the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 30 Jun 2023
Opinion: Stratospheric Ozone – Depletion, Recovery and New Challenges
Martyn Chipperfield
Slimane Bekki
Abstract. We give a personal perspective on recent issues related to the depletion of stratospheric ozone and some newly emerging challenges. We first provide a brief review of historic work on understanding the ozone layer where we highlight some work from the late Paul Crutzen as a contribution to the special issue in his honour. We then review the status of ozone recovery from the effects of halogenated source gases and discuss the undoubted effectiveness of the Montreal Protocol and its challenges from renewed production of controlled substances and short-lived uncontrolled substances. We then discuss, in some detail, the recent observations of ozone depletion through injection of smoke particles from Australian fires in early 2020. Further unexpected perturbations to the ozone layer are occurring at the moment through injection of very large amounts of water vapour (and some sulphur dioxide) from the Hunga Tonga-Hunga Ha`apai volcano in January 2022. We conclude with some thoughts on the urgent need to ensure continuity in observations and on how to exploit ever more complex and expensive models. Overall, the stratospheric ozone layer continues to produce novel research challenges and reveal more processes that threaten this essential component of the Earth system.
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Martyn Chipperfield and Slimane Bekki
Status: final response (author comments only)
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CC1: 'Comment on egusphere-2023-1409', Albert Ansmann, 18 Jul 2023
The article on the stratospheric ozone layer (including recent research activities) comes to the right time after all the strong and partly record-breaking ozone depletion events during the last years, especially in polar regions (Arctic, spring 2020, and Antarctica, springs of 2020 and 2021). Because written by renowned authors, this article will be read and considered in future work and publications by many researchers. This fact especially motivated us to write this comment.
The new wildfire smoke aspect (impact on ozone depletion over the mid latitudes as well as over polar regions) must be considered in all detail in such an ozone survey article. However, the smoke impact on ozone depletion over the polar regions is left out.
We (Kevin Ohneiser and Albert Ansmann, TROPOS, Leipzig, Germany) were quite surprised that you did not include our recent publications on Siberian and Australian UTLS wildfire smoke over the Arctic (Ohneiser et al., ACP, 2021, MOSAiC expedition) and in the southern hemisphere (Ohneiser et al, ACP, 2022, Punta Arenas, Southern Chile) in your discussion. In Ohneiser et al. (2021), we pointed, in the literature for the first time, to a potential contribution of stratospheric wildfire smoke to ozone depletion in polar regions. In a comment in Science, Paul Voosen (P. Voosen, High-flying wildfire smoke poses potential threat to ozone layer, doi = 10.1126/science.acx9655) highlighted this hypothesis in a short report.
In Ohneiser et al. (ACP, 2022), we clearly showed the Australian wildfire smoke impact on Antarctic ozone depletion. In Ansmann et al. (ACP, 2022), we provided a deep and quantitative overview of ozone depletion in the presence of wildfire smoke over Antarctica as well as over the Arctic. Ozone depletion over the polar regions was found to be much stronger (20%-30% additional ozone loss over Antarctica caused by wildfire smoke) than over the mid latitudes (a few percent ozone reduction) as discussed by, e.g., Solomon et al. (2022).
Over Antarctica, the strong springtime ozone reduction was observed during two consecutive years 2020 and 2021 (very unusual that strong springtime ozone loss was observed in two years in a row). In both years, the southern part of the Southern Hemisphere was significantly polluted by Australian wildfire smoke as shown in Ohneiser et al. (ACP, 2022) and Ansmann et al. (ACP, 2022).
Final point: In your summary section, you mention: We need to continue to observe, understand and model these processes; Atmospheric Chemistry and Physics will continue to provide a primary journal for our community's major advances in these areas.
With other words, you recommend that researchers and scientists, working in this exciting stratospheric ozone research field, should permanently check ACP for new papers on these exciting directions. However, it seems that you do not follow your own suggestion. I think, it is very important to check out the most recent publications and include the most recent findings in our investigations, discussions, and communications to further advance current research.
In summary, the opinion paper is very good and will become a very useful addition to the present discussion about the stratospheric ozone layer, but one should clearly mention the recent impact of wildfire smoke on ozone depletion in polar regions.
Citation: https://doi.org/10.5194/egusphere-2023-1409-CC1 -
RC1: 'Comment on egusphere-2023-1409', Anonymous Referee #1, 01 Aug 2023
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2023/egusphere-2023-1409/egusphere-2023-1409-RC1-supplement.pdf
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RC2: 'Comment on egusphere-2023-1409', Anonymous Referee #2, 24 Sep 2023
This opinion piece on stratospheric ozone is an interesting read. The authors briefly recount the basics of the ozone story, the Montreal Protocol and ozone recovery along with the substantial contributions of Paul Crutzen. They further discuss more recent events like the Australian wildfire, the Hunga Tonga eruption, observations from space and modelling challenges. The authors are well qualified to discuss these topics. As such, the manuscript will be acceptable for publication after the authors consider the comments below.
1) The text is uneven as an opinion piece. Section 3, 5 and 6 have a number of valuable opinion statements (e.g., ln122, ln147, ln157, ln393, ln411, ln421, ln454). In contrast, they are lacking in Section 4 which reads more as a tutorial of recent impacts on the stratosphere. I recommend adding opinions to Sect 4 or shortening considerably and relying on references to describe these processes.
And the impact of the manuscript would be enhanced if the authors review the text to make sure their existing opinions clearly stated and consider where opinions could be added and that the combined portfolio of opinions has a reasonable coherence and story line.
2) The conclusions ‘threaten this essential component’ and ‘threaten further depletion’ seem overstated since they are not usefully quantified or justified. I recommend eliminating that word without further explanation.
3) The role of greenhouse gas (GHG) increases in influencing future ozone is not well discussed, especially since large GHG increases will lead to ozone super recovery for which the impacts are poorly known. Suggest emphasizing that ODS and ODS substitute emissions also enhance climate change which will influence ozone in multiple ways.
4) A useful additional opinion would be to note the importance of communicating ODS and ozone layer science to policy makers to guide future decision making to protect ozone and climate. For example, the ‘effectiveness’ comment on ln 142 implicitly includes the effective communication of scientists and policymakers over a number of years after the emissions were documented.
5) The statement “However, recent discoveries related to increased emissions of controlled ODSs and uncontrolled shorter-lived halogenated source gases have raised some concerns on the continued success of the treaty and the outlook for ozone recovery.” seems alarmist. A more valid perspective, for example, is that the success of dealing with CFC-11 emissions has uniquely demonstrated the resilience of the Mont Prot, the effectiveness of their provisions, and the importance of continued vigilance of atmospheric abundances.
6) As an aside, a real vulnerability of the Mont Prot is that the atmospheric observations of ODSs and other gases that provide essential information for the foundation of MP regulations are not controlled/supported/managed by the Mont Prot. Instead, they are provided independently by governments, ie NOAA and AGAGE networks.
Suggest pointing to the white paper prepared in part by the Scientific Assessment Panel of the Mont Prot that offers recommendations to mitigate critical gaps in ODS observations. The white paper was motivated by the discovery of unreported CFC-11 emissions by the global networks. https://ozone.unep.org/system/files/documents/ORM11-II-4E.pdf
6a) Section 5 has an extremely important and timely message about continued observations that could be elevated to the abstract. It is truly unfortunate that, for example, the HT impacts diagnosed in the Santee et al paper cited below will not be repeatable for a future eruption after the MLS instrument is retired.
Other comments:
7) Suggest adding a reference to the Fishman et al tribute to Crutzen in BAMS (2022).
8) ln14 change ‘Further unexpected perturbations to the ozone layer are occurring at the moment through injection of very large amounts…’’ to ‘Further perturbations to the ozone layer are occurring at the moment through unexpected injection of very large amounts…’’
9) ln116: Suggest changing to: “ODS production controls have caused a net reduction in the tropospheric source gases (Figure 1a) that transport chlorine and bromine to the stratosphere.”
10) ln125 Suggest adding clarification that dichloromethane has a substantial non-anthropogenic source.
11) Suggest changing to “The history of the MP since its signing in 1987 (and ratification in 1989) is one of continued success – as evidenced…”
12) ln144 change to ‘ODSs’ and delete ‘it’
13) ln163 Change ‘can be’ to ‘was’
14) ln 189 Recovery estimates already include ongoing and future ODS abundances and emissions. Suggest changing to ‘Clearly, ongoing emissions of chlorine and bromine from ODSs or VSLS that are not already accounted for will act to slow down the estimated rate of recovery…’
15) ln235 Suggest changing to ‘…relatively limited impact on global ozone and, unlike the anthropogenic halogen emissions, are only expected to…’
16) ln249 Suggest ‘when aerosols are enhanced’. Suggest choosing a more contemporary reference than Hofmann and Solomon.
17) ln262 Suggest citing how recent observed stratospheric aerosol perturbations have offset climate change with the recent Yu et al. paper, which derives in part from Fig. 3 of this manuscript: Yu, P., et al. (2023). Radiative forcing from the 2014–2022 volcanic and wildfire injections. Geophysical Research Letters, 50, e2023GL103791. https://doi.org/10.1029/2023GL103791
18) ln332 Suggest citing the recent Santee et al paper which diagnoses the HT impact on het processes that alter ozone chemistry. Santee, M. L., et al. (2023). Strong evidence of heterogeneous processing on stratospheric sulfate aerosol in the extrapolar Southern Hemisphere following the 2022 Hunga Tonga-Hunga Ha'apai eruption. Journal of Geophysical Research: Atmospheres, 128, e2023JD039169. https://doi. org/10.1029/2023JD039169
19) ln392 Suggest changing to ‘In recent years we have benefitted from a wealth of observations from instruments in ground-based networks and on balloon, aircraft and satellite platforms.’
20) It would be great to update the H2O time series in Fig. 4 as a private communication
20) ln412 suggest replacing ‘that we have been used to’ to ‘that has been so valuable’ or ‘that has been so essential to understanding ozone depletion’ or similar.
21) ln426 Suggest omitting ‘and simulators’ since it is a tool.
22) ln814. This figure is outdated. Suggest updating to Fig ES-1 of the 2022 assessment.
Citation: https://doi.org/10.5194/egusphere-2023-1409-RC2 - AC1: 'Comment on egusphere-2023-1409', Martyn Chipperfield, 07 Nov 2023
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AC2: 'Comment on egusphere-2023-1409', Martyn Chipperfield, 27 Nov 2023
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2023/egusphere-2023-1409/egusphere-2023-1409-AC2-supplement.pdf
Martyn Chipperfield and Slimane Bekki
Martyn Chipperfield and Slimane Bekki
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