the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 04 Mar 2025
Surface Ozone: Seasonal cycles, trends and events, a new perspective from the OPE station in France over the 2012–2023 period
Abstract. A comprehensive atmospheric dataset including hourly meteorological parameters, pollutants and methane mixing ratio and weekly beryllium-7 and sodium-22 activity concentration measured at the Observatoire Pérenne de l’Environnement (OPE) station from 2012 to 2023 was used to assess the interannual and intraseasonal variability of ozone. Weekly parameters were computed to characterize mean afternoon, mean nighttime, baseline and peak events for ozone as well as for NOX, CO, CH4. The mean afternoon O3 concentrations did not show a significant trend, while baseline O3 exhibited a significant increase of 0.7 µg.m-3 per year. The intra-seasonal variability of surface weekly ozone anomalies revealed two different periods. During the November to February period, ozone anomalies were related to CO, CH4 and NOx anomalies but not with meteorological parameters or Stratosphere to Troposphere Transport (STT) proxies. From April to September, the relations between CH4, CO and NOX diminished while stronger association of O3 residuals were observed with STT proxies, but also with temperature, solar radiation and relative humidity. The surface O3 enhancement associated with positive anomalies of 7Be and 22Na suggests a direct stratospheric O3 contribution but also a local photochemical contribution which is favored by warmer and drier air masses and increased solar radiations.
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RC1: 'Comment on egusphere-2025-148', Anonymous Referee #1, 17 Mar 2025
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Revision of “Surface Ozone: Seasonal cycles, trends and events, a new perspective
from the OPE station in France over the 2012 - 2023 period” by Conil S. et al.
General comments
As from the title, the paper aims to investigate a long-term time series of ozone data measured in the 2012-2023 period at the OPE station in France, shedding light on the causes of the signals detected. To this aim, the ozone dataset is integrated with data on pollutants, methane, meteorological variables, and radiotracers. Although the topic is surely interesting, the work remains superficial and lacks some logic even in the structure but also deeply reflected in the methodological approach.
Starting with the introduction, the authors present a long list of papers and processes responsible of different things (ozone variability, radiotracers, …) but at the end the reader remains unsatisfied with the missing gaps and main motivation of the research paper. The presentation and discussion of results present significant drawbacks, which can be summarized as follows:
- The results are often compared with other papers, but the results are seldomly discussed in view of what they could imply in terms of the processes responsible or what is already known on the topic;
- STT events and their connections with both ozone and radiotracers variability at midlatitude high-altitude stations have been the subject of several research studies, which are never cited here;
- Circulation at both local and synoptic scales, and not only vertical transport, may have significant implications on ozone variability: not only this is not studied here, but also this is never cited as a potential additional process responsible for at least some of the observed variabilities;
- I have not seen any results (e.g., spectral analysis) clearly pointing out the main periods appearing in the different time series;
- At the same time, the correlation analysis presented remains superficial and does not consider the presence of (potential) spurious correlations (the presence of correlation in principle only highlights the presence of similar time patterns in the time series);
- There is no effort in highlighting limitations (e.g., the use of very coarse resolution data for boundary layer height) and in drawing general conclusions from the study.
In particular, while reading the results section, I had the feeling that the authors were trying to put together all the analyses done, but the logic behind the structure and the connection between the different analyses remain very unclear and for sure not deep. To conclude, and summarizing, the results section seems an extensive list of different results without any efforts to put them together or investigate in depth what they are telling us.
Specific comments
Page 1
Lines 16-25: It is not clear how meteorological parameters were used: does transport (not only in the vertical, but for instance, mountain-valley breeze regimes) play a role in the O3 patterns? The relation with temperature and radiation seems straightforward and not particularly new. It is also not clear if the diurnal and seasonal pattern was studies (e.g., O3 higher in the afternoon, O3 higher in spring, …). Finally, there is no quantification apart from the overall trend of the baseline, while the influence of specific processes remains unquantified at least in the abstract.
Lines 19-21: Are anomalies and peaks utilized as synonyms? Anomalies in principle can be positive or negative, but here it seems implicit that all are positive.
Lines 18-19: not clear why afternoon and nighttime, and not diurnal and nighttime.
Line 29: Ozone in the stratosphere is beneficial because its absorption of UV radiation (and which type?) shields life on Earth’s SURFACE from it. This has also a climate (radiative forcing) effect. Please explain better.
Lines 35-38: Please explain better that only with NOx the net cycle would be null (production = destruction).
Lines 40-41: In principle, this is very rare as normally the two layers are essentially isolated because of stability conditions. Downward transport from the stratosphere require specific conditions which are not mentioned here. Please explain better (with references).
Page 2
Lines 15-17: Please try to describe these.
Page 3
Lines 35: Which type of aerosols, and in particular, of which dimensions? This has important implications.
Page 4
Line 14: What do you mean by “sometimes”?
Line 38: I assume “temperature” and “pressure” are atmospheric temperature and barometric pressure.
Page 5
Lines 3-5: not clear if these results are from elsewhere (in which case, a reference is missing), or if they are from this specific paper, and in this case what conclusion the reader should draw (e.g., type of climate? Type of circulation?). A reference to the mean wind intensity is also missing.
Lines 5-7: The use of such coarse resolution data may represent a clear drawback: are you sure that the reanalysis with its coarse resolution can catch the complex terrain features in the study area, clearly impacting on the boundary layer height variability?
Lines 10-12: The change of instrumentation may have several drawbacks or at least effects on the signal, with discontinuities and inhomogeneity appearing. Please discuss.
Lines 15-17: Not clear how the different instruments are used. If utilized as for NOx, meaning changing instruments, this may have implications as discussed above.
Lines 19-23: not clear, please explain better.
Lines 25-26: This can be omitted as already described thoroughly in the Introduction.
Lines 25-32: This is not really relevant, the important thing is to describe how they are measured in your case/dataset.
Page 6
Line 5: It is not only the height of the mixing layer that is relevant, but the type (convective, residual, …) that matters.
Lines 13-14: What do you mean by “standard parameters”? Maybe they result from some spectral analysis and not extendable to other datasets where the time variability is different.
Lines 25-27: I think it should be mentioned and described better that the purpose is to investigate the processes behind O3 anomalies, using proxies. And then describe the types of proxies used and the processes they are representative of.
Lines 31-40 and page 7, lines 1-2: I do not understand the reason to compare with other stations and in particular for different time periods. Please explain better.
Page 7
Figure 1, caption: Could you please explain the meaning of the shaded area?
Lines 7-16: Could you then discuss how these results help in discovering processes related to O3 diurnal variability in the different seasons?
Page 8
Lines 4-18: I am not sure where these results come from (i.e., to which figure can they be related?).
Page 9
Lines 9-10; What do you mean by “more important”?
Lines 12-24: Can you please discuss which are the processes responsible for such seasonal patterns?
Lines 30-32: There are other studies presenting seasonal patterns of 7Be at midlatitude, and also high-altitude stations. Please revise.
Figure 2, 3 and 4: the figures present not only average values but also uncertainty bars which are not discussed.
Page 11
Line 1: I cannot see this ratio plotted in Figure 3.
Lines 3-12: The seasonal variability of STT processes have been already thoroughly analysed at midlatitude high-altitude stations in several papers, also, but not restricted to, applications of cosmogenic radiotracers. Please discuss your findings in view of literature.
Lines 11-12: Not clear, please revise.
Lines 18-19 and page 12, lines 1-2: How does this can be related to other results? If it cannot be linked to other results, then it is only another part of the study site characterization in Section 2.
Page 12
Lines 4-8: It is not clear how this relates to Figures and results presented in other sections.
Line 10: Where are these thresholds coming from?
Lines 4-17: The discussion remains quite superficial and subjective, without any references to previous supporting literature.
Page 13
Line 7 and 9: How was significance analysed? What do you mean by marginal significance? Could you discuss what is the process related with such increases?
Page 13
Lines 27-29: They have been also associated with changing circulation patterns in some studies.
Page 14
Lines 12-16: Either you present and discuss the results, either you omit them directly, this halfway of presenting the results without discussing and after that switch to another topic saying that these results are not meaningful for the purposes of this work is not understandable and leave the reader with many doubts.
Page 15
Lines 19-20: The impact of a particular transport pattern comes here out of the blue.
Page 16
Lines 11-18: How are the classes and their link with quantiles defined?
Figure 10, caption: Please explain or remind to the reader what is the meaning of AMJJAS and NDJF, which is a quite unusual separation (normally, the seasons are studies)
Page 22
Line 41: Saharan dust air masses have a particular link with O3, in particular they have been connected with O3 reduction in some studies.
Page 23
Lines 11-22: Not clear how this connects to the rest of the paper.
Citation: https://doi.org/10.5194/egusphere-2025-148-RC1 -
RC2: 'Comment on egusphere-2025-148', Anonymous Referee #2, 25 Mar 2025
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My comments are included in the Reviewer comments in a pdf file.
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RC3: 'Comment on egusphere-2025-148', Anonymous Referee #3, 28 Mar 2025
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The paper presents a large and comprehensive data set regarding the concentration of ozone in surface air.
The introduction is detailed. Perhaps a figure showing the interplay of all the parameters might be helpful for readers with less background on this topic (or a reference to a review paper with such a figure).
Much emphasis is given in the paper to the role of STT using 7Be and 22Na as proxies. This certainly is a valid and interesting approach. However, the interpretation of the cosmic radionuclides can be rather complex. Before averaging different years the 7Be and 22Na data should ideally be corrected for the changing intensity of cosmic rays. This could be done based on annual means of cosmic-ray neutron flux which is measured at various locations. (Perhaps such a correction would not change the seasonal cycles shown in Fig. 3 fundamentally, but in any case it should be mentioned that the correction is not applied.) Moreover, the 7Be/22Na ratio is dependent on the residence time, since 7Be decays much faster than 22Na. This may be important with atmospheric residence times longer than one week.
With respect to the role of STT I would be interested in a quantitative estimate of its contribution to the surface O3 concentrations, especially for the June 2019 event case study. Should such more quantitative conclusions not be possible, the authors should explain why and perhaps say which additional information might enable one to do so. It seems that the correlation of the cosmogenic radionuclides and O3 in summer is also due to solar radiation (i.e. sunnier and warmer weather) which enhances both vertical advection (i.e. higher cosmogenic RN) as well as photochemical O3 production. Therefore, a stratospheric contribution of O3 may be difficult to be discerned. Is it possible to estimate the fraction of stratospheric ozone, e.g. based on typical O3/22Na ratios in the troposphere or upper stratosphere?
In this context, it would be helpful to give some quantitative information of the vertical profiles of O3 from the stratosphere to the surface and to compare these with 7Be and 22Na. As the authors mention, it is not straight forward to distinguish stratospheric from upper tropospheric inputs based on 7Be an 22Na.
Figures: There are probably too many figures in the paper. Some of the sub plots in Figs. 2-4 could go to the SI. The boxplots in Figs. 10-12 are not ideal to represent the data. The colors do not add any information. I think scatter plots with the indication of the R-value like in Figs. 13 and 14 are more informative. And again, some of the these plots might go to the SI.
Overall, in my opinion, the paper could be improved by addressing the key processes contributing to surface ozone in a more focused and quantitative way.
Some more specific remarks and minor points:
page 5, line 40: "solar contribution" sounds like the particles from the sun would contribute to the production of Be-7 and Na-22. "solar modulation" would be a more appropriate term.
page 6, line 12: "We fit the weekly ..." this sentence is not clear.
first paragraph of 3.1.1: MHD is cited twice.
Figure 1: How are the values normalized? To annual average and standard deviation?
page 9, line 31: replace "that" by "than". In the same sentence it is not clear which stations are compared.
page 11, top paragraph: "stratosphere-troposphere transport is larger between March and August ..." How do the 7Be and 7Be/22Na data include March and August into a period of larger stratosphere-troposphere transport?
page 12, line 10: the threshold value for 7Be should be variable from year to year in order to reflect the varying source term (soalar modulation of cosmic rays).
Figure 5: add a legend
page 13, line 31: what about boreal wetlands?
page 15, line 7: the cited figures are not correct
page 15, line 10: March is not really transitional but quite well in line with AMJJAS in Figs. S11 and S12
page 24, line 3: I would suggest to replace "sun radiation activity intensity changes" by "solar modulation of cosmic rays intensity". Moreover, this is not really an important conclusion in the context of surface ozone concentrations.
page 24, line 14: "These weather conditions and synoptic situations also favour more active photochemical processes, suggesting that situations with a significant ozone component from stratospheric origin could frequently be associated with a larger ozone regional near surface photochemical production." Do the authors imply any cause and effect relationship here, or is that simply the co-incidence that warmer, sunnier weather affects both, vertical advection and photochemical ozone production?
Citation: https://doi.org/10.5194/egusphere-2025-148-RC3
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