Atmospheric Chemistry and Physics Discussions www.atmos-chem-phys-discuss.net 1680-7367 1680-7375 9 1 2009 10.5194/acpd-9-4873-2009 http://www.atmos-chem-phys-discuss.net/9/4873/2009/ http://www.atmos-chem-phys-discuss.net/9/4873/2009/acpd-9-4873-2009.html http://www.atmos-chem-phys-discuss.net/9/4873/2009/acpd-9-4873-2009.pdf 4873 4898 2009-02-24 Influence of line mixing on the retrievals of atmospheric CO₂ from spectra in the 1.6 and 2.1 μm regions J.-M. Hartmann hartmann@lisa.univ-paris12.fr H. Tran G. C. Toon Laboratoire Inter-universitaire des Systèmes Atmosphériques (LISA) UMR CNRS/INSU 7583, Universités Paris VII et Paris XII, 94010 Créteil Cedex, France Jet Propulsion Laboratory (JPL), California Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA 91109, USA We present the first study of the influence of line mixing among CO₂ lines on the remote sensing retrieval of atmospheric carbon dioxide. This is done in the bands near 1.6 and 2.1 μm which have been retained by the Orbiting Carbon Observatory (OCO) and Greenhouse Gases Observatory Satellite (GOSAT) instruments. A purely theoretical analysis is first made, based on simulations of atmospheric spectra. It shows that line mixing cannot be neglected since disregarding this process induces significant errors in the calculated absorption coefficients, leading to systematic structures in the spectral fit residuals and airmass-dependent biases in the retrieved CO₂ amounts. These theoretical predictions are then confirmed by using atmospheric solar-absorption spectra measured by a ground-based Fourier transform spectrometer. Indeed, it is first shown that including line mixing in the forward model used for the inversion leads to a very significant reduction of the residuals in the 2.1 μm region. 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