A Comparison of the Atmospheric CO2Concentrations Obtained by an Inverse Modeling System and Passenger Aircraft Based Measurement

In this study, the atmospheric CO₂ concentrations estimated by CT2013B, a recent version of CarbonTracker, are compared with CO₂ measurements from the Comprehensive Observation Network for Trace gases by Airliner (CONTRAIL) project during 2010-2011. CarbonTracker is an inversion system that estimates surface CO₂ fluxes using atmospheric CO₂ concentrations. Overall, the model results represented the atmospheric CO₂ concentrations well with a slight overestimation compared to observations. In the case of horizontal distribution, variations in the model and observation difference were large in northern Eurasia because most of the model and data mismatch were located in the stratosphere where the model could not represent CO₂ variations well enough due to low model resolution at high altitude and existing phase shift from the troposphere. In addition, the model and observation difference became larger in boreal summer. Despite relatively large differences at high latitudes and in boreal summer, overall, the modeled CO₂ concentrations fitted well to observations. Vertical profiles of modeled and observed CO₂ concentrations showed that the model overestimates the observations at all altitudes, showing nearly constant differences, which implies that the surface CO₂ concentration is transported well vertically in the transport model. At Narita, overall differences were small, although the correlation between modeled and observed CO₂ concentrations decreased at higher altitude, showing relatively large differences above 225 hPa. The vertical profiles at Moscow and Delhi located on land and at Hawaii on the ocean showed that the model is less accurate on land than on the ocean due to various effects (e.g., biospheric effect) on land compared to the homogeneous ocean surface.