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 ± 1.81ppm, 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 4ppm, 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 1ppm, 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.