Fiducial Reference Measurements for Greenhouse Gases (FRM4GHG): Validation of Satellite (Sentinel-5 Precursor, OCO-2, and GOSAT) Missions Using the COllaborative Carbon Column Observing Network (COCCON)

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

Line 318 – this is a reasonable criteria to define coincidence. It would be stronger if there was some physical basis related to the rates that the gas concentrations are changing.

 

397 – Interesting point about the surface albedo contributing to the bias. It would be more persuasive if a physical mechanism was proposed.

 

480 - “We observe a high positive bias during periods with high CO events, and a 481 negative bias during periods with low CO events.” CO retrieval errors are correlated with signal strength. Low CO introduces uncertainty. How can that be accounted for?

 

484 – Can the sites around Mexico City be identified more precisely?

 

601 – Again, explain the sensitivity of the results to the criteria condition. What happens if you expand the region to 4 hours?

 

706 – It would be interesting to see the effects of SZA on bias. Can you provide a plot?

Author Response

Please see the attachment.

Author Response File: Author Response.docx

Reviewer 2 Report

Comments and Suggestions for Authors

Dear authors,

I've read your manuscript on "Fiducial Reference Measurements for Greenhouse Gases 2 (FRM4GHG): Validation of Satellite (Sentinel-5 Precursor, 3 OCO-2 and GOSAT) Missions using COCCON ". I think this is an interesting work also in the light of forthcoming satellites missions as the Sentinel 4 and 5. The manuscript is clear, the number of references is adequate as well as the number of figures and tables reporting the results. In my opinion only minor revisions are required before publication.

Indeed I've one general comment I think you should address: in the abstract you state that "The random uncertainties of the validation results are found to be similar to the comparison with the TCCON. " However this is supported in the manuscript only for the comparison with TROPOMI XCH4. I think you should better discuss this point also for other satellites and target before stating that. This point on the similarity of COCCON and TCCON random uncertainties is relevant, since, as you state, allows to "expand on the coverage of the already existing ground-based reference remote sensing sites from the TCCON and NDACC network "

Here below a list of minor comments:

lines 175-176: Please add dome details, what cause this bias?

line 204: please add crossing time as for other missions

line 208-209: remove one digit 1.3kmx2.3km

line 214: For TROPOMI you also add a description of channels, spectral ranges etc.. the same should be done here.

line 216: Why is it insensitive? please explain

line 288: Possibly it will be useful to add a map with the stations position to see the distribution over the globe.

lines 324-327: Is this correction applied to both standard and bias-corrected data?

Figure 2: if possible improve the quality of this figure

Figure 3 etc: You plot the stations as a function of latitude. What about if you sort the data as function of target amounts? How does the used distance coincidence criteria affects the validation results in urban areas or in region where dishomogeneity are present? have you done some test on this?  

 

Author Response

Please see the attachment.

Author Response File: Author Response.docx

Reviewer 3 Report

Comments and Suggestions for Authors

The manuscript provides a detailed study on the validation of satellite-based greenhouse gas measurements (Sentinel-5 Precursor, OCO-2, and GOSAT) using the COCCON network. It effectively demonstrates the utility of COCCON's ground-based measurements as Fiducial Reference Measurements (FRM) for satellite data, complementing existing networks like TCCON and NDACC. The paper meticulously discusses methodologies, results, and the implications of COCCON's contributions to satellite validation, particularly for XCH4, XCO2, and XCO products.

Strengths:

The authors present a comprehensive dataset, integrating observations from COCCON sites across diverse geographic regions. The validation methodology is robust, addressing sources of bias systematically. Results are well-supported with extensive statistical analyses and clear visualizations (e.g., bar charts, mosaic plots). 

Potential improvements:

1. Include a comparative discussion of COCCON's performance relative to TCCON and NDACC in terms of satellite validation.

2. Clarify the impact of potential biases or limitations in COCCON's measurement protocols on the validation results.

3. The unit of SD in Table 2 and Table 3 is not explicitly stated. Clarifying whether it represents a ratio, percentage, or another metric would help readers better interpret the results.

4. The known albedo-dependent bias in TROPOMI methane retrievals is not compared to findings from other studies. Including such comparisons would contextualize the observed biases and strengthen the discussion.

5. The study lacks a visual representation of the site locations. A map displaying COCCON station sites would improve the understanding of the spatial coverage and regional representativeness of the validation results.

6. The temporal variability in TROPOMI methane biases is not quantified. Including a detailed analysis of how biases evolve seasonally or annually would enhance the robustness of the findings.

7. The reason for observed carbon monoxide (CO) biases is not thoroughly explored. A discussion on potential factors, such as instrument limitations, retrieval algorithms, or environmental conditions, would add depth to the analysis.

Author Response

Please see the attachment.

Author Response File: Author Response.docx