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

Abstract

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.