Toward accurate CO2 and CH4 observations from GOSAT

The column-average dry air mole fractions of atmospheric carbon dioxide and methane (X_CO2 and X_CH4) are inferred from observations of backscattered sunlight conducted by the Greenhouse gases Observing SATellite (GOSAT). Comparing the first year of GOSAT retrievals over land with colocated ground-based observations of the Total Carbon Column Observing Network (TCCON), we find an average difference (bias) of -0.05% and -0.30% for X_CO2 and X_CH4 with a station-to-station variability (standard deviation of the bias) of 0.37% and 0.26% among the 6 considered TCCON sites. The root-mean square deviation of the bias-corrected satellite retrievals from colocated TCCON observations amounts to 2.8 ppm for X_CO2 and 0.015 ppm for X_CH4. Without any data averaging, the GOSAT records reproduce general source/sink patterns such as the seasonal cycle of X_CO2 suggesting the use of the satellite retrievals for constraining surface fluxes.