The IAGOS programme is unique in providing routine observations of the vertical profiles of CO, O3 and other compounds in the troposphere and UT/LS. As such it is a key source of information to evaluate profiles retrieved from satellite sensors. The paper of Barret et al. is, to my opinion, an important addition to the existing literature on IASI-CO retrievals and validation efforts. It is well written and was a pleasure to read, providing insights in the two retrievals and how they compare with IAGOS. As such I am in favour of publishing these results, after my comments below have been addressed.

General comment: 

The other important source of validation data are the surface remote sensing FTIR instruments (NDACC). These are mentioned briefly but I would like to read more about the comparisons with the findings from these evaluations (e.g. EUMETSAT validation report). Are the main conclusions similar? Is the underestimate reported for the TCC quantitatively in agreement with FTIR evaluations?

Abstract:

l 5: It would be good to mention that MLS CO is used for the stratospheric part.

l 9: Does the period "2008-2020" cover 12 or 13 years?

l 12: What is meant by "to capture the CO variabilities"? Variability in time, or in the profile? I suggest to replace "variabilities" by "variability".

l 13: Same question for "correlation coefficients". Does this refer to temporal correlations or vertical profile correlations?

l 21: The last sentence "Our validation results will provide a better characterisation of IASI-CO data to the users and help improve the retrievals for future versions" can be formulated better. I would replace "will provide" by "do provide". What does "better characterisation of IASI-CO data to the users" mean? Please reformulate "improve the retrievals for future versions".

l 36: Remove the "(" before "Hurtmans".

l 47: The introduction gives the impression that the study of De Wachter 2012 is the only comparison with IAGOS. It would be good to mention that George 2015 also includes comparisons with IAGOS profiles, although this comparison is somewhat limited.

Introduction: The CO retrievals are introduced with a good set of references. But I am missing a paragraph in the introduction on IAGOS. What is it, what can be measured, some key achievements and key references, including validation work, to provide the reader with a background and further reading.

Sec 2.1: The key reference for SOFRID is De Wachter. But this paper is more than 10 years old. Has there been any CO retrieval development in the meantime based on SOFRID? Are the updates made to SOFRID CO documented somewhere in more detail (is there a recent ATBD)?

l 70: "instead of operational EUMETSAT Level 2 IASI products". Why? 

l 71: Could you please motivate why "The noise of the measurement covariance matrix has been reduced from 1.4 to 1.0 · 10−8 W/(cm2 sr cm−1)" I assume that the performance of IASI has not improved over time?

l 80: Same question as for SOFRID. The key ref for FORLI is from 2012 as well, again more that 10 years old. I noted the ATBD is from 2014. Are there updates compared to the 2012 Hurtmans paper? Any relevant evaluations of the FORLI CO retrieval published after 2012? Does FORLI v20151001 introduce any important changes compared to Hurtmans 2012?

l 97: What is the reason that "only airports providing at least 60 days with valid data " were selected? Is a number of 60 linked to the quality of the comparison? Even just one profile can still provide a useful comparison.

l 122: The difference in DFS is striking. The statement that "the reduction of the noise of the measurement covariance matrix relative to De Wachter et al. (2012)" is partly responsible askes for some more explanation. How can the noise be reduced by such a large factor? How does this compare to the a-priori noise assumed in FORLI? Please provide more detail.

l 119, 133: I note that TCC is used in two ways, either as total column (retrieval of 1 quantity) or as total atmosphere (for DFS = 2.9). Maybe better to use "Total atmosphere" instead of TCC in table 1.

l 177: I find the statement "because they provide the best assessment of the real differences between the in-situ and the remote sensed data" a bit dubious. Equation 1 describes how the retrieval relates to the real profile, and provides the best way of comparing. This is also evidenced by the validation results, e.g. l 238.

l 297: This mentions "two major updates of EUMETSAT Level 2 data processing". Could you please provide details on these updates in Sec. 2. 

l 377: "timeseries"

l 387: What does "not statistically significant" refer to? From the paper I got the message that TCC globally is not really different between SOFRID/FORLI, but that negative biases are observed.

General comments:

IASI now provides an impressive database of CO observations from space, here covering more than 12 years. Given the importance of CO as a major tracer for atmospheric composition and air quality, independent validation of the complex retrieval algorithms available (SOFRID and FORLI) is crucial. In this manuscript, profiles of IAGOS airplane ascents and descents at 33 airports have been used for this purpose. The unique in-situ dataset from IAGOS is one of the few observational data sources of tropospheric CO and has a good coverage, at least for the Northern Hemisphere, with a focus on urbanized regions over Europe, North America and East Asia. The profile information is of particular interest for the validation, as the satellite retrievals exhibit some (limited) information on the CO vertical profile. Although a preliminary validation study of this kind has been performed already (De Wachter et al., 2012), this has been done with only two years of data and two airports. Thus, an updated validation study is due and should be able to reflect the developments in observational techniques, retrieval algorithms, and representation of spatio-temporal variability of CO.

While the authors achieve to provide an extensive validation and present datasets which reveal relevant information, the manuscript also show some weaknesses: First, some information is missing about the details of the IASI retrievals and its development over time. There are several quality and error flags as well as data processing updates mentioned without explanation and discussion of potential impact on the validation. It is also not clear what IASI-B/C will provide and if there are any differences to IASI-A. It would also be intriguing to learn more about the differences between SOFRID and FORLI which lead to such fundamentally different averaging kernels and its implications when using the data. Smoothed IAGOS data is shown in Figures 6-12 as well as in Table 2 and in the appendix, but hardly ever discussed in the main text. I propose to either omit this discussion (which would also make the figures more readable) or to move respective figures and tables to the appendix. Instead, one could combine the IAGOS raw information from Tables 2, A2, and A3 into one table, replacing actual Table 2. Coming to my main point of criticism, the Figures are not carefully analysed and partly contain information which do not match the corresponding Tables and main text. I observed such clear mismatches in Figures 1, 5 and 6, which does not exactly strengthen my confidence in the veracity of the other figures. Taking these concerns into account, I’d propose to publish the manuscript after some revision, which can be considered as substantial.

Specific comments:

Line 9: To be fair and also to align with the title, you should limit yourself to validation to the years 2008-2019, although a few 2020 validation data is available.

Line 10: What is a “1.5 independent piece of information“? I think you should handle these as integers and talk of 1 to 3 independent pieces of information (also line 120).

Line 19-20 and several other occasions: A location specification such as “south of Bangkok“ also implies spatial proximity to the location, which is not the case here. Please use latitude information to delineate the regions of interest.

Line 29: Please note that about 40-50% of methane sources are of natural origin.

Line 44: The link to the pdf document should be transformed into a reference.

Line 47: I am missing an overview of the versions of IASI-CO retrieval and its potential effects on the validation. Are there any differences between IASI-A/B/C CO retrievals?

Line 50-52: I would not agree that anthropogenic pollution is generally the most important source of CO over Asia. You should rephrase the sentence to something like: “… covering a number of regions especially over Asia where anthropogenic pollution is enhanced …“.

Line 62: RTTOV needs to be explained.

Line 82: Please add Schlüssel et al. (2005) as reference to the IASI Level 2 Product Processing Facility.

Line 84: You should explain what a “Level 1C error“ stands for.

Lines 86-87: Are the general quality flag and the error profiles in any way taken into account for this study?

Line 97: The study is about validation up to 2019. Please give more information on which 2020 data has been used and how it is included in the validation.

Line 103: The Near and Middle East is usually counted as Asia. Please be more precise in the definition of the regions.

Lines 105-106: Name the African airports here.

Line 118: You rather mean here the validation time period or the validation days?

Line 121-124: Describing the differences between SOFRID and FORLI algorithms are one of the major outcomes of the study. Elaborate more precisely on possible reasons which lead here to the fundamentally different profiles of the averaging kernels from the two algorithms.

Lines 126-128: Instead of speculating about grouping of the AKs one could simply give another figure displaying only the peak altitudes of the AKs as a function of their nominal height. Without this information I can’t judge if two groups can clearly be distinguished for JJA.

Line 130: I’d rather see the lowest peak height at 800 hPa. Again, an additional figure as proposed before would help.

Line 187: TCC marker for New York in Figure 5 has approximately the same distance from the reference point in SOFRID and FORLI, resulting in a similar standard deviation.

Line 194: I guess you wanted to give also the number of airports with R<0.5 but missed to do so.

Lines 195-196: I cannot find confirmation of the R values for Düsseldorf in Fig. 5.

Line 196: Is this always the case or just on average (as it can be deduced from Table 2)?

Lines 207-208: The only airport markers lying outside Fig. 5e are New York and Dallas.

LINE 251: Fig. 6 reveals median differences between -23% and 3%.

Lines 255-256: Please use latitude information here instead of airport information.

Line 273: Fig. 6 reveals median differences between -20% and -1%.

Lines 281-284: This is an observation, not a conclusion. Of course, the reader would be interested in the conclusions from this finding. Use latitude bands!

Line 289: Which datasets are you referring to? There are in total 5 datasets (SOFRID, FORLI, IAGOS raw and IAGOS smoothed in two ways, all for three atmospheric column types). All of these datasets are involved in Figures 7-12.

Line 293: Can you confirm that summer biases are still not significant for time periods after 2012? In my opinion, this would require a statistical analysis.

Lines 313-314: The seasonal and interannual bias variations are not as prominent than over Frankfurt due to the more incomplete temporal sampling.

Lines 315-316: This can be omitted.

Line 355: 2020 -> 2019

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Table A1: Add the number of profiles to each airport.

Figure 1: The colour/size of the symbols used to characterize the airports do not agree with the numbers given in Table A1. E.g, three airports have more than 960 valid days as displayed in Figure 1, while in Table A1 only Frankfurt has more than 960 days with valid IAGOS profiles. In Table A1, 17 airports have less than 120 IAGOS days, while in Figure 1 only four airports have this characterization.

Figure 2: What do the colours stand for? Ideally they should characterize latitude bands. Please explain and add an additional colour bar as legend.

Figure 3: “validation database -> “validation days“?

Figure 4: “AvKs“ ->“AKs“. IASI retrieval quality flag as well as MLS profile selection details and IAGOS data gap need to be explained (as part of the data section in the main text).

Figures 5-12: You always start the discussion with the total column, followed by the partial columns. I’d therefore also group the figures in the same order (from left to right or from top to bottom, resp.). At least for Figures 6-12 it would be more intuitive to show 600-200 hPa in the middle and surface-600 hPa at the bottom. You should of course also alter the discussion order accordingly.

Figures 7-12: Timeseries for IAGOS raw and smoothed can mostly not be distinguished. Also IAGOS raw timeseries are partly covered by the smoothed timeseries. The only part of the text where you refer to the smoothed timeseries is lines 315/316. I propose that you remove the smoothed timeseries from the figures and also from the discussion.

Technical comments:

Line 26: Please remove “still“.

Line 28: Please add the year 2000 to the publication of Bergamaschi et al. .

Line 31: “makes of CO“ -> “makes CO“

Line 34: “developped“ -> “developed“

Line 36: add “)“ at the end of the sentence.

Line 77: Please add a blank space before “cm“.

Lines 94-95: “ascend and descend“ -> “ascent and descent“.

Line 95: “m.s^-1“ -> “m s^-1“

Line 103: “important“ -> “major“ or “larger“.

Line 113: AK has been explained before.

Line 181: “graduation“ -> “marker“?

Line 273: “Instead of“ -> “In contrast to“

Line 300: The same behaviour is observed …

Line 305: “against“ -> “compared to“

Line 360: “behaviours“ -> “behaviour“

Line 382: For Taipei which is the airport with the second longest …

Line 394: from May 14, 2019 onwards …

Line 425: Add the publication year (2000).

References:

De Wachter, E., Barret, B., Le Flochmoen, E., Pavelin, E., Matricardi, M., Clerbaux, C., Hadji-Lazaro, J., George, M., Hurtmans, D., Coheur, P. F., Nedelec, P., and Cammas, J. P.: Retrieval of MetOp-A/IASI CO profiles and validation with MOZAIC data, Atmospheric Measurement Techniques, 5, 2843–2857, https://doi.org/10.5194/amt-5-2843-2012, 2012.

Schlüssel, P., Hultberg, T. H., Phillips, P. L. T., August, T., and Calbet, X.: The operational IASI Level 2 processor, Adv. Space Res., 36, 982, doi:10.1016/j.asr.2005.03.008, 2005.