Global CO2 Analyses from Assimilation of AIRS Channels that Peak in the Middle to Lower Troposphere

We present an evaluation of a two-year (2005-2006) assimilation of atmospheric carbon dioxide, in which the observations are middle-to-lower tropospheric partial columns derived from Atmospheric Infrared Sounder (AIRS). The CO2 retrievals are derived using eight AIRS channels in the 4-micron range, along with NWP profiles of temperature and moisture; The eight channels togther constrain both the CO2 and land surfce emission. This retrieval has peak sensitivity to CO2 at about 450hPa. The assimilation consists of the GEOS-5 GCM, which uses specified surface CO2 fluxes, coupled with a Physical-space Statistical Analysis System (PSAS) for the assimilation. An evaluation of the atmospheric CO2 concentrations is made using in-situ measurements, including aircraft measurements from the NOAA/ESRL network and NASA's Intex-Bfield mission over North America during March to May of 2006. Compared to the free-running model, the bias was reduced at all levels, with the most pronounced improvements between 3000 to 6000m (about 700 to 500hPa), with smaller reductions in standard deviations. The evaluation implies that the net CO2 flux into the atmosphere over North America is overesimated in the model and that the AIRS observations can correct this model bias. This indicates that the AIRS 4-micron channels contain useful information on the abundance of CO2 in the middle to lower troposophere and could potentially be useful for carbon cycle flux inversion.