GENERAL COMMENTS

GOSAT provides the longest continuous record and its data have been processed by various algorithms. GOSAT-2 has higher signal offers higher signal-to-noise ratio than GOSAT. Therefore, the detailed comparison between GOSAT and GOSAt-2, validated against the TCCON data is essential for the seamless integration of both satellites. The manuscript should provide GOSAT and GOSAT-2 difference in sampling pattern, number of successful retrievals in more detail. In addition, the authors should check the observation condition, particularly the use of intelligent pointing function of GOSAT-2. Usually, intelligent pointing is not applied allied to TCCON validation sites in order to repeatedly target the same footprint. Overall, several aspects of the manuscript are insufficiently described. Major revisions are needed before publication.

SPECIFIC COMMENTS

(1) Abstract, lines 22 and 23

Description of “more accurate” and “better performance” are unclear.

(2) Abstract, Lin 26

“due to bias in raw XCH4” is not clear

(3) Page 4 lines 130 and 132

“The spectral resolution in the SWIR of approximately 0.2 cm⁻¹ is better than that of TANSO-FTS, is not accurate. The value is the spectral sampling interval.  The spectral resolution and optical throughput of GOSAT and GOSAT-2 are approximately the same.  

Signal-to-noise ratio of GOSAT-2 is slightly better.

The mission requirements for GOSAT launched in 2009 and GOSAT-2 launched in 2018 are defined differently. The authors should refer to Suto, et al, AMT 2014 for GOSAT-2 rather than Kuze et al., 2009.

(4) Page 6, Line 205

OCO-2 does not have the CH4 absorption band. Is the term “OCO-2 Proxy” correct?

(5) Page 13, Table 3

Observation period of GOSAT is much longer than that of GOSAT-2. Does the table 3 data use the same period for both satellites?

In addition, the geolocation matching criteria for example the maximum allowable distance between the satellite footprint center and the TCCON site should be described in more detail.

TECHNICAL CORRECTIONS　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　

(1) Reference

Add Suto et al., Atmospheric Measurement Techniques 14, 2013-2039, 2021

GENERAL COMMENTS

The manuscript presents a comprehensive evaluation of multiple GOSAT and GOSAT-2 XCO₂ proxy XCH₄ products using TCCON observations over China. The study provides valuable information on the performance of satellite methane products and their potential applications in regional greenhouse gas monitoring. However, several aspects of the manuscript require further clarification and refinement to improve the overall presentation and interpretation of the results.

SPECIFIC COMMENTS

(1) Page 1, Line 31

“impact on the Earth’climate (Hansen and Sato, 2001)” should be “impact on the Earth’s climate (Hansen and Sato, 2001)”.

(2) Page 4, Line 148-150

The authors should cite the appropriate updated TCCON data version documentation and include information on the accuracy of TCCON measurements.

(3) Page 9, Line 286

“the frequency of high-AOD observations”: Fig 3 shows only few and disparate periods with AOD observations at both site and only a very short overlapping period in 2023. Hence is is awkward to speak about “frequency” of observations. Rather, it seems that the AOD at Hefei is higher during most of the available measurements. In addition, the AOD has been reported to decrease over China. Why would tracking failures occur at during high AOD conditions at Xianghe and not at Hefei?

(4) Page 14, Line 419-429

The manuscript analyzes seasonal variations of product biases. However, seasonal comparisons may be affected by uneven availability of observations among seasons and thus influence the derived seasonal statistics. Was this accounted for by the authors.

(5) Page 18, Line 524

The terms “raw XCH₄” and “raw XCO₂” require clarification. The authors should define these variables and explain their roles in the proxy XCH₄ retrieval process, as they are used to interpret the observed biases in proxy products.

(6) Page 23, Line 701

The conclusion that GOSAT-2 SRPR outperforms other products may be somewhat overstated, since the validation periods of GOSAT and GOSAT-2 are not fully consistent, in particular due to the shorter period of observations of GOSAT-2 and the changed conditions and higher CH4 concentrations.  Likewise, the FOCAL time series are much shorter (Fig 3) how does that influences comparison with other algorithms and the statical metrics? Please discuss.

(7) Page 24, Line 709-712

The validation is based on only two TCCON sites in China, which represent different atmospheric conditions but cannot fully characterize the performance of satellite products across the entire country. The authors should acknowledge this limitation and further discuss the implications of the results for future satellite-based methane monitoring applications in China.