An inverse problem of atmospheric transport has been solved as applied to the estimation of monthly average surface CO2 fluxes for 2009 using ground-based observation data and GOSAT data beginning from June, 2009. Corrections to the flux fields for each kind of sources are represented as a linear combination of main flux components of the corresponding surface gas-exchange fields. To calculate the atmospheric transport, the coupled Eulerian-Lagrangian model (GELCA) is used. A large array of observation data is used (3000–5000 observations per month); therefore, the Fix-Lag Kalman Smoother is used, which allows monthly fluxes to be estimated sequentially, according to a chosen size of assimilation window. The calculation results are represented in the form of 2D fields of monthly average flux fields, and recalculated for chosen regions. The calculations show a significant decrease in the posterior flux uncertainty when using GOSAT observations.