Atmospheric CO2 concentrations have been obtained from the Atmospheric Infrared Sounder (AIRS) radiance data within the European Centre for Medium-Range Weather Forecasts data assimilation system. A subset of channels from the AIRS instrument on board the NASA Aqua platform has been assimilated providing estimates of tropospheric and stratospheric column-average CO2 mixing ratios. Although global estimates are obtained, the information content of the tropospheric estimates at middle and high latitudes is limited, and results are therefore only presented for the tropical region. First results for February and August 2003 show considerable geographical variability compared to the background with values ranging between 371 and 380 ppmv. These CO2 values are representative for a layer between the tropopause and about 600 hPa. The monthly mean random error is about 1%. Careful error analysis has been carried out to minimize any systematic errors. This study has demonstrated the feasibility of global CO2 estimation using AIRS data in a numerical weather prediction data assimilation system. In the future the system will be improved to treat CO2 as a full three-dimensional atmospheric variable, including transport.