Tropospheric Ozone Assessment Report (TOAR): 16-year ozone trends from the IASI Climate Data Record

Boynard, Anne; Wespes, Catherine; Hadji-Lazaro, Juliette; Sinnathamby, Selviga; Hurtmans, Daniel; Coheur, Pierre-François; Doutriaux-Boucher, Marie; Onderwaater, Jacobus; Steinbrecht, Wolfgang; Pennington, Elyse A.; Bowman, Kevin; Clerbaux, Cathy

doi:https://doi.org/10.5194/egusphere-2025-1054

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https://doi.org/10.5194/egusphere-2025-1054

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01 Apr 2025

| 01 Apr 2025

Tropospheric Ozone Assessment Report (TOAR): 16-year ozone trends from the IASI Climate Data Record

Anne Boynard, Catherine Wespes, Juliette Hadji-Lazaro, Selviga Sinnathamby, Daniel Hurtmans, Pierre-François Coheur, Marie Doutriaux-Boucher, Jacobus Onderwaater, Wolfgang Steinbrecht, Elyse A. Pennington, Kevin Bowman, and Cathy Clerbaux

Abstract. Assessing tropospheric ozone (O₃) variability is essential for understanding its impact on air quality, health, and climate change. The Infrared (IR) Atmospheric Sounding Interferometer (IASI) mission onboard the Metop platforms, has been providing global measurements of O₃ concentrations since 2007. This study presents the first comprehensive analysis of the 16-year O₃ Climate Data Record (CDR) from IASI/Metop (2008–2023), a homogeneous dataset offering valuable insights into the variability and long-term trends of tropospheric O₃. The IASI-CDR ozone product is evaluated against TROPESS (TRopospheric Ozone and its Precursors from Earth System Sounding) O₃ retrievals from the Cross-track Infrared Sounder (CrIS). The comparison shows excellent agreement for total ozone (biases < 1.2 %, correlations > 0.97) and good agreement for tropospheric ozone (biases 10–12 %, correlations 0.77–0.91). Comparisons with ozonesonde data indicate that IASI underestimates tropospheric ozone by 2 % in the tropics and by up to 10 % in mid and high latitudes. Spatiotemporal analysis of IASI data from 2008 to 2023 reveals a global negative trend in tropospheric O₃ (‑0.40 ± 0.10 % year^-1), with the most pronounced decreases observed in the tropics and in Europe. Despite differing from positive trends in ultraviolet (UV) satellite data, both UV and IR satellite instruments show a significant drop in tropospheric ozone starting in 2020, partly due to pandemic-related emission reductions. This study emphasizes the importance of long-term, consistent datasets for tracking ozone trends and the need for improved data retrieval and integration to address regional and temporal discrepancies.

Received: 05 Mar 2025 – Discussion started: 01 Apr 2025

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CC1:
'Comment on egusphere-2025-1054: TOAR-II guidelines', Martin Schultz, 10 Apr 2025

Dear authors,
The TOAR Steering Committee wishes to express their gratitude to you (as well as all authors of articles in the inter-journal Community Special Issue for the second Tropospheric Ozone Assessment Report) for submitting your manuscript to Atmospheric Chemistry and Physics. Even though your article was submitted after the special issue deadline of November 30th, the scientific community will very likely associate it with the other articles from this special collection. Therefore, your article should also follow the rules that are defined in the guidelines for submitting articles to the TOAR-II Community Special Issue (see https://igacproject.org/sites/default/files/2023-04/TOAR-II_Community_Special_Issue_Guidelines_202304.pdf): “To avoid confusion with the final assessment papers, “TOAR” or “Tropospheric Ozone Assessment Report” may not appear in the title of the paper, nor should the paper claim to be an official TOAR publication. However, it is acceptable to refer to TOAR in the manuscript as, for example, “In the context of the Tropospheric Ozone Assessment Report (TOAR) Phase Two focus working group on XYZ, etc.”.”
We therefore ask you to change the title of your article and remove the explicit reference to TOAR from the title.
This comment, of course, has no bearing whatsoever concerning the scientific quality or potential impact of your manuscript.
We kindly ask for your understanding.
Martin Schultz and Helen Worden on behalf of the TOAR-II Steering Committee

Citation: https://doi.org/10.5194/egusphere-2025-1054-CC1
- AC3: 'Reply on CC1', Anne Boynard, 12 Jul 2025
  
  The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2025/egusphere-2025-1054/egusphere-2025-1054-AC3-supplement.pdf
  
  Citation: https://doi.org/10.5194/egusphere-2025-1054-AC3
RC1:
'Comment on egusphere-2025-1054', Anonymous Referee #1, 05 May 2025

Boynard et al. present a comprehensive analysis on the quality and reliability of ozone columns observed from IASI, highlighting its consistency with another IR instrument (CrIS) and sonde observations, while pointing out discrepancies from UV instruments in terms of long-term trend analysis. This work includes rigorous validation and comparison by incorporating various datasets which is essential for understanding the spatiotemporal variability of tropospheric ozone, which has been under debate especially regarding recent global/regional trends. The manuscript is overall well written but there are some points to be considered and clarified before publication.
1. Section 2.1.1: Can the authors include a brief description on how the IASI-CDR O3 L2 products are retrieved? AK, DOFs, and a priori profiles are discussed as possible contributors to observed discrepancies but readers may not be familiar on how they would affect the retrievals.
2. L123-125: Please clarify what IASI-A/B refer to. Are they the products from Metop-A/B?
3. L230-234: Are the differences in tropospheric columns related to uncertainties in the tropopause height?
4. L260: O3 -> O₃
5. L338 "IASI pixels have been processed (only 3.5 out of 10)": Can you clarify what this "processed" means?
6. L375: a secondary second smaller peak -> a secondary smaller peak
7. L400-402: The two lines look nearly similar at around 200 hPa, they seem to diverge above that altitude. Assuming that IASI is more sensitive to temperature, I'm curious why the smoothed sonde observations (which I understood as to have applied IASI's vertical sensitivity, i.e., AK) show larger discrepancies than the raw sonde?
8. L443-445, Figure 12: The difference between the left/right panels over Southeast Asia is surprising. There should be a lot of biomass burning emissions in that region, which wouldn't have been particularly affected by the pandemic. Has the Lanina affected fire frequencies?
9. Figure 13: How significant was the drop in precursor emissions during the pandemic? It also looks like O₃ is bouncing back up recently. This has been observed in surface concentrations over the US (US EPA). Can the authors briefly comment on observed changes of NOx/VOCs and implications for surface O₃ trends?
10. L474: troposphere (450 hPa to thermal tropopause) troposphere. -> troposphere (450 hPa to thermal tropopause).
11. L479, Figure 14: Does cp450 refer to the lower troposphere? I suggest adjusting the figure labels so that they are consistent with the phrases used in the text.

Citation: https://doi.org/10.5194/egusphere-2025-1054-RC1
- AC1: 'Reply on RC1', Anne Boynard, 12 Jul 2025
  
  The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2025/egusphere-2025-1054/egusphere-2025-1054-AC1-supplement.pdf
  
  Citation: https://doi.org/10.5194/egusphere-2025-1054-AC1
RC2:
'Comment on egusphere-2025-1054', Anonymous Referee #2, 09 May 2025
The manuscript “Tropospheric Ozone Assessment Report (TOAR): 16-year ozone trends from the IASI Climate Data Record” presents and validates the IASI-CDR O₃ product, a reprocessed and homogenized version of IASI-FORLI, providing a consistent dataset of tropospheric ozone spanning 2008–2023. The dataset is validated through comparisons with the TROPESS-CrIS product globally and with ozonesonde measurements at selected sites. A comprehensive trend analysis is conducted across multiple vertical layers using 16 years of observations. The manuscript is well-structured, and it is informative, offering a valuable contribution to the understanding of tropospheric ozone dynamics and trends over the past two decades. Below, I provide several comments for the authors to consider before the manuscript can be recommended for publication:
The authors employ a 1° × 1° gridded resolution for comparing IASI and CrIS datasets. Given that both instruments have native spatial resolutions on the order of 12–14 km at nadir, it would be helpful for the authors to justify the use of this relatively coarse grid. Was this choice driven by data availability, computational efficiency, or other methodological considerations?

Was validation at finer spatial resolutions considered? The authors might comment on whether such an approach was explored and how it might influence the interpretation of inter-satellite differences.

The trend analysis is performed using quantile regression at the 50th percentile (median), which is robust. However, applying the same method to higher percentiles would help assess whether the trends differ when considering extreme ozone events. This would be especially relevant considering the reported decreasing trends in tropical regions, which diverge from findings using UV-based instruments like OMI that often suggest stable or increasing trends in similar regions.

The use of 1° × 1° resolution in the trend analysis may mask finer-scale changes, especially in urban regions where surface ozone levels are known to be increasing in many parts of the world. Since urban areas occupy a small fraction of a 1° grid cell, their signal could be diluted by surrounding lower-concentration regions, potentially biasing the trend analysis toward decreases. Could this resolution choice be contributing to the observed negative trends? Discussing this potential limitation would strengthen the interpretation of the trend results.

Lines 360-365. The manuscript notes that differences in tropospheric ozone columns between IASI and CrIS are partly driven by the a priori information used—fixed for IASI and seasonally/latitudinally variable for CrIS. Given the importance of the a priori in shaping retrievals, particularly under conditions of low sensitivity, could the authors comment on what might constitute a more optimal or standardized approach? For example, would using dynamic, seasonally and geographically resolved climatologies for both instruments help reduce inter-product differences?

In Figure C1, the most pronounced positive trends in lower tropospheric ozone appear in arid regions of Africa and Australia. It would be helpful for the authors to comment on the possible drivers behind these trends briefly. Are they associated with natural emissions (e.g., soil NOₓ), biomass burning, or transport processes?
Citation: https://doi.org/10.5194/egusphere-2025-1054-RC2
- AC2: 'Reply on RC2', Anne Boynard, 12 Jul 2025
  
  The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2025/egusphere-2025-1054/egusphere-2025-1054-AC2-supplement.pdf
  
  Citation: https://doi.org/10.5194/egusphere-2025-1054-AC2
CC2:
'Comment on egusphere-2025-1054: HEGIFTOM perspective', Roeland Van Malderen, 13 May 2025

Dear authors,

I'm writing this comment mainly from my perspective as HEGIFTOM co-chair.

First of all, I want to congratulate you with your nice paper. What I really like about the paper is the honest approach. From the first TOAR activity, and in the paper by Gaudel et al. (2018), the inconsistency in tropospheric ozone time variability (and even in the sign of the trend) among different satellite ozone retrievals (and even between different retrieval methods of the same satellite data) popped up. Your paper tries to contribute to this issue, and you do not hide the overall negative IASI trends, which contrast with UV-VIS satellite tropospheric ozone measurements and ground-based measurements, but really try to find explanations for it (nicely summarized in the conclusions). And the tropospheric ozone decline after 2019 is really a common feature in all those datasets.

However, what I do miss in the introduction (lines 50-60) and in the conclusions (lines 550-554) are more references to results from TOAR-II papers. Please look for more relevant references (also for (regional) trends) in the TOAR-II SI collection of papers: https://acp.copernicus.org/articles/special_issue1256.html.

With my HEGIFTOM hat on, I want to draw your attention to some additional clarifications that are needed when using the HEGIFTOM ozonesonde data. First of all, your Table 2 contains some errors, e.g. Lindenberg, Prague, Tateno, Hongkong, Broadmeadows, and Macquarie are not available as HEGIFTOM time series, because these data have not been homogenized. So, they can be only available as WOUDC data. It might be important to explicitly mention that only the HEGIFTOM (and SHADOZ) data have been homogenized (i.e. corrected for biases), which is very important for use in trend estimation. Also important to note is that the HEGIFTOM and SHADOZ data are identical: the SHADOZ data have been copied on the HEGIFTOM ftp-server. Also, I found it very surprising that only 7 stations fulfill your criteria of having at least 3 monthly launches and > 70% sampling. Knowing that most stations launch weekly and overall do not reveal large, consistent, gaps in their time series, this looks rather an underestimation to me. In our HEGIFTOM individual trends paper (see Van Malderen et al., 2025), with at least 2 monthly launches and more or less > 50% sampling in the 2000-2022 time period, we end up with 34 homogenized ozonesonde stations! Are QA/QC criteria driving this small sample? Which ones? Please give more details about the site selection.

Your mostly negative IASI tropospheric ozone trends in the 2008-2023 time period contrasts somewhat with the mixture of insignificant, positive and negative trends that we found for 55 HEGIFTOM sites (IAGOS, FTIR, ozonesonde, Umkehr, Lidar) for the time period 2000-2022 and the sfc - 300 hPa ozone column. To reconcile those trend differences between both studies, trends for the same time period (2008-2019) and for exactly the same metric might be compared (another comment posted by co-authors from our study might follow). W.r.t. this last comment, as you are looking at the time behaviour/trends of different tropospheric ozone metrics (sfc-tropopauze, sfc - 450 hPa, 450 hPa - tropopauze), as we did, although we used different metrices (sfc-300 hPa, sfc-700 hPa, 300-700 hPa), some reference or discussion w.r.t. our results might be appropriate.

In our HEGIFTOM trends paper, we compared and discussed the Payerne tropospheric ozone time series (also used DLM for it) quite extensively, so you might use of this information when discussing the drift between the Payerne ozonesonde and IASI time series.

Recently, as you are aware, Dufour et al. (2025) also looked at the comparison and (regional) trends of another IASI product, IASI-O3 KOPRA, with a limited sample of ozonesondes (Boulder, Payerne, and Uccle are common). You do refer to this study, but I would expect a more detailed comparison of the results of both studies! To my opinion, at least a paragraph might be needed explaining the similarities or the differences between the results of both studies.

Also, looking at Fig. B1 (the constant a priori used in the IASI retrievals vs. the latitudinal and seasonal variable a priori used for CrIS), I wonder what the impact of such a time invariant (right?) a priori would have on the calculated trends.... The authors might comment on this.

Finally, I am also quite surprised by your small trend uncertainties (e.g. in comparison with ours, which are quite consistent between the QR and MLR trend estimation tools used). You might want to provide some additional information on how exactly those have been calculated.

Thank you for taking my comments into consideration and I wish you good luck in the review of your manuscript!

With kind regards,

Roeland Van Malderen

References:
Dufour, G., Eremenko, M., Cuesta, J., Ancellet, G., Gill, M., Maillard Barras, E., and Van Malderen, R.: Performance assessment of the IASI-O3 KOPRA product for observing midlatitude tropospheric ozone evolution for 15 years: validation with ozone sondes and consistency of the three IASI instruments, EGUsphere [preprint], https://doi.org/10.5194/egusphere-2024-4096, 2025.
Van Malderen, R., Thompson, A. M., Kollonige, D. E., Stauffer, R. M., Smit, H. G. J., Maillard Barras, E., Vigouroux, C., Petropavlovskikh, I., Leblanc, T., Thouret, V., Wolff, P., Effertz, P., Tarasick, D. W., Poyraz, D., Ancellet, G., De Backer, M.-R., Evan, S., Flood, V., Frey, M. M., Hannigan, J. W., Hernandez, J. L., Iarlori, M., Johnson, B. J., Jones, N., Kivi, R., Mahieu, E., McConville, G., Müller, K., Nagahama, T., Notholt, J., Piters, A., Prats, N., Querel, R., Smale, D., Steinbrecht, W., Strong, K., and Sussmann, R.: Global Ground-based Tropospheric Ozone Measurements: Reference Data and Individual Site Trends (2000–2022) from the TOAR-II/HEGIFTOM Project, EGUsphere [preprint], https://doi.org/10.5194/egusphere-2024-3736, 2025.

Citation: https://doi.org/10.5194/egusphere-2025-1054-CC2
- AC4: 'Reply on CC2', Anne Boynard, 12 Jul 2025
  
  The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2025/egusphere-2025-1054/egusphere-2025-1054-AC4-supplement.pdf
  
  Citation: https://doi.org/10.5194/egusphere-2025-1054-AC4
CC3:
'Comment on egusphere-2025-1054', Anne Thompson, 13 May 2025
IASI-Boynard Paper Comment – 13 May 2025
Anne Thompson, Debra Kollonige, Ryan Stauffer

SUMMARY COMMENTS AND SUGGESTIONS FOR REVISION
This Comment follows that of HEGIFTOM Co-Chair R. Van Malderen and amplifies his major points. We refer to the same “Van Malderen et al, in press, 2025” that he does and designate it as “HEGIFTOM-1” because there is a second HEGIFTOM paper in review in the TOAR II collection. We go beyond Van Malderen’s comments to 3 summary recommendations:
The selection and number of ground-based stations for which ozone data are compared to the IASI-CDR product is inadequate in number and ‘non-homogenized’ datasets should not be used. In contrast to the 7 stations used in this paper, more than 25 stations, homogenized over the 2008-2019 period, need to be used in evaluation of the IASI products and the trends. The 7 sites included no equatorial stations and far too few northern mid-latitude stations, in some cases where more than sonde data are available. The recommended ozonesonde 27 stations appear in Table 1 at the end of this Comment.

The authors conclude with general, not well-defined speculation on why there is little progress in how computed trends from the new IASI products diverge from the UV-based product trends from the Gaudel et al. (2018) TOAR I paper. More analysis and insights on this issue are needed before the paper is worthy of publication in a quality Copernicus journal. Specific questions are raised for consideration.

HEGIFTOM-1 is now “THE Reference dataset” for trends comparisons. The mismatch of years (2008-2019 in Boynard et al. vs 2000-2022 in HEGIFTOM-1) is speculated as one reason for why the trends in this paper differ from the ground-based trends in HEGIFTOM-1. We reran the trends in HEGIFTOM-1 for 27 stations for 2008-2019, see Table below, to support a valid comparison. Some of the HEGIFTOM-1 site trend signs changed and uncertainties increased, leading to a clear TOAR-worthy conclusion that trends computed from 12 or 16 years of IASI or ground-based data is inadequate. A revision must include this important result!

SYNOPSIS – This study presents an IASI product over 16 years, consisting of contributions from METOP-A, METOP-B and METOP-C, merged to create the IASI-CDR (Climate Data Record, 2008-2023). Several tropospheric ozone columns (to 450, to tropopause) are presented and compared on a monthly mean basis with (1) the CrIS IR ozone product and with (2) comparable ozonesonde columns from 43 stations over the period. Trends for global mid-latitudes and the tropics are computed using Quantile Regression (QR) for the period 2008-2019 (pre-COVID) and 2008-2023; the latter trends reflect an apparent COVID impact. As in the TOAR I paper in which Gaudel et al. (2018) summarized satellite product trends (2005/2008 – 2016) showing IASI (FORLI version) to be an outlier compared to UV-type satellite records, it appears that the IASI-CDR trends (now 2008-2019, omitting COVID period) is an outlier with the corresponding UV-based time-series. In both cases the greatest discrepancies are in the tropics except for SE Asia where all products display increases of ~1-2 DU/decade. The UV-type satellite products tend to be more variable with regions in the tropical Americas, Africa and Atlantic showing some increases (~2005-2019; Gaudel et al., 2024; Thompson et al., 2024). The IASI-CDR TOAR II-period (Fig. 12 in paper) shows little increase over Europe, a decrease over North America and only modest increases in east Asia, again in disagreement with more variable UV-type satellite trends. IASI 2008-2019 trend comparisons are made with 7 ozonesonde time-series: one subtropical site, 4 northern mid-latitude (majority European), 1 southern mid-latitude (Fig. 16). The authors speculate briefly on causes of the persistent discrepancy between the IASI vs UV trends and what is emerging as the prevailing view of tropospheric changes over the prior ~15 years. Comparisons among comparable IR products eg. IASI-CDR, IASI-KOPRA, and CrIS global ozone products are described with varying degrees of detail or with Figures that are suggestive but not conclusive.

OVERALL COMMENT – The paper poses good questions, presents a reasonable approach for its calculations and selection of results and is well-arranged. However, it leaves too many unanswered questions and does not advance the scientific understanding of its ozone trends beyond the first TOAR study. The three most important aspects of the paper that require additional analysis are summarized as follows:
1>> Quality assurance and evaluation of the IASI-CDR. There is reference to a larger set of sonde stations used for comparisons (43 stations; Fig 8 and 10) but Table 2 is inaccurate. The following stations are not available on HEGIFTOM archive because they are not homogenized datasets: Lindenberg, Prague, Tateno, Hong Kong, Broadmeadows, and Macquarie. We recommend using only homogenized datasets for reference datasets and trends. Comparison of trends is restricted to 7 stations, none truly tropical, 15 degrees or less, despite the fact that a number of cited and other TOAR II studies, both satellite and ground-based, focus on the tropics e.g., Froidevaux et al., 2025; Gaudel et al., 2024; Thompson et al., 2025. Time-series comparisons in Fig. 16 show only 7 stations with limited geographical coverage.
Concerning the evaluation of IASI-CDR:
Extensive discussion of CDR vs earlier FORLI product appears – but no illustrations of why you expect the CDR product to perform better than the earlier one.

IASI-KOPRA product mentioned but why is there no extensive comparison with this product or at least a paragraph comparing results of Dufour et al. (https://egusphere.copernicus.org/preprints/2025/egusphere-2024-4096/) to those shown here, particularly for sonde comparisons?

Vertical discrepancies mentioned in comparisons with sondes (tropical, mid-latitudes, and polar) appear in Fig. 10. Although overall IASI column amounts are compared favorably to the sondes (Line 22, only 2% offset in tropics) can this be due to a cancelling of offsets illustrated in the Figure? Discuss the potential impact on the trends.

The CrIS-TROPESS a priori looks so much better than the IASI (varies with season and latitude). Although the IASI climatology (Fig. 7) looks reasonable, can the IASI a priori (Fig. B1) – with apparently little seasonal information and only latitude dependence- be a cause of the discrepancies with other products? Does inadequate representation of seasonality (monthly variations in trends are typical and significant in the tropics, for example: Stauffer et al., 2024; Thompson et al., 2021) propagate to trends that disagree with both sondes and UV-products? What additional insights can you derive from scatterplots with ozonesondes (Figs. A1-4)? Comparing to sondes seasonally can help identify discrepancies. There is extensive discussion of similarities and differences with CrIS but it doesn’t get to the crux of understanding the large negative ozone trends here that are at odds with other products.

2>> Inadequate number of ground-based (GB) reference sites. There are two aspects of this issue.
First, the authors show 43 potential stations (Table 2, Fig. 8) but make trend comparisons with only 7 ozonesonde data sets. Perhaps they don’t understand the fine points of which data are appropriate to use (see Van Malderen comment). There is no need to speculate, as the authors have done, on why their results do not resemble those from mostly UV sensors. All satellites now have the HEGIFTOM-1 trends at individual stations (some with multiple instruments, not only sondes) as the gold standard independent reference at 55 sites total for 2000-2022. The HEGIFTOM reprocessing includes references of the data for each instrument type (sonde, FTIR, UV Umkehr) to a global absolute standard. Your trends analysis should add at least 20 stations (exclude polar sites where IASI-CDR struggles with DOFS) to have a more representative picture of IASI performance. Note that of the 7 reference sites (Boulder, Hilo, Lauder), there is more than one GB record for comparison. The stations in the Table below have sufficiently temporally dense records for comparison.

Second, Line 330 states that months with only 1-2 sondes/month give ‘inadequate’ results for 50%-ile trends. In the accepted version of HEGIFTOM-1, it is shown that these trends (computed with QR or MLR) are unaffected by cutting from 4-5 sondes/ month to 2; only the uncertainty changes (increases). That is a second justification for using more sonde locations for the GB comparisons - candidates in the Table.

3>> Brevity of the IASI record is a concern for 2008-2019 trends. For comparison, we re-ran the QR trends for the authors’ selected 7 stations as well as 20 other HEGIFTOM reference ozonesonde stations (excluding polar sites). The results are listed in the Table below, which also includes results from the HEGIFTOM-1 2000-2022 trends for surface-300 hPa (in ppbv/decade) as well as XO3 (ppbv/decade) and DU (DU/decade) for surface to tropopause tropospheric columns as a reference.
Note for the recalculated 2008-2019 ozonesonde trends, that 4 out of the 7 author selected stations change sign of trend from the longer time series to the shorter time series. For example, Boulder, a station with a high certainty (p<0.05) associated with a negative trend for the longer time series (2000-2022), has a slightly positive trend with large uncertainties for the 2008-2019 period. Of the 27 ozonesonde stations listed in the table, 9 sites have trend sign changes. Discussion on the point of reduced reliability and value of the shorter time series (12 or 16 years vs 23 years in HEGIFTOM and sonde studies*) is needed and is now a view that can be made with confidence. In a sort of “reversal” of your paper’s message, a significant advance and outcome of your paper, with the contracted (2008-2019) HEGIFTOM calculation, is that TOAR II needs to recognize the limitations of datasets that cover fewer than ~20 years!

The uncertainties in the trends also increase with the shorter time series (ie. double those for the 2000-2022 time period – see Table below).

On your paper (also noted by R. Van Malderen) the reported uncertainties in Fig. 16 seem small for only 12 years of data. Can you check those and discuss your bootstrap method in more detail?

*In addition to the sondes studies you have referenced, Stauffer et al., 2024; Thompson et al., 2021; Thompson et al., 2024, there is an excellent new sonde trends paper submitted on Réunion SHADOZ and SAOZ time-series (1998-2021) submitted to Earth and Space Science: https://essopenarchive.org/doi/full/10.22541/essoar.174594999.98715985/v1

‘HEGIFTOM-1” below is posted. Final version is in press.
Van Malderen, R., Thompson, A. M., Kollonige, D. E., Stauffer, R. M., Smit, H. G. J., Maillard Barras, E., Vigouroux, C., Petropavlovskikh, I., Leblanc, T., Thouret, V., Wolff, P., Effertz, P., Tarasick, D. W., Poyraz, D., Ancellet, G., De Backer, M.-R., Evan, S., Flood, V., Frey, M. M., Hannigan, J. W., Hernandez, J. L., Iarlori, M., Johnson, B. J., Jones, N., Kivi, R., Mahieu, E., McConville, G., Müller, K., Nagahama, T., Notholt, J., Piters, A., Prats, N., Querel, R., Smale, D., Steinbrecht, W., Strong, K., and Sussmann, R.: Global Ground-based Tropospheric Ozone Measurements: Reference Data and Individual Site Trends (2000–2022) from the TOAR-II/HEGIFTOM Project, EGUsphere [preprint], https://doi.org/10.5194/egusphere-2024-3736, 2025.
Also referred to is Thompson et al., submitted, 2024, Posted in 2025:
Thompson, A. M., Stauffer, R. M., Kollonige, D. E., Ziemke, J. R., Cazorla, M., Wolff, P., and Sauvage, B.: Tropical Ozone Trends (1998 to 2023): A Synthesis from SHADOZ, IAGOS and OMI/MLS Observations, EGUsphere [preprint], https://doi.org/10.5194/egusphere-2024-3761, 2025.

TABLE. This is essentially an update of Table 1 in HEGIFTOM-1, Van Malderen et al., in press, 2025, run again with the QR method, same as employed in Boynard et al. Ozonesonde stations with homogenized data and sufficient sample size are listed and exclude near-polar regions. Yellow-coded lines represent the 7 author-selected IASI-sonde comparison stations. Orange-coded lines indicate where the sign of the trend changes based on the different periods of trends calculations (23 years vs. 12 years).
TABLE IS POSTED IN SUPPLEMENT
Citation: https://doi.org/10.5194/egusphere-2025-1054-CC3
- AC5: 'Reply on CC3', Anne Boynard, 12 Jul 2025
  
  The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2025/egusphere-2025-1054/egusphere-2025-1054-AC5-supplement.pdf
  
  Citation: https://doi.org/10.5194/egusphere-2025-1054-AC5

Status: closed

CC1:
'Comment on egusphere-2025-1054: TOAR-II guidelines', Martin Schultz, 10 Apr 2025

Dear authors,
The TOAR Steering Committee wishes to express their gratitude to you (as well as all authors of articles in the inter-journal Community Special Issue for the second Tropospheric Ozone Assessment Report) for submitting your manuscript to Atmospheric Chemistry and Physics. Even though your article was submitted after the special issue deadline of November 30th, the scientific community will very likely associate it with the other articles from this special collection. Therefore, your article should also follow the rules that are defined in the guidelines for submitting articles to the TOAR-II Community Special Issue (see https://igacproject.org/sites/default/files/2023-04/TOAR-II_Community_Special_Issue_Guidelines_202304.pdf): “To avoid confusion with the final assessment papers, “TOAR” or “Tropospheric Ozone Assessment Report” may not appear in the title of the paper, nor should the paper claim to be an official TOAR publication. However, it is acceptable to refer to TOAR in the manuscript as, for example, “In the context of the Tropospheric Ozone Assessment Report (TOAR) Phase Two focus working group on XYZ, etc.”.”
We therefore ask you to change the title of your article and remove the explicit reference to TOAR from the title.
This comment, of course, has no bearing whatsoever concerning the scientific quality or potential impact of your manuscript.
We kindly ask for your understanding.
Martin Schultz and Helen Worden on behalf of the TOAR-II Steering Committee

Citation: https://doi.org/10.5194/egusphere-2025-1054-CC1
- AC3: 'Reply on CC1', Anne Boynard, 12 Jul 2025
  
  The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2025/egusphere-2025-1054/egusphere-2025-1054-AC3-supplement.pdf
  
  Citation: https://doi.org/10.5194/egusphere-2025-1054-AC3
RC1:
'Comment on egusphere-2025-1054', Anonymous Referee #1, 05 May 2025

Boynard et al. present a comprehensive analysis on the quality and reliability of ozone columns observed from IASI, highlighting its consistency with another IR instrument (CrIS) and sonde observations, while pointing out discrepancies from UV instruments in terms of long-term trend analysis. This work includes rigorous validation and comparison by incorporating various datasets which is essential for understanding the spatiotemporal variability of tropospheric ozone, which has been under debate especially regarding recent global/regional trends. The manuscript is overall well written but there are some points to be considered and clarified before publication.
1. Section 2.1.1: Can the authors include a brief description on how the IASI-CDR O3 L2 products are retrieved? AK, DOFs, and a priori profiles are discussed as possible contributors to observed discrepancies but readers may not be familiar on how they would affect the retrievals.
2. L123-125: Please clarify what IASI-A/B refer to. Are they the products from Metop-A/B?
3. L230-234: Are the differences in tropospheric columns related to uncertainties in the tropopause height?
4. L260: O3 -> O₃
5. L338 "IASI pixels have been processed (only 3.5 out of 10)": Can you clarify what this "processed" means?
6. L375: a secondary second smaller peak -> a secondary smaller peak
7. L400-402: The two lines look nearly similar at around 200 hPa, they seem to diverge above that altitude. Assuming that IASI is more sensitive to temperature, I'm curious why the smoothed sonde observations (which I understood as to have applied IASI's vertical sensitivity, i.e., AK) show larger discrepancies than the raw sonde?
8. L443-445, Figure 12: The difference between the left/right panels over Southeast Asia is surprising. There should be a lot of biomass burning emissions in that region, which wouldn't have been particularly affected by the pandemic. Has the Lanina affected fire frequencies?
9. Figure 13: How significant was the drop in precursor emissions during the pandemic? It also looks like O₃ is bouncing back up recently. This has been observed in surface concentrations over the US (US EPA). Can the authors briefly comment on observed changes of NOx/VOCs and implications for surface O₃ trends?
10. L474: troposphere (450 hPa to thermal tropopause) troposphere. -> troposphere (450 hPa to thermal tropopause).
11. L479, Figure 14: Does cp450 refer to the lower troposphere? I suggest adjusting the figure labels so that they are consistent with the phrases used in the text.

Citation: https://doi.org/10.5194/egusphere-2025-1054-RC1
- AC1: 'Reply on RC1', Anne Boynard, 12 Jul 2025
  
  The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2025/egusphere-2025-1054/egusphere-2025-1054-AC1-supplement.pdf
  
  Citation: https://doi.org/10.5194/egusphere-2025-1054-AC1
RC2:
'Comment on egusphere-2025-1054', Anonymous Referee #2, 09 May 2025
The manuscript “Tropospheric Ozone Assessment Report (TOAR): 16-year ozone trends from the IASI Climate Data Record” presents and validates the IASI-CDR O₃ product, a reprocessed and homogenized version of IASI-FORLI, providing a consistent dataset of tropospheric ozone spanning 2008–2023. The dataset is validated through comparisons with the TROPESS-CrIS product globally and with ozonesonde measurements at selected sites. A comprehensive trend analysis is conducted across multiple vertical layers using 16 years of observations. The manuscript is well-structured, and it is informative, offering a valuable contribution to the understanding of tropospheric ozone dynamics and trends over the past two decades. Below, I provide several comments for the authors to consider before the manuscript can be recommended for publication:
The authors employ a 1° × 1° gridded resolution for comparing IASI and CrIS datasets. Given that both instruments have native spatial resolutions on the order of 12–14 km at nadir, it would be helpful for the authors to justify the use of this relatively coarse grid. Was this choice driven by data availability, computational efficiency, or other methodological considerations?

Was validation at finer spatial resolutions considered? The authors might comment on whether such an approach was explored and how it might influence the interpretation of inter-satellite differences.

The trend analysis is performed using quantile regression at the 50th percentile (median), which is robust. However, applying the same method to higher percentiles would help assess whether the trends differ when considering extreme ozone events. This would be especially relevant considering the reported decreasing trends in tropical regions, which diverge from findings using UV-based instruments like OMI that often suggest stable or increasing trends in similar regions.

The use of 1° × 1° resolution in the trend analysis may mask finer-scale changes, especially in urban regions where surface ozone levels are known to be increasing in many parts of the world. Since urban areas occupy a small fraction of a 1° grid cell, their signal could be diluted by surrounding lower-concentration regions, potentially biasing the trend analysis toward decreases. Could this resolution choice be contributing to the observed negative trends? Discussing this potential limitation would strengthen the interpretation of the trend results.

Lines 360-365. The manuscript notes that differences in tropospheric ozone columns between IASI and CrIS are partly driven by the a priori information used—fixed for IASI and seasonally/latitudinally variable for CrIS. Given the importance of the a priori in shaping retrievals, particularly under conditions of low sensitivity, could the authors comment on what might constitute a more optimal or standardized approach? For example, would using dynamic, seasonally and geographically resolved climatologies for both instruments help reduce inter-product differences?

In Figure C1, the most pronounced positive trends in lower tropospheric ozone appear in arid regions of Africa and Australia. It would be helpful for the authors to comment on the possible drivers behind these trends briefly. Are they associated with natural emissions (e.g., soil NOₓ), biomass burning, or transport processes?
Citation: https://doi.org/10.5194/egusphere-2025-1054-RC2
- AC2: 'Reply on RC2', Anne Boynard, 12 Jul 2025
  
  The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2025/egusphere-2025-1054/egusphere-2025-1054-AC2-supplement.pdf
  
  Citation: https://doi.org/10.5194/egusphere-2025-1054-AC2
CC2:
'Comment on egusphere-2025-1054: HEGIFTOM perspective', Roeland Van Malderen, 13 May 2025

Dear authors,

I'm writing this comment mainly from my perspective as HEGIFTOM co-chair.

First of all, I want to congratulate you with your nice paper. What I really like about the paper is the honest approach. From the first TOAR activity, and in the paper by Gaudel et al. (2018), the inconsistency in tropospheric ozone time variability (and even in the sign of the trend) among different satellite ozone retrievals (and even between different retrieval methods of the same satellite data) popped up. Your paper tries to contribute to this issue, and you do not hide the overall negative IASI trends, which contrast with UV-VIS satellite tropospheric ozone measurements and ground-based measurements, but really try to find explanations for it (nicely summarized in the conclusions). And the tropospheric ozone decline after 2019 is really a common feature in all those datasets.

However, what I do miss in the introduction (lines 50-60) and in the conclusions (lines 550-554) are more references to results from TOAR-II papers. Please look for more relevant references (also for (regional) trends) in the TOAR-II SI collection of papers: https://acp.copernicus.org/articles/special_issue1256.html.

With my HEGIFTOM hat on, I want to draw your attention to some additional clarifications that are needed when using the HEGIFTOM ozonesonde data. First of all, your Table 2 contains some errors, e.g. Lindenberg, Prague, Tateno, Hongkong, Broadmeadows, and Macquarie are not available as HEGIFTOM time series, because these data have not been homogenized. So, they can be only available as WOUDC data. It might be important to explicitly mention that only the HEGIFTOM (and SHADOZ) data have been homogenized (i.e. corrected for biases), which is very important for use in trend estimation. Also important to note is that the HEGIFTOM and SHADOZ data are identical: the SHADOZ data have been copied on the HEGIFTOM ftp-server. Also, I found it very surprising that only 7 stations fulfill your criteria of having at least 3 monthly launches and > 70% sampling. Knowing that most stations launch weekly and overall do not reveal large, consistent, gaps in their time series, this looks rather an underestimation to me. In our HEGIFTOM individual trends paper (see Van Malderen et al., 2025), with at least 2 monthly launches and more or less > 50% sampling in the 2000-2022 time period, we end up with 34 homogenized ozonesonde stations! Are QA/QC criteria driving this small sample? Which ones? Please give more details about the site selection.

Your mostly negative IASI tropospheric ozone trends in the 2008-2023 time period contrasts somewhat with the mixture of insignificant, positive and negative trends that we found for 55 HEGIFTOM sites (IAGOS, FTIR, ozonesonde, Umkehr, Lidar) for the time period 2000-2022 and the sfc - 300 hPa ozone column. To reconcile those trend differences between both studies, trends for the same time period (2008-2019) and for exactly the same metric might be compared (another comment posted by co-authors from our study might follow). W.r.t. this last comment, as you are looking at the time behaviour/trends of different tropospheric ozone metrics (sfc-tropopauze, sfc - 450 hPa, 450 hPa - tropopauze), as we did, although we used different metrices (sfc-300 hPa, sfc-700 hPa, 300-700 hPa), some reference or discussion w.r.t. our results might be appropriate.

In our HEGIFTOM trends paper, we compared and discussed the Payerne tropospheric ozone time series (also used DLM for it) quite extensively, so you might use of this information when discussing the drift between the Payerne ozonesonde and IASI time series.

Recently, as you are aware, Dufour et al. (2025) also looked at the comparison and (regional) trends of another IASI product, IASI-O3 KOPRA, with a limited sample of ozonesondes (Boulder, Payerne, and Uccle are common). You do refer to this study, but I would expect a more detailed comparison of the results of both studies! To my opinion, at least a paragraph might be needed explaining the similarities or the differences between the results of both studies.

Also, looking at Fig. B1 (the constant a priori used in the IASI retrievals vs. the latitudinal and seasonal variable a priori used for CrIS), I wonder what the impact of such a time invariant (right?) a priori would have on the calculated trends.... The authors might comment on this.

Finally, I am also quite surprised by your small trend uncertainties (e.g. in comparison with ours, which are quite consistent between the QR and MLR trend estimation tools used). You might want to provide some additional information on how exactly those have been calculated.

Thank you for taking my comments into consideration and I wish you good luck in the review of your manuscript!

With kind regards,

Roeland Van Malderen

References:
Dufour, G., Eremenko, M., Cuesta, J., Ancellet, G., Gill, M., Maillard Barras, E., and Van Malderen, R.: Performance assessment of the IASI-O3 KOPRA product for observing midlatitude tropospheric ozone evolution for 15 years: validation with ozone sondes and consistency of the three IASI instruments, EGUsphere [preprint], https://doi.org/10.5194/egusphere-2024-4096, 2025.
Van Malderen, R., Thompson, A. M., Kollonige, D. E., Stauffer, R. M., Smit, H. G. J., Maillard Barras, E., Vigouroux, C., Petropavlovskikh, I., Leblanc, T., Thouret, V., Wolff, P., Effertz, P., Tarasick, D. W., Poyraz, D., Ancellet, G., De Backer, M.-R., Evan, S., Flood, V., Frey, M. M., Hannigan, J. W., Hernandez, J. L., Iarlori, M., Johnson, B. J., Jones, N., Kivi, R., Mahieu, E., McConville, G., Müller, K., Nagahama, T., Notholt, J., Piters, A., Prats, N., Querel, R., Smale, D., Steinbrecht, W., Strong, K., and Sussmann, R.: Global Ground-based Tropospheric Ozone Measurements: Reference Data and Individual Site Trends (2000–2022) from the TOAR-II/HEGIFTOM Project, EGUsphere [preprint], https://doi.org/10.5194/egusphere-2024-3736, 2025.

Citation: https://doi.org/10.5194/egusphere-2025-1054-CC2
- AC4: 'Reply on CC2', Anne Boynard, 12 Jul 2025
  
  The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2025/egusphere-2025-1054/egusphere-2025-1054-AC4-supplement.pdf
  
  Citation: https://doi.org/10.5194/egusphere-2025-1054-AC4
CC3:
'Comment on egusphere-2025-1054', Anne Thompson, 13 May 2025
IASI-Boynard Paper Comment – 13 May 2025
Anne Thompson, Debra Kollonige, Ryan Stauffer

SUMMARY COMMENTS AND SUGGESTIONS FOR REVISION
This Comment follows that of HEGIFTOM Co-Chair R. Van Malderen and amplifies his major points. We refer to the same “Van Malderen et al, in press, 2025” that he does and designate it as “HEGIFTOM-1” because there is a second HEGIFTOM paper in review in the TOAR II collection. We go beyond Van Malderen’s comments to 3 summary recommendations:
The selection and number of ground-based stations for which ozone data are compared to the IASI-CDR product is inadequate in number and ‘non-homogenized’ datasets should not be used. In contrast to the 7 stations used in this paper, more than 25 stations, homogenized over the 2008-2019 period, need to be used in evaluation of the IASI products and the trends. The 7 sites included no equatorial stations and far too few northern mid-latitude stations, in some cases where more than sonde data are available. The recommended ozonesonde 27 stations appear in Table 1 at the end of this Comment.

The authors conclude with general, not well-defined speculation on why there is little progress in how computed trends from the new IASI products diverge from the UV-based product trends from the Gaudel et al. (2018) TOAR I paper. More analysis and insights on this issue are needed before the paper is worthy of publication in a quality Copernicus journal. Specific questions are raised for consideration.

HEGIFTOM-1 is now “THE Reference dataset” for trends comparisons. The mismatch of years (2008-2019 in Boynard et al. vs 2000-2022 in HEGIFTOM-1) is speculated as one reason for why the trends in this paper differ from the ground-based trends in HEGIFTOM-1. We reran the trends in HEGIFTOM-1 for 27 stations for 2008-2019, see Table below, to support a valid comparison. Some of the HEGIFTOM-1 site trend signs changed and uncertainties increased, leading to a clear TOAR-worthy conclusion that trends computed from 12 or 16 years of IASI or ground-based data is inadequate. A revision must include this important result!

SYNOPSIS – This study presents an IASI product over 16 years, consisting of contributions from METOP-A, METOP-B and METOP-C, merged to create the IASI-CDR (Climate Data Record, 2008-2023). Several tropospheric ozone columns (to 450, to tropopause) are presented and compared on a monthly mean basis with (1) the CrIS IR ozone product and with (2) comparable ozonesonde columns from 43 stations over the period. Trends for global mid-latitudes and the tropics are computed using Quantile Regression (QR) for the period 2008-2019 (pre-COVID) and 2008-2023; the latter trends reflect an apparent COVID impact. As in the TOAR I paper in which Gaudel et al. (2018) summarized satellite product trends (2005/2008 – 2016) showing IASI (FORLI version) to be an outlier compared to UV-type satellite records, it appears that the IASI-CDR trends (now 2008-2019, omitting COVID period) is an outlier with the corresponding UV-based time-series. In both cases the greatest discrepancies are in the tropics except for SE Asia where all products display increases of ~1-2 DU/decade. The UV-type satellite products tend to be more variable with regions in the tropical Americas, Africa and Atlantic showing some increases (~2005-2019; Gaudel et al., 2024; Thompson et al., 2024). The IASI-CDR TOAR II-period (Fig. 12 in paper) shows little increase over Europe, a decrease over North America and only modest increases in east Asia, again in disagreement with more variable UV-type satellite trends. IASI 2008-2019 trend comparisons are made with 7 ozonesonde time-series: one subtropical site, 4 northern mid-latitude (majority European), 1 southern mid-latitude (Fig. 16). The authors speculate briefly on causes of the persistent discrepancy between the IASI vs UV trends and what is emerging as the prevailing view of tropospheric changes over the prior ~15 years. Comparisons among comparable IR products eg. IASI-CDR, IASI-KOPRA, and CrIS global ozone products are described with varying degrees of detail or with Figures that are suggestive but not conclusive.

OVERALL COMMENT – The paper poses good questions, presents a reasonable approach for its calculations and selection of results and is well-arranged. However, it leaves too many unanswered questions and does not advance the scientific understanding of its ozone trends beyond the first TOAR study. The three most important aspects of the paper that require additional analysis are summarized as follows:
1>> Quality assurance and evaluation of the IASI-CDR. There is reference to a larger set of sonde stations used for comparisons (43 stations; Fig 8 and 10) but Table 2 is inaccurate. The following stations are not available on HEGIFTOM archive because they are not homogenized datasets: Lindenberg, Prague, Tateno, Hong Kong, Broadmeadows, and Macquarie. We recommend using only homogenized datasets for reference datasets and trends. Comparison of trends is restricted to 7 stations, none truly tropical, 15 degrees or less, despite the fact that a number of cited and other TOAR II studies, both satellite and ground-based, focus on the tropics e.g., Froidevaux et al., 2025; Gaudel et al., 2024; Thompson et al., 2025. Time-series comparisons in Fig. 16 show only 7 stations with limited geographical coverage.
Concerning the evaluation of IASI-CDR:
Extensive discussion of CDR vs earlier FORLI product appears – but no illustrations of why you expect the CDR product to perform better than the earlier one.

IASI-KOPRA product mentioned but why is there no extensive comparison with this product or at least a paragraph comparing results of Dufour et al. (https://egusphere.copernicus.org/preprints/2025/egusphere-2024-4096/) to those shown here, particularly for sonde comparisons?

Vertical discrepancies mentioned in comparisons with sondes (tropical, mid-latitudes, and polar) appear in Fig. 10. Although overall IASI column amounts are compared favorably to the sondes (Line 22, only 2% offset in tropics) can this be due to a cancelling of offsets illustrated in the Figure? Discuss the potential impact on the trends.

The CrIS-TROPESS a priori looks so much better than the IASI (varies with season and latitude). Although the IASI climatology (Fig. 7) looks reasonable, can the IASI a priori (Fig. B1) – with apparently little seasonal information and only latitude dependence- be a cause of the discrepancies with other products? Does inadequate representation of seasonality (monthly variations in trends are typical and significant in the tropics, for example: Stauffer et al., 2024; Thompson et al., 2021) propagate to trends that disagree with both sondes and UV-products? What additional insights can you derive from scatterplots with ozonesondes (Figs. A1-4)? Comparing to sondes seasonally can help identify discrepancies. There is extensive discussion of similarities and differences with CrIS but it doesn’t get to the crux of understanding the large negative ozone trends here that are at odds with other products.

2>> Inadequate number of ground-based (GB) reference sites. There are two aspects of this issue.
First, the authors show 43 potential stations (Table 2, Fig. 8) but make trend comparisons with only 7 ozonesonde data sets. Perhaps they don’t understand the fine points of which data are appropriate to use (see Van Malderen comment). There is no need to speculate, as the authors have done, on why their results do not resemble those from mostly UV sensors. All satellites now have the HEGIFTOM-1 trends at individual stations (some with multiple instruments, not only sondes) as the gold standard independent reference at 55 sites total for 2000-2022. The HEGIFTOM reprocessing includes references of the data for each instrument type (sonde, FTIR, UV Umkehr) to a global absolute standard. Your trends analysis should add at least 20 stations (exclude polar sites where IASI-CDR struggles with DOFS) to have a more representative picture of IASI performance. Note that of the 7 reference sites (Boulder, Hilo, Lauder), there is more than one GB record for comparison. The stations in the Table below have sufficiently temporally dense records for comparison.

Second, Line 330 states that months with only 1-2 sondes/month give ‘inadequate’ results for 50%-ile trends. In the accepted version of HEGIFTOM-1, it is shown that these trends (computed with QR or MLR) are unaffected by cutting from 4-5 sondes/ month to 2; only the uncertainty changes (increases). That is a second justification for using more sonde locations for the GB comparisons - candidates in the Table.

3>> Brevity of the IASI record is a concern for 2008-2019 trends. For comparison, we re-ran the QR trends for the authors’ selected 7 stations as well as 20 other HEGIFTOM reference ozonesonde stations (excluding polar sites). The results are listed in the Table below, which also includes results from the HEGIFTOM-1 2000-2022 trends for surface-300 hPa (in ppbv/decade) as well as XO3 (ppbv/decade) and DU (DU/decade) for surface to tropopause tropospheric columns as a reference.
Note for the recalculated 2008-2019 ozonesonde trends, that 4 out of the 7 author selected stations change sign of trend from the longer time series to the shorter time series. For example, Boulder, a station with a high certainty (p<0.05) associated with a negative trend for the longer time series (2000-2022), has a slightly positive trend with large uncertainties for the 2008-2019 period. Of the 27 ozonesonde stations listed in the table, 9 sites have trend sign changes. Discussion on the point of reduced reliability and value of the shorter time series (12 or 16 years vs 23 years in HEGIFTOM and sonde studies*) is needed and is now a view that can be made with confidence. In a sort of “reversal” of your paper’s message, a significant advance and outcome of your paper, with the contracted (2008-2019) HEGIFTOM calculation, is that TOAR II needs to recognize the limitations of datasets that cover fewer than ~20 years!

The uncertainties in the trends also increase with the shorter time series (ie. double those for the 2000-2022 time period – see Table below).

On your paper (also noted by R. Van Malderen) the reported uncertainties in Fig. 16 seem small for only 12 years of data. Can you check those and discuss your bootstrap method in more detail?

*In addition to the sondes studies you have referenced, Stauffer et al., 2024; Thompson et al., 2021; Thompson et al., 2024, there is an excellent new sonde trends paper submitted on Réunion SHADOZ and SAOZ time-series (1998-2021) submitted to Earth and Space Science: https://essopenarchive.org/doi/full/10.22541/essoar.174594999.98715985/v1

‘HEGIFTOM-1” below is posted. Final version is in press.
Van Malderen, R., Thompson, A. M., Kollonige, D. E., Stauffer, R. M., Smit, H. G. J., Maillard Barras, E., Vigouroux, C., Petropavlovskikh, I., Leblanc, T., Thouret, V., Wolff, P., Effertz, P., Tarasick, D. W., Poyraz, D., Ancellet, G., De Backer, M.-R., Evan, S., Flood, V., Frey, M. M., Hannigan, J. W., Hernandez, J. L., Iarlori, M., Johnson, B. J., Jones, N., Kivi, R., Mahieu, E., McConville, G., Müller, K., Nagahama, T., Notholt, J., Piters, A., Prats, N., Querel, R., Smale, D., Steinbrecht, W., Strong, K., and Sussmann, R.: Global Ground-based Tropospheric Ozone Measurements: Reference Data and Individual Site Trends (2000–2022) from the TOAR-II/HEGIFTOM Project, EGUsphere [preprint], https://doi.org/10.5194/egusphere-2024-3736, 2025.
Also referred to is Thompson et al., submitted, 2024, Posted in 2025:
Thompson, A. M., Stauffer, R. M., Kollonige, D. E., Ziemke, J. R., Cazorla, M., Wolff, P., and Sauvage, B.: Tropical Ozone Trends (1998 to 2023): A Synthesis from SHADOZ, IAGOS and OMI/MLS Observations, EGUsphere [preprint], https://doi.org/10.5194/egusphere-2024-3761, 2025.

TABLE. This is essentially an update of Table 1 in HEGIFTOM-1, Van Malderen et al., in press, 2025, run again with the QR method, same as employed in Boynard et al. Ozonesonde stations with homogenized data and sufficient sample size are listed and exclude near-polar regions. Yellow-coded lines represent the 7 author-selected IASI-sonde comparison stations. Orange-coded lines indicate where the sign of the trend changes based on the different periods of trends calculations (23 years vs. 12 years).
TABLE IS POSTED IN SUPPLEMENT
Citation: https://doi.org/10.5194/egusphere-2025-1054-CC3
- AC5: 'Reply on CC3', Anne Boynard, 12 Jul 2025
  
  The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2025/egusphere-2025-1054/egusphere-2025-1054-AC5-supplement.pdf
  
  Citation: https://doi.org/10.5194/egusphere-2025-1054-AC5

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Short summary

This study analyzes 16 years of global ozone data to assess its impact on air quality and climate. Using satellite measurements, we observed a global decrease in tropospheric ozone, particularly in tropical and European regions. The study highlights the importance of long-term data for tracking trends, especially during events like the pandemic. We emphasize the need for improved data processing and integrating multiple datasets to better understand ozone trends.


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