More than a year of Atmospheric Infrared Sounder (AIRS) radiance observations have been assimilated in the European Centre for Medium-Range Weather Forecasts four-dimensional variational (4D-Var) data assimilation system to estimate tropospheric CO2. The assimilation of a set of 18 spectral channels provides a mean tropospheric mixing ratio representing a layer between ~700 hPa and the tropopause. Analysis errors for a 5-day mean on a 6¿ by 6¿ averaging grid box are on the order of 1%. Comparisons with independent flight data from Japanese Airlines and National Oceanic and Atmospheric Administration Climate Monitoring and Diagnostics Laboratory are favorable. Differences between the averaged assimilation estimates and the onboard flask observations are generally within the 1-σ error bars. Currently, this work is being extended by introducing CO2 as a full assimilation model tracer variable, which will allow the operational monitoring of atmospheric CO2 using AIRS observations and observations from upcoming instruments.