Comparing simulated atmospheric carbon dioxide concentration with GOSAT retrievals

Satellite observations of atmospheric carbon dioxide (CO2) provide a useful way to improve the understanding of global carbon cycling. In this paper, we present a comparison between simulated CO2 concentrations from an inversion model of the CarbonTracker Data Assimilation System (CTDAS) and satellite-based CO2 measurements of column-averaged dry air mole fraction (denoted XCO2) derived from version 3.3 Atmospheric CO2 Observations from Space retrievals of the Greenhouse Gases Observing SATellite (ACOS-GOSAT) L2 data products. We examine the differences of CTDAS and GOSAT to provide important guidance for the further investigation of CTDAS in order to quantify the corresponding flux estimates with satellite-based CO2 observations. We find that the mean point-by-point difference (CTDAS-GOSAT) between CTDAS and GOSAT XCO2 is −0.11 ± 1.81 ppm, with a high agreement (correlation r = 0.77, P < 0.05) over the studied period. The latitudinal zonal variations of CTDAS and GOSAT are in general agreement with clear seasonal fluctuations. The major exception occurs in the zonal band of 0°-15°N where the difference is approximately 4 ppm, indicating that large uncertainty may exist in the assimilated CO2 for the low-latitude region of the Northern Hemisphere (NH). Additionally, we find that the hemispherical/continental differences between CTDAS and GOSAT are typically less than 1 ppm, but obvious discrepancies occur in different hemispheres/continents, with high consistency (point-by-point correlation r = 0.79, P < 0.05) in the NH and a weak correlation (point-by-point correlation r = 0.65, P < 0.05) in the Southern Hemisphere. Overall, the difference of CTDAS and GOSAT is small, and the comparison of CTDAS and GOSAT will further instruct the inverse modeling of CO2 fluxes using GOSAT.