Quantitative Assessment of Satellite-Observed Atmospheric CO2 Concentrations over Oceanic Regions

Atmospheric carbon dioxide in mole fraction (XCO2) is one of the key parameters in estimating CO2 fluxes at the air–sea interface. Satellite-derived column-averaged XCO2 has been widely used in the estimates of air–sea CO2 fluxes, yet the uncertainties induced by using column-averaged XCO2 instead of atmospheric XCO2 in the ocean boundary layer have been generally unknown. In this study, based on an extensive dataset of atmospheric XCO2 measured in the ocean boundary layer from global ocean mooring arrays (N = 945,243) and historical cruises (N = 170,000) between 2002 and 2024, for the first time, we quantitatively evaluated the performance of four satellites, including the Greenhouse gases Observing SATellite (GOSAT and GOSAT-2), the Orbiting Carbon Observatory-2 (OCO-2), and the Atmospheric InfraRed Sounder (AIRS), in monitoring the atmospheric XCO2 over oceanic regions. The atmospheric XCO2 has been increasing from 375 ppm in 2002 to 417 ppm in 2024 based on the longest data record from AIRS. We found that the column-averaged atmospheric XCO2 can serve as a good proxy for atmospheric XCO2 in the ocean boundary layer, with associated uncertainties of 2.48 ppm (0.46%) for GOSAT, 1.01 ppm (0.24%) for GOSAT-2, 2.45 ppm (0.45%) for OCO-2, and 4.22 ppm (0.83%) for AIRS. We also investigated the consistency of these satellites in monitoring the growth rates of atmospheric XCO2 in the global ocean basins. Based on the longest data record from AIRS, the atmospheric XCO2 has been increasing at a rate of 1.87–1.97 ppm year−1 over oceanic regions in the past two decades. These findings contribute to improving the reliability of satellite-derived column-averaged XCO2 observations in the estimates of air–sea CO2 fluxes and support future efforts in monitoring ocean carbon dynamics through satellite remote sensing.