1) General comments

This study compares the accuracy of two algorithms in determining methane (CH4) concentrations from IASI sensor data. The retrieval results with the two algorithms are compared with each other and with independent observations and model data using various methods.

It is crucial to validate IASI CH4 data comprehensively for scientific use, and this paper's scientific significance is high. Therefore, I believe this paper's content deserves to be published in AMT.

However, the structure and description of this paper are somewhat confusing. In particular, the intercomparison part in Section 4 needs a more organized structure. The "short summary" subsection should be included in each intercomparison subsection, with tables, to aid readers in comprehending the validation results.

Also, regarding the gap in the data for 2013, it is important to include clear communication in the data section that this issue exists. Then, a comparative validation distinguishing between before and after data, or at least an intercomparison between before and after RAL and LMD, should be performed to show the presence or absence of an impact.

In addition, several figures need more legends or are unreadable. Please improve visibility.

Considering the above, I recommend that this paper should be published after examining the issues and making necessary revisions.

2) Specific comments

・Page3 Line20 (P3L20): about IFOV of RAL measurements

Why use the IFOV with the highest brightness temperature among the four IFOVs instead of the average of the four?

・P8L19: about vertical sensitivity of LMD and RAL_LMDavk

You note that there is a difference in altitude concerning the sensitivity of the two, but how much difference would this be in mtCH4? Can mtCH4 be evaluated using, for example, climatological values?

・P8L23: about grid averaging

Why did you use a 1-degree grid for intercomparison? This corresponds to a 100 km grid on a spatial scale, which might be too large, considering that the source of CH4 is more localized than CO2.

・P9L4 and P9L10-L17: about selection criteria

The time frame for simultaneous observation with IASI is currently at ±6 hours. However, I believe this interval may be too long, considering the horizontal and vertical transport of CH4. CH4 distribution can vary significantly over such a period, so it may not be accurate to consider the observation as genuinely simultaneous.

Additionally, comparing ground-based FTIRs to the satellite dataset is unfair due to the differences in time and longitude ranges for each comparison. Can the conditions be as consistent as possible?

・P9L5: about the mean of the satellite values

How many satellite data points are usually averaged? And how large is the variability (stdv) of these data?

・P10L21: about eq.(7)

Please tell me how this equation was derived. Also, please explain what C'_r,R on the left side represents.

・P11L9, L24, and L28: about equations (9), (11), (12)

What does the right-hand side of equations (9), (11), and (12) represent, respectively?　Please add an explanation.

・P13L5-L6: about internal consistency

Why was the internal consistency only checked in October 2014? Could there be differences depending on the season, especially summer and winter? Also, is there any impact of the 2013 gap?

・P13L30: IFOV selection

The authors claim that "RAL selects the best IFOV among 4 of them.", but in the previous section they mention "the one with the highest brightness temperature". Why is this the best?

・P14L16: about SAT

What is “SAT”? Does SAT mean RAL and/or LMD measurements?

・P15L13-L14: about the correlation between HIPPO and RAL measurements

What is the correlation between the two values obtained from the best fit?

・P15L15-L18: comparison with IAGOS

Why is the correlation coefficient with IAGOS lower than other comparisons? Is such a low correlation coefficient due to the spatial bias of IAGOS measurements?

・P17L24-P19L9: about subsection 4.7

This subsection repeats previous statements, making it redundant. It would be more useful to list each table in a related subsection.

・P19L10-P19L15: about discontinuity in RAL L2 data in mid-2013

Does the discontinuity in mid-2013 also affect the intercomparisons made in Section 4? For example, the comparison with AirCore and IAGOS was made using all data before and after the discontinuity. Would there be a difference in bias before and after?

Also, on page 23, the authors only compared partial columns between RAL and CAMS in 2012. Is there any difference in partial column bias before and after 2013?

・Figure 5, 6, 7, 12 and 13:

The legend should be relocated or resized to avoid overlapping the plots. In Figure 13, please add a legend.

・Figure 5, 6 and 7:

Could you please explain the meaning of the dotted lines in the correlation diagram?

3) Technical corrections

P2L9: andh => and

P4L23: (about 30 km; (Karion et al., 2010)) => (about 30 km) (Karion et al., 2010).

P5L3: CAMS model) as => CAMS model as

P7L9: in (Massart et al., 2014) => in Massart et al. (2014)

P9L20: According to (Rodgers and Connor, 2003) => According to Rodgers and Connor (2003),

P10L4: “c_i” in bold should be plain.

P10L7: respectively; => respectively,

P10L7: XCH4 => XCH4. (need period)

P15L23, L28 and L29: XCH4 => qCH4

Table 1: ParkFalls => Park Falls?

Figure 1: Rikubetsu should be “TCCON (yellow)”, not “NDACC (green)”.

Figure 4: “The last three rows show …” maybe “The last two rows show …”.

Figure 6: In the x- and y-axis labels, “XCH4” should be “qCH4”.  

Comment on “Independent validation of IASI/METOP-A LMD and RAL CH₄ products using CAMS model, in situ profiles and ground-based FTIR measurements” by Bart Dils et al.

General:

This paper describes the validations of two methane products from IASI/METOP-A satellite sensor. This work is important, but I couldn’t understand why the authors think that it is enough to use RAL data without correction. They pointed out there is a discontinuity in mid-2013 and compared some trends in Section 5.4. The corrected trend (5.6 ppb/yr) is significantly higher than the original one (4.2 ppb/yr) and the difference is larger than the standard deviation of original one. I think the trend analyses in Section 5.1 and 5.2 should be done using corrected data, or at least separately for the periods before and after mid-2013. Furthermore, the discontinuity should affect the results of validation in Section 4 but there is no explanation. The validation of RAL should also be done using corrected data, or separately for the periods before and after mid-2013.

The paper should be published after major revisions.

Comments and questions:

Abstract

p1, l17

 The long-term stability --> The long-term trend

Section 1

p2, l5 and 8

 IPCC, 2013 --> It is better to refer newer report (IPCC, 2021)

Section 2

p6, l8

 largly --> largely

p7 2.4

It is better to describe the original spatial resolution of CAMS because it was averaged onto a 1-degree latitude and 1-degree longitude grid before comparison.

Section 3

p9, l8-9

Please discuss on the impact of this difference to the validation results in Section 4.7.

Section 4

p13, l4

 What is ‘pixel’? Is it mostly the same as IFOV? Please explain it in Section 2.

p14, l28

 June --> July

p15, l24

 … and IAGOS … : IAGOS isn’t used for Figure 6. This is misleading.

l28-29

 XCH₄ generally …: What does this sentence mean? Are there any relations to the validation results?

p16, l32

 Figure 8 --> Figure 8 (left)

p17, l7

 Figure 8 --> Figure 8 (right)

l10

 Tule --> Thule

l11-13

 Why this sentence is put here and the content is different from that described in l2-3.

p18, l3-6

 … temporally …: Does HIPPO observation limited to some season? What figure in the top row of Figure 4 should be referred? This sentence is too vague.

Section 5

p20, l11

 It is found that … --> At land regions, it is found that ...

l33-p21, l1

 What about Maido?

p21, l25-27

 What is the definition of ‘similar’ and ‘different’? Why Wollongong is ‘similar’ but Lauder isn’t categorized?

Section 6

p23, l3

 Section 4.1 --> Section 4.3.1

Section 7

p26, l8

 The long-term stability --> The long-term trend

Many figures

 The legends covered some of data plot. Move them not to cover the data.

 Size of the figure is too small.

Figure 1

There are many mistakes. For example,

 I couldn’t find ‘AirCore’ mark at Sodankylä.

 I found ‘NDACC’ mark at Ny-Ålesund but Ny-Ålesund isn’t listed in Table 2.

 I found ‘NDACC’ mark at Rikubetsu but Rikubetsu is listed in Table 1 (TCCON site).

Please check carefully.

Figure 2 caption

 The solid data --> The solid line

Figure 5, 6, and 7

 The correlation plots should be written with HIPPO values in horizontal axis. The values to be validated (RAL or LMD) should be in the vertical axis.

Figure 5 caption

 the scatter plot between the RAL and HIPPO XCH₄.

--> the scatter plot between the RAL and HIPPO XCH₄ (right).

Please add the explanation of the gray bar in the left figure, black bar, black solid line, and pink dashed line in the right figure.

Figure 6 caption

Same as the comments for Figure 5.

Figure 7 legend

 GB --> AIR

Figure 14

 There is no error bar for Rikubetsu.