Evaluation of the GOSAT/GOSAT-2 XCO ₂ proxy XCH ₄ product by comparison with TCCON observations over China

Methane (CH ₄ ) is the second most important well-mixed greenhouse gas after carbon dioxide (CO ₂ ) and its accurate monitoring supports China's “dual carbon” goals. Satellites enable global CH ₄ monitoring using various sensors and retrieval techniques. The XCO ₂ proxy method derives the column-averaged dry-air mole fraction of CH ₄ (XCH ₄ ) from the XCH4/XCO2 ratio and an independent XCO ₂ estimate, exploiting the overlapping absorption bands of CH ₄ and CO ₂ near 1.6 µm and assuming similar vertical distributions to reduce common retrieval errors. In this study, four XCO ₂ proxy methane products, GOSAT SRPR, OCPR, FOCAL Proxy, and GOSAT-2 SRPR, are evaluated using Total Carbon Column Observing Network (TCCON) ground-based observations at the Hefei and Xianghe sites in China. GOSAT products show lower bias and RMSE at Xianghe than at Hefei, while GOSAT-2 products are similar at both sites, with lower bias, RMSE, and standard deviation compared with the GOSAT products and approximately 51 % of retrievals meeting GCOS requirements. All products, except OCPR, overestimate XCH ₄ at Hefei and underestimate XCH ₄ at Xianghe, due to XCO ₂ model errors and spatial differences associated with regional and local methane emissions affecting both sites. Furthermore, the overestimation of the ratio implies that the fundamental assumption of the XCO ₂ proxy method is not fulfilled, primarily due to systematic positive biases in the satellite-retrieved XCH ₄ used in the proxy calculation. This study provides key error characteristics and spatial-seasonal biases in current satellite proxy methane products over China, supporting future retrieval optimization.