Validation of XCH₄derived from SWIR spectra of GOSAT TANSO-FTS with aircraft measurement data

Column-averaged dry-air mole fractions of methane (XCH₄), retrieved from Greenhouse gases Observing SATellite (GOSAT) short-wavelength infrared (SWIR) spectra, were validated by using aircraft measurement data from the National Oceanic and Atmospheric Administration (NOAA), the US Department of Energy (DOE), the National Institute for Environmental Studies (NIES), the HIAPER Pole-to-Pole Observations (HIPPO) program, and the GOSAT validation aircraft observation campaign over Japan. In the calculation of XCH₄from aircraft measurements (aircraft-based XCH₄), other satellite data were used for the CH₄profiles above the tropopause. We proposed a data-screening scheme for aircraft-based XCH₄for reliable validation of GOSAT XCH₄. Further, we examined the impact of GOSAT SWIR column averaging kernels (CAK) on the aircraft-based XCH₄calculation and found that the difference between aircraft-based XCH₄with and without the application of the GOSAT CAK was less than ±9 ppb at maximum, with an average difference of -0.5 ppb. We compared GOSAT XCH4 Ver. 02.00 data retrieved within ±2° or ±5° latitude-longitude boxes centered at each aircraft measurement site with aircraft-based XCH₄measured on a GOSAT overpass day. In general, GOSAT XCH₄was in good agreement with aircraft-based XCH₄. However, over land, the GOSAT data showed a positive bias of 1.5 ppb (2.0 ppb) with a standard deviation of 14.9 ppb (16.0 ppb) within the ±2° (±5°) boxes, and over ocean, the average bias was 4.1 ppb (6.5 ppb) with a standard deviation of 9.4 ppb (8.8 ppb) within the ±2° (±5°) boxes. In addition, we obtained similar results when we used an aircraft-based XCH₄time series obtained by curve fitting with temporal interpolation for comparison with GOSAT data.