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
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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
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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
Data sets
HEGIFTOM homogenized ozone profile and TrOC datasets R. Van Malderen et al. http://hegiftom.meteo.be
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