Tropical upper-tropospheric trends in ozone and carbon monoxide (2005–2020): observational and model results

Froidevaux, Lucien; Kinnison, Douglas E.; Gaubert, Benjamin; Schwartz, Michael J.; Livesey, Nathaniel J.; Read, William G.; Bardeen, Charles G.; Ziemke, Jerry R.; Fuller, Ryan A.

doi:https://doi.org/10.5194/acp-25-597-2025

Articles | Volume 25, issue 1

https://doi.org/10.5194/acp-25-597-2025

© Author(s) 2025. This work is distributed under
the Creative Commons Attribution 4.0 License.

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Tropospheric Ozone Assessment Report Phase II (TOAR-II) Community...

https://doi.org/10.5194/acp-25-597-2025

© Author(s) 2025. This work is distributed under
the Creative Commons Attribution 4.0 License.

Articles | Volume 25, issue 1

Research article

|

17 Jan 2025

Research article |

| 17 Jan 2025

Tropical upper-tropospheric trends in ozone and carbon monoxide (2005–2020): observational and model results

Lucien Froidevaux, Douglas E. Kinnison, Benjamin Gaubert, Michael J. Schwartz, Nathaniel J. Livesey, William G. Read, Charles G. Bardeen, Jerry R. Ziemke, and Ryan A. Fuller

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Final revised paper (published on 17 Jan 2025)
Supplement to the final revised paper
Preprint (discussion started on 11 Mar 2024)
Supplement to the preprint

Interactive discussion

Status: closed

RC1:
'Comment on egusphere-2024-525', Anonymous Referee #1, 18 Apr 2024

Citation: https://doi.org/10.5194/egusphere-2024-525-RC1
- AC2: 'Reply on RC1', Lucien Froidevaux, 19 Jul 2024
  
  The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2024/egusphere-2024-525/egusphere-2024-525-AC2-supplement.pdf
  
  Citation: https://doi.org/10.5194/egusphere-2024-525-AC2
RC2:
'Comment on egusphere-2024-525', Yann Cohen, 17 May 2024

The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2024/egusphere-2024-525/egusphere-2024-525-RC2-supplement.pdf

Citation: https://doi.org/10.5194/egusphere-2024-525-RC2
- AC1: 'Reply on RC2', Lucien Froidevaux, 19 Jul 2024
  
  The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2024/egusphere-2024-525/egusphere-2024-525-AC1-supplement.pdf
  
  Citation: https://doi.org/10.5194/egusphere-2024-525-AC1

Peer review completion

AR: Author's response | RR: Referee report | ED: Editor decision | EF: Editorial file upload

AR by Lucien Froidevaux on behalf of the Authors (19 Jul 2024) Author's response Author's tracked changes Manuscript

ED: Referee Nomination & Report Request started (06 Aug 2024) by Gabriele Stiller

RR by Anonymous Referee #3 (26 Aug 2024)

Suggestions for revision or reasons for rejection

This paper by Froidevaux et al. explores a unique opportunity to evaluate upper tropospheric ozone and carbon monoxide trends over the past 15 years based on the comprehensive Aura-MLS satellite observations and offers in addition a comparison to model simulations.

The revisions, particularly the restructuring of the manuscript in my opinion has helped remedy much of the previous reviewers’ concerns that the results are very complex and hard to grasp. I have only a few more comments the authors should consider, which I hope will further help to clarify the content before I would recommend publication of this manuscript in ACP.

Minor suggestions:

L21 Not sure why WACCM-CEDS is all of a sudden introduced. Without more explanation, this is confusing. Authors should decide on whether to use WACCM6, WACCM, or WACCM-CEDS in the abstract. I suggest just ‘WACCM’ for better readability and then to introduce and explain WACCM6 or WACCM-CEDS only in the methods section. Similarly, I suggest to delete the specification ‘(CAM-chem-CAMS simulation)’ since it is non-necessary information at this point or simply say instead ‘using emissions from the CAMS chemical reanalysis’.

L35 The discussion of these results in terms of model-measurement differences still needs the caveat that these differences are not statistically significant. The trends could be considered consistent with each other within the uncertainty bounds given a statistical z-test, and are not excluding each other as the authors imply.

L40 Small wording issue. I don’t think the way you phrase the comparison of MLS UT CO trends to previously published total column CO trends is correct. It factually is like comparing apples to oranges. It would need some caveat that these measures are implying ‘consistent, but muted behaviour’ between total column and UT trends or something alike.

L76 Suggest to use CO for Carbon monoxide, since you introduced it already in L64.

L278 This description is still hard to follow… The CMIP6 emissions are not relevant, I suggest not even mentioning them. Then simply explain. Are both CAM-chem simulations run with CAMS-GLOB-ANT_v5.1, but one of them with CO emissions from CEDS? Is WACCM run with CAMS-GLOB-ANT_v5.3 and also CO emissions from CEDS?

L284 I don’t understand why you can ‘exclude a change in CO secondary formation or sink between these two simulations…’. If the interpretation of the emissions used in the different simulations (previous comment) is correct, then differences may be due to using once v5.3 and once v5.1 that may affect the CO lifetime indirectly through chemical processing.

L298 It would be good to provide a link or reference to the CMIP6 aircraft emissions source.

L345 here you say 24S-24N, in the caption to figure 1 it is 26S-26N. Please correct. Correspondingly, it is not good practice to not have all axes labelled. Please add the titles (latitude) and tickmarks on the x-axes for rows 1 and 3 in Fig. 1, and all rows in Fig. 4.

L361 I would have expressed this more positively for MLS, i.e., that the MLS-ozonesonde differences support the finding of the models’ positive biases.

L383 I would say the difference is definitely ‘not significant’ and not ‘not very significant’.

L394 ff This is a very long-winded justification and I do agree that significance is overrated in atmospheric journal publications. However, I would be happier to see this simplified and that you just would say that while the results are not statistically significant, we still can learn about the general directions in the trends and model-observation differences. No need to address this comment, but I would like to give you and the editor a choice to simplify this section again.

L488 I really like this sensitivity analysis! What would help the reader is to add in one sentence what a positive/negative ozone-ENSO correlation means. i.e., does a positive correlation mean that during El Niño (la Niña), ozone levels increase (decrease), correct?

L587 This result (and particulary hypothesis about the dynamical influence) is consistent with the results of a recent study by Ma et al (https://doi.org/10.5194/egusphere-2023-2411, 2024). These authors find similar trends and provide in addition a model-based quantification of its drivers. In particular, stratosphere-troposphere exchange explains around 10-40% and tropospheric precursor emissions around 60-90% of the observed ozone increases in the South-East Asian tropical region. A reference could be included here to substantiate the claims by Thompson et al. and to provide additional interpretation to your results.

L645/676 Suggest to add specific years here, particularly 2016 which is an extreme example!

L742/3 please ask the typesetting people to make sure to not break between the minus sign and the trend number!

L841 delete extra space between ‘to’ and ‘increased’

L877 again I take some issue in the wording that CO trends from UT level measurements should be agreeing with total column CO trends. They *are* consistent but perhaps muted. Please rephrase.

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RR by Anonymous Referee #1 (16 Sep 2024)

ED: Publish subject to minor revisions (review by editor) (24 Sep 2024) by Gabriele Stiller

AR by Lucien Froidevaux on behalf of the Authors (24 Oct 2024) Author's response Author's tracked changes Manuscript

ED: Publish subject to technical corrections (11 Nov 2024) by Gabriele Stiller

AR by Lucien Froidevaux on behalf of the Authors (15 Nov 2024) Manuscript

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Short summary

We compare observed changes in ozone (O₃) and carbon monoxide (CO) in the tropical upper troposphere (10–15 km altitude) for 2005–2020 to predictions from model simulations that track the evolution of natural and industrial emissions transported to this region. An increasing trend in measured upper-tropospheric O₃ is well matched by model trends. We find that changes in modeled industrial CO surface emissions lead to better model agreement with observed slight decreases in upper-tropospheric CO.