Comparison of CO2 retrievals from IASI-A, IASI-B and GOSAT in the thermal infrared for nearly coincident measurements over the Arctic ocean in summer

The capabilities to retrieve reliable information on the concentration of greenhouse gases in the lower atmosphere from thermal infrared (TIR) spectra collected by nadir sounders is still to be assessed. We have selected the two months period of July-August in the high latitude polar region where it is possible to observe almost coincident or superimposed footprints (IFOV) of the three infrared sounders considered in this study, namely IASI-A, IASI-B (on the MetOp platforms) and TANSO-FTS (on GOSAT). Retrievals of the column averaged mixing ratio of carbon dioxide XCO2 (and of the surface temperature) have been performed for three years i.e. 2010, 2013 and 2014 over Arctic waters. The summer period was chosen because ice free IFOVs (in the latitude band 68N to 82N) can be selected for which retrievals are less sensitive to surface inhomogeneity (as compared to IFOVs located over land). The emissivity of sea water is also better constrained. The inversion configuration (using the atmospheric window covering the so-called CO2 laser band in the interval 940-980 cmwill be described. The sensitivity of the retrieved XCO2 to the different layers of the lower atmosphere as a function of thermal contrast, temperature and humidity profiles will be presented. The precision/accuracy of the retrieved XCO2 will be discussed and compared between sounders. The CO2 trends is clearly captured over the years analysed in this work. The retrieved values will be compared to similar XCO2 products available from other sources (Leicester Univ., NIES, SRON/KIT). Some remaining spectroscopic issues in the vicinity of 948 cm-1 have been identified and circumvented. The retrieved sea surface temperature Tsurf used as a control variable is also providing an additional check of the performances of the retrievals and is compared to the Eumetsat IASI Tsurf product. These results are interesting starting points for preparing future missions like IASI-NG on MetOp-SG as well as GOSAT-2.