This manuscript by Keppens et al. reports on harmonisation efforts applied to sixteen tropospheric ozone satellite data products which also contributes to CEOS activities and the second TOAR assessment. In contrast to common harmonisation approaches to perform spatiotemporal resampling or bias corrections, here the vertical perspective is harmonised to account for differences in the vertical smoothing and sampling between the different ozone satellite products. The authors first define and then address several harmonisation needs. This is done by harmonising tropospheric top level definitions and a-priori information of several types of ozone data products (nadir profile products, limb/reanalysis-nadir matching products and convective cloud differential products). Large efforts are put in the harmonisation of the nadir profile products for which the CDF framework is used and two methods are eventually selected. The column products are finally harmonised using a fill-in method exploiting CAMSRA data. To investigate the effect of the harmonisation on the ozone products, global maps and time series are compared before and after harmonisation. It is concluded that, despite improvements, the presented harmonisation efforts only partially account for the observed differences between the satellite products and that not only differences in spatiotemporal smoothing and sampling but differences in the measurement uncertainty or other instrument and/or retrieval specific effects are the main challenges in the assessment of different tropospheric ozone satellite products.

Given this comprehensive study demonstrating the challenges of interpreting, using and comparing different (tropospheric ozone) satellite data products the manuscript is well-suited for publication in AMT. However, I suggest to address several (mostly minor and technical) aspects before final publication to further improve the quality of the manuscript.

General Comments:

Sometimes the British English (BE) and sometimes the American English (AE) spelling of individual words is used. As an example, in the abstract both “harmonisation” and “harmonization” can be found separated only by three lines. Although there will be a thorough language editing by AMT, I suggest already now to be consistent throughout the manuscript and to decide on one of both options (probably BE as it is also used in the title).

In general, the consistency of several aspects might be improved throughout the manuscript and the presentation style might be further harmonised. Firstly, this refers to the way things are named (e.g., GOME2 vs. GOME-2). Secondly, there should be a consistent use of abbreviations which are defined. Lastly, also the way information is provided might be addressed. An example here is the consistent representation of geographical coordinates (e.g., 60° N vs. 60°N vs. 60N or 1° x 1° vs. 1° by 1° vs. 1 x 1 degree). See also the specific comments below for other examples.

Main comments:

Section 2 (especially Sections 2.1, 2.2 and 2.3) + Table 1: In this section, the different data products relevant for this work are summarised and detailed descriptions of the individual products are given. However, there are some inconsistencies between the information provided in Table 1 and in the text. Also, not for all of the different data products the same information content is provided. For example, for the TROPOMI ozone profile product, no information on the vertical grid resolution and the top level is given, while this information is provided for others, for instance for IASI. So, the authors may consider harmonising the information content provided for the different products (as far as possible). Further, many abbreviations are used in this section (and in general throughout the manuscript). I suggest to consistently introduce them at their first occurrence and then to mostly use these abbreviations. Another point regarding the naming of the data products: consistent naming should be used for the same product, e.g., GOME2 (in Table 1 and also Fig. 2, 3 + 4) vs. GOME-2 (in the text). The authors might consider revising Section 2 with respect to consistency of naming conventions etc., I think this will be helpful for future readers of this paper. Thereby, some of the specific comments listed below (which is likely not a complete list of small action points) might be considered.

Figures in general: Several different styles are used for the different figures. While this does not pose a major problem, some harmonisation in the figures styles and layouts might be still considered. The authors may also consider using consistent/similar colours for Fig. 3 + 4 and Fig. 5. In the current version, the colours used for the results of the different harmonisation approaches are contra intuitive, e.g., USR is shown in blue in Fig. 3 + 4, while in Fig. 4 blue is used for the WAV results. Several figures lack the units of the shown quantities or even do not contain the latter at all. I suggest to add this information to the axes-labels or colourbars (see also specific comments below). Lastly, some figures might be enlarged which particularly applies to the font sizes of the axes- and colourbar-labels (again see also specific comments below).

Result part: To assess the effects of the harmonisation efforts on the ozone results, a large part focuses on analyses of the spatiotemporal inter-product dispersion. In my understanding the term “dispersion” does not always describe the same “quantity” throughout the manuscript and refers sometimes to slightly different definitions. Although this might be a misunderstanding, I strongly recommend to introduce and define the term “dispersion” somehow. This should include a brief description of how it was determined and what it describes in the respective (sub-) section. This could be done in a similar way as in Section 6.1, where the term “dispersion” for the multi-annual mean maps is explained. I have the impression that this would help future readers of the paper.

Minor/specific and technical comments:

Page 1, line 1: The first Tropospheric Ozone Assessment Report is sometimes referred to as TOAR (mainly in the first part) and sometimes as TOAR-I (mainly in the second part). Please consider a consistent naming throughout the manuscript.

Page 3, line 45: It is mentioned that the different ozone products are spanning over 21 years (January 2003 to December 2023). In Table 1, however, some data products start before 2003, e.g., ERS2/GOME v3 starts in 1995. In my understanding these 21 years refer to the final harmonised products. Probably cross-check and comment on that.

Page 3, lines 53-56: Does a reference for this product already exists? If yes, maybe add it here.

Page 3, lines 59-60: Probably replace “The last section before the conclusions…” by “Section 6….”.

Page 4, line 76: Already introduce the abbreviation LISA here and leave the long version out in line 122 on page 5.

Page 4, line 77 + Table 1: Please consider a consistent naming for GOME-2 (in the text vs. GOME2 in the table) here and also in the rest of the manuscript (e.g., lines 71 or 81).

Page 4, line 81: Please also consider a consistent naming for ERS-2 (vs. ERS2 in the table).

Page 4, line 85: Consider consistency between BE and AE throughout the manuscript.

Page 5, line 88: Cross-check the value of 10¹² molecules of ozone per cubic meter in the troposphere. I feel it should read “molecules per cubic centimeter”.

Page 5, line 91: Like above for the Hartley band, you might add here the wavelength range for the Huggins bands.

Page 5, line 95-96: Can this negative bias and its implications easily be estimated. If so, probably add some information here?

Page 5, lines 100-102: In Table 1, the native resolution is listed to be 28 x 28 km². Probably clarify that.

Page 5, line 110: The time periods given here are not consistent to the ones listed in Table 1.

Page 5, line 112: Consider using the already introduced abbreviation OE for optimal estimation here.

Page 5, line 114: I guess it should be FORLI-O3.

Page 6, line 126: The sentence regarding the synergism of IASI and GOME2 is basically the same as the first sentence of this paragraph (page 5, line 121). You may consider skipping one of them or rephrasing a little bit.

Page 6, lines 139-140: RAL was already introduce and the long version might be skipped here.

Page 6, lines 134-149: The descriptions in this paragraph give the impression that this product also requires a lot of harmonisation efforts. In case this is not a complete misunderstanding. Could you briefly comment on what was done differently in their harmonisation efforts?

Page 6, line 154: Probably add “(IUP-UB)” after University Bremen to make a link to Table 1.

Page 7, line 164: Probably add “…ranging from 2012 to 2022…”.

Page 7, line 180: Please introduce the abbreviation EPIC somewhere.

Page 7, line 183: Please also introduce the abbreviation LRT here (used in Table 1).

Page 8, line 191: The abbreviation CCD was already introduced earlier.

Page 8, line 192: 1 x 1 degree might be consistently represented by 1° x 1° as done mainly in the rest of the manuscript.

Page 8, line 201: Please change NO2 to NO₂.

Page 8, line 203: In Table 1, a 6 h resolution is given. Probably comment on this in the text.

Page 8, line 208: Please specify SBUV/2.

Page 8, lines 207-220: This paragraph gives an overview on effects on the quality of the ozone analysis field caused by changes in the observing system. I wonder how the mentioned effects affect the study presented here? You may comment on that.

Page 9, lines 224-230: As stated in the text differences are in general below 1 DU. Nevertheless, there seems to be indeed a systematic trend from ca. 2010 onwards which was already mentioned by the authors as a possible risk of this approach. Could you comment on the effect this choice might have on the final harmonised products?

Figure 1: Please add a space between y-axis label and “[DU]”.

Page 10, line 266: Consider rephrasing the end of the sentence. Suggestion: “…, including the use of the CAMSRA….”

Page 10, line 271: Please consider the consistent use of 1° x 1° instead of 1° by 1°.

Page 10, line 272: Add the unit (°) to the numbers.

Page 10, line 273: Compare above (5° x 5°).

Page 11, line 289: I guess it should read: “Such a process…”

Page 11, line 299: “…use of an a-priori…”

Page 11, line 304: You might consider using the above-defined abbreviation OE here.

Page 12, lines 326-335: Please also define the I used in the equations.

Tables 2 + 3: I find it a little bit confusing that CDF is marked as “Not applied in this work.”, whereas on page 12, lines 322-324 it reads “The latest formulation of the CDF framework has therefore been applied in this work…”. In my understanding this is no contradiction as all methods rely on the CDF framework. Nevertheless, you might consider commenting on this and make the text/Tables clearer on that point. Further, I find the table captions a little bit confusing but do not have a clear opinion how to improve that, so this remains just a comment. A last point regarding the tables (just a suggestion): You might consider changing the order of the different methods according to the order in the text (basically putting USR to the end).

Page 13, line 347: You might add “(second row)” to that sentence.

Page 14, lines 387-388: After reading this sentence several times, I still have the impression that I do not get it right. You might consider rephrasing it.

Page 14, line 389: Can you briefly comment on how large the expected discrepancies by this approximation are?

Page 14, line 391: See above (1° x 1°).

Page 14 + 15, lines 400-404: Consider slightly rephrasing this sentence (i.e., description of what is depicted in Fig. 2) to make it clearer. This would then also apply to the first sentence of the caption of Fig. 2. Further, I wonder why WAV is chosen as “reference” of the comparison. Probably, I missed it but is the WAV method the common approach? Maybe you could briefly comment on that.

Figure 2, caption: “…closer to each other…”

Page 15, line 403: This means…

Figures 3 + 4: You might consider adding “Year” as a y-axis-label.

Page 16, lines 426-428: Probably discuss (or at least comment) that this increase of ΔSMOD apparently depends also on the top level definition, i.e., there is no clear increase for GOME2 in the middle column of Fig. 3.

Page 16, lines 434-436: Probably briefly repeat these reasons. I think keywords might be sufficient.

Page 16, line 436: Please remove “)”.

Figure 5: If possible, consider enlarging the figure or at least the font size of the x- and y-axes-labels. In a printed version, the y-labels are really hard to read. The same applies to the legend of the figure. Like for Fig. 3 + 4 the top level definition might be used as titles for the two columns. As mentioned above (see main comments), you may consider using consistent colours for the different methods for Fig. 3 + 4 and Fig. 5. You also might specify the abbreviation GND somewhere.

Page 16, line 449: I would argue that the dispersions reach higher than 10 DU for some cases. Probably increase the range mentioned in the text.      

Page 17, line 464: Probably insert the word “only” before +1 % to stress that this is not of great relevance.

Page 17, line 467: “spatio-temperal“ should be changed to “spatiotemporal” to be consistent to other parts of the manuscript.

Page 17, line 483: Probably, briefly mention the two selected (out of four) approaches. (In my understanding APR and CAMSRA fill-in.)

Figures 6 + 7: Both figures should be enlarged as they are (even on a screen) hard to read. This in particular applies to the font size of the subplot titles. Further, I recommend adding the shown quantity and its unit to the colourbars. This could be done once in the middle for each figure.

Page 18, Title of Sect. 6.1: I suggest introducing the abbreviation MAM here (or somewhere else in the beginning of this section). It is used several times in the following but not introduce before.

Page 18, line 492: In my understanding it’s an inter-product dispersion. Probably, mention it explicitly. See also main comments above.

Page 18, line 495: Probably call it consistently near-global rather than “pseudoglobal” also in the equation.

Page 18, line 496: Consider adding one or two sentences on what can be learned from Fig. 9. For example, why is the change for GOP-ECV so large or why are their no values for GOME and SCIAMACHY. Although the first aspect is explained below, I think there should be in general a sentence on Fig. 9 (might be a different aspect).

Figure 8: Again, consider adding the shown quantity and its unit to the colourbars (once should be sufficient). It would be also helpful for the reader to add titles to the columns indicating the difference between the columns (like mentioned in the caption). Probably, change “…after minus before harmonisation” to something like “upon harmonisation” (just a suggestion).

Figure 9: Consider adjusting the y-label to make the representation of geophysical coordinates consistent to the caption and the rest of the manuscript (see also general comment above).

Page 18, lines 506-507: Probably, add in brackets where this can be seen. In my understanding Fig. 8, left column.

Page 18, lines 509-512: Also here, please add where (which plot) this can be seen.

Page 19, line 518: I think it should read: left and right in the brackets instead of blue and red. Please cross-check.

Figure 10: Please consider enlarging the font size of the axes-labels and the legend (at least the size of the ones in Fig. 11). Further, I suggest to adjust the y-axis-label to a more practical name for tropospheric column. 

Figure 11: Same as Fig. 10 for the y-axis-label. In my understanding not the tropospheric column but the dispersion is shown. Further, please add the unit DU to the y-axis-label. Why does it say “L3” in the legend although L2 is in the title and should be correct (similar for the titles in Fig. 10)?

Section 6.2. in general: Consider also adding some statements on Fig. 10 in the text or include the figure in the considerations in the paragraph on page 19, lines 524-532.

Page 19, lines 524-532: Consider adding where the different conclusions can be seen, e.g., compare Fig. 11, upper panel or so.

Figure 12: Please add the unit to the y-axis-label and enlarge the font sizes of labels and legend. I think the dispersion is shown, please add this information to the caption. Further, consider using consistent representations of the geographical coordinates in the legend (compare above). I am again little bit confused about the L3 in the title of the panels.

Page 19, lines 533-536: For some regions (e.g., 40° S – 60° S) there seem to be systematic seasonal cycles of the inter-product dispersion. I wonder if these seasonal cycles are understandable? If so, you might comment on that.

Page 20, lines 553-554: You might briefly repeat why these two methods were chosen. While this was done in the main part, I think it would be helpful for the reader to get a short reminder here in the conclusions section.

Page 20, lines 555-559: Like the previous comment: consider briefly mention which of the two was chosen and why.