the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 13 Jan 2025
Ground-based Tropospheric Ozone Measurements: Regional tropospheric ozone column trends from the TOAR-II/ HEGIFTOM homogenized datasets
Abstract. The quantification of long-term free-tropospheric ozone trends is essential for understanding the impact of human activities and climate change on atmospheric chemistry, but is challenged by the diversity between satellite tropospheric ozone records and the sparse temporal and spatial sampling of ground-based measurements. Here, we explore if a more consistent understanding of the geographical distribution of tropospheric ozone column (TrOC) trends can be obtained by focusing on regional trends calculated from ground-based measurements. Regions were determined with a correlation analysis between modelled TrOCs at the site locations. For those regions, TrOC trends were estimated with Quantile Regression and Dynamical Linear Modelling for the Trajectory-mapped Ozonesonde dataset for the Stratosphere and Troposphere (TOST), and with a linear mixed-effects modelling (LMM) approach to calculate synthesized trends from the homogenized HEGIFTOM (Harmonization and Evaluation of Ground-based Instruments for Free-Tropospheric Ozone Measurements) individual site trends. For different periods (1990–2021/22, 1995–2021/22, 2000–2021/22), both approaches give increasing (partial) tropospheric ozone column amounts over almost all Asian regions (median confidence), and negative trends over the Arctic regions (very high confidence). Trends over Europe and North America are mostly weakly positive (LMM method) or negative (TOST). For both approaches, the 2000–2021/22 trends decreased in magnitude compared to the 1995–2021/22 for most of the regions, and for all time periods and regions, the pre-COVID trends are larger than the post-COVID trends. Our results enable the validation of global satellite TrOC trends, and assessment of the performance of atmospheric chemistry models to represent the distribution and variation of TrOC.
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CC1: 'Comment on egusphere-2024-3745', Gabriele Pfister, 05 Feb 2025
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2025/egusphere-2024-3745/egusphere-2024-3745-CC1-supplement.pdf
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RC1: 'Comment on egusphere-2024-3745', Anonymous Referee #1, 13 Mar 2025
The manuscript has presented the regional mean tropospheric ozone trends using the currently available homogenized ground-based and in-situ tropospheric ozone measurements, as a research outcome from the TOAR-II HEGIFTOM working group. Several statistical methods, including spatial correlation analysis, quantile regression, and linear mixed-effects modeling, are applied to test the consistency of tropospheric ozone trends as estimated from different regions, periods and datasets. Overall, the study is a valuable addition to our current understanding by summarizing the free tropospheric ozone trends revealed by HEGIFTOM datasets. Such datasets and estimated trends will provide benchmarks for model evaluation.
Here I have several comments that the authors should address to further improve the manuscript.
Specific Comments
1) Page 2, Line 58-59, Abstract -
Here, median confidence and high confidence were used to describe the robustness of the trends; however, how were these terms defined in the main text? Some explanation was mentioned in Table 5, yet I think some more details are needed to clarify their statistical meaning.
2) Page 7, Line 225 -
Twenty-four well-correlated regions were identified here using the spatial correlation analysis; however, it was unclear how these 24 regions were located and their covered areas. Such information was unclear by looking at Figure 1 as an example.
And in Figure 1, I suggest showing the locations of IAGOS FRA airport and the FTIR Hefei station.
3) Page 10, Figure 3 -
Figure 3 shows the locations of 19 different regions. Were these regions identified by the spatial correlation analysis (which said 24 different regions)? Please clarify.
4) Page 14, Line 350-357 -
For the TOST-based trends, 12 regions were selected and analyzed, as shown in Figure 4. It was not clear why the 19 regions as defined in Figure 3 were not used to facilitate the comparisons between TOST-based and HEGIFTOM-based trends. Please clarify.
5) Figures 10-13
Figures 10-13 appear to need large revisions. The font size was difficult to read, and the font type was different from that of other figures, such as Figure 9.
6) Since several regression methods were utilized in the study, a Table or some paragraphs summarizing their applications and differences would be helpful to smooth the paper structure. Right now, it is not clear in the manuscript why Quantile Regression was used in one case, while Dynamical Linear Modelling was used in another case. Would these regression methods return different findings?
7) Page 20, Figure 8
The legends p300 and FT need to be defined in the figure caption. DLM trends were presented here and in the Section 5.1.2, right? I do not see anywhere else described the DLM results (not in Sect 5.3 as said on Line 378).
8) Page 30, Line 667
“The difference is, at least for some regions, driven by the positive trends from measurement techniques other than ozonesondes.” What did the statement mean by trends from measurement techniques? Please better explain it.
9) “TO BE FURTHER COMPLETED” in the Acknowledgements shall be noted.
Citation: https://doi.org/10.5194/egusphere-2024-3745-RC1 -
RC2: 'Comment on egusphere-2024-3745', Anonymous Referee #2, 16 Apr 2025
This paper presents a careful quantification of regional trends in tropospheric ozone from a range of ground-based measurements. The paper is well written, the analysis is thorough and the results are a valuable addition to the field.
My comments are minor. See below.
General comments:
I didn’t catch why the chosen time periods run through 2021 for the TOST analysis but through 2022 for the HEGIFTOM analysis. I would suggest that the authors make it clear why a different date range was chosen for TOST vs HEGIFTOM.
I did not follow the reasoning as to why different statistical approaches were applied to the TOST vs the HEGIFTOM datasets. It would be helpful to include some more explanation for the rationale for the choice of these particular approaches for application to these particular datasets.
Specific comments:
Abstract: “challenged by the diversity between satellite tropospheric ozone records and the sparse temporal and spatial sampling of ground-based measurements”.
I had to read this quite a few times before I could understand what this sentence was supposed to mean, particularly the phrase “challenged by the diversity”. I think that the authors are pointing to two different issues. One issue is that there are numerous satellite products for tropospheric ozone and that there are differences between those records that have not yet been well understood or accounted for. The other issue is that the “reference” ground-based datasets have limitations in their spatial and temporal coverage. I think it would be much better to split this into two sentences to make the points clear.
Line 87: “available satellite products disagreed on the sign of the trend”. This may be overly picky, but I would say that the analysis in the Gaudel et al. (2018) paper was not sufficient to determine whether there was truly disagreement or whether the differences between the trends presented were due to inherent characteristics of the different satellite datasets. I would suggest saying something like “early analysis of available satellite products did not provide a consistent picture of the sign of the trend”.
Line 89 and Conclusions: Can you state at the end whether or not your conclusions are consistent with those from the IPCC AR6?
Line 122: It would be helpful to state in this paragraph what the 5 different measurement techniques are, since this isn’t explained until later.
Section 2.2: Is it correct to say that there is no chemistry in TOST? Ozone is assumed to be constant along a given trajectory? Is that what “assigned along its forward and backward trajectory” means? Please clarify.
Caption for Figure 5: Figures show trends from 1995-2021 but statistics are based on median values over 1990-2021. Is this correct or should the date ranges be consistent?
Line 543: There is a sentence that starts with “And”. Please check grammar.
Line 580: Consider adding a sentence or two that says something about possible reasons for free tropospheric increases that are not driven by precursor emissions/lower tropospheric increases. Changes in dynamics? Changes in OH availability?
Caption for Figure 13: Please state what the gray points represent. I see that this is stated in the text, but it should also be made clear in the figure caption.
Citation: https://doi.org/10.5194/egusphere-2024-3745-RC2
Data sets
HEGIFTOM homogenized ozone profile and TrOC datasets R. Van Malderen et al. http://hegiftom.meteo.be
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