Evaluation of the GOSAT/GOSAT-2 XCO₂ proxy XCH₄ product by comparison with TCCON observations over China

Abstract. Methane (CH₄) is the second most important greenhouse gas after carbon dioxide (CO₂) and its accurate monitoring supports China's "dual carbon" goals. Satellites enable global monitoring of CH₄, supported by various sensors and retrieval techniques. Among them, the XCO₂ proxy method derives the column-averaged dry-air mole fraction of CH₄ (XCH₄) from the XCH₄/XCO₂ ratio and an independent estimate of XCO₂. By exploiting the overlapping absorption bands of CH₄ and CO₂ near 1.6 μm and assuming similar vertical distributions, the ratio eliminates common retrieval errors, providing an accurate estimate of XCH₄. In this study, four XCO₂ proxy methane products, GOSAT SRPR, OCPR, FOCAL Proxy, and GOSAT-2 SRPR, are evaluated using TCCON ground-based observations at the Hefei and Xianghe sites in China. The comparison shows that the GOSAT products (OCPR, SRPR, FOCAL Proxy) are more accurate at Xianghe, while GOSAT-2 products (SRPR) are similar at both sites with better performance than each of the GOSAT products (>50 % meeting GCOS requirements). All products, except OCPR, overestimate XCH₄ at Hefei and underestimate XCH₄ at Xianghe, which stems from both XCO₂ prior model errors and spatial inconsistency between satellite and ground-based measurements. Furthermore, the overestimation of the ratio implies that the fundamental assumption of the XCO₂ proxy method is not fulfilled, primarily due to bias in raw XCH₄. This study provides key error characteristics and spatial-seasonal biases in current satellite proxy methane products over China, providing a scientific basis for future retrieval optimization.