Assimilation of GOSAT XCO2 data to optimize surface CO2 flux in East Asia

Because East Asia is the third-largest source region of CO₂ after North America and Europe, there is a need to estimate surface CO₂ fluxes accurately over East Asia. Nevertheless, since the number of surface CO₂ observations in East Asia is relatively small compared to that in North America and Europe, the estimation of surface CO₂ fluxes in East Asia has relatively large uncertainties. To supplement sparse surface CO₂ observations, satellite observations can be used.

In this study, the column-averaged dry-air mole fraction (XCO₂) concentration data from the Greenhouse gas Observing SATellite (GOSAT) Project was used to estimate the surface CO₂ fluxes in East Asia. CarbonTracker developed by Earth System Research Laboratory was used as an inverse modeling system. To assimilate GOSAT XCO₂ data in CarbonTracker, the observation operator for GOSAT XCO₂ data was developed. To determine the appropriate Model-Data-Mismatch (MDM) for GOSAT XCO₂ data, a sensitivity test was conducted. The experiment assimilating GOSAT data showed lower BIAS and RMSE than that without assimilating GOSAT data. In addition, the experiment using 2 ppm MDM for GOSAT data showed lower BIAS and RMSE than that using 3 ppm MDM.

The surface CO₂ fluxes over East Asia from the experiments with and without GOSAT data were also compared. By assimilating GOSAT observations, the absorption of surface CO₂ fluxes in the ocean became strong and that in land became weaker. Especially, the absorption of surface CO₂ fluxes in the Eurasian Boreal region became much weaker than in other regions. The uncertainty reduction was also the largest in the Eurasian Boreal region where the surface CO₂ observations are sparse.

Therefore GOSAT XCO₂ data have a profound impact on estimating the surface CO₂ fluxes in East Asia where the surface observations are insufficient.

 

 

Acknowledgments

This study was supported by the Korea Meteorological Administration Research and Development Program under grant KMI2018-03712 and a National Research Foundation of Korea (NRF) grant funded by the South Korean government (Ministry of Science and ICT) (Grant 2017R1E1A1A03070968). The authors thank Andrew R. Jacobson for providing the CarbonTracker and JAXA/NIES/MOE for providing GOSAT data.