A method for evaluating bias in global measurements of CO{sub 2} total columns from space

We describe a method of evaluating systematic errors in measurements of total column dry-air mole fractions of CO{sub 2} (X{sub CO{sub 2}} ) from space, and we illustrate the method by applying it to the Atmospheric CO{sub 2} Observations from Space retrievals of the Greenhouse Gases Observing Satellite (ACOS-GOSAT) v2.8. The approach exploits the lack of large gradients in X{sub CO{sub 2}} south of 25{degree}#14; S to identify large-scale offsets and other biases in the ACOS-GOSAT data with several retrieval parameters and errors in instrument calibration. We demonstrate the effectiveness of the method by comparing the ACOS-GOSAT data in the Northern Hemisphere with ground truth provided by the Total Carbon Column Observing Network (TCCON). We use the correlation between free-tropospheric temperature and X{sub CO{sub 2}} in the Northern Hemisphere to define a dynamically informed coincidence criterion between the ground-based TCCON measurements and the ACOS-GOSAT measurements. We illustrate that this approach provides larger sample sizes, hence giving a more robust comparison than one that simply uses time, latitude and longitude criteria. Our results show that the agreement with the TCCON data improves after accounting for the systematic errors. A preliminary evaluation of the improved v2.9 ACOS-GOSAT data is also discussed.