regard with some quantitative estimation?
Citation: https://doi.org/10.5194/egusphere-2025-1799-RC2
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