Validation of GOSAT SWIR XCO2 and XCH4 Retrieved by PPDF-S Method and Comparison with Full Physics Method

Column-averaged concentrations of carbon dioxide (XCO2) and methane (XCH4) were retrieved from spectra observed by the Greenhouse gases Observing SATellite (GOSAT) using the so-called Photon path length Probability Density Function-Simultaneous (PPDF-S) retrieval method, which explains cloud/aerosol effects in terms of light path modification. The PPDF-S data, as well as the standard products for General Users (GU) of XCO2 and XCH4 retrieved using the full physics (FP) method, were validated through comparison with Total Carbon Column Observing Network (TCCON) data. Results show that bias and its standard deviation of XCO2 over the land are 0.73 and 1.83 ppm for the PPDF-S data, and −0.32 and 2.16 ppm for GU products. For XCH4, they are 1.4 and 14.1 ppb, and −1.9 and 12.5 ppb, respectively. Although the magnitude relations between XCO2 and XCH4 retrieved by the PPDF-S and GU products are identical over the land, they differ over the ocean. This fact emphasizes the importance of additional validation data over the ocean. Results also show that 68% of FP data that were screened out through an Aerosol Optical Thickness (AOT) test passed all screening tests for the PPDF-S method, implying the applicability of the PPDF-S method to denser aerosol conditions.