Atmos. Chem. Phys. Discuss., 9, 4873-4898, 2009
www.atmos-chem-phys-discuss.net/9/4873/2009/
doi:10.5194/acpd-9-4873-2009
© Author(s) 2009. This work is distributed
under the Creative Commons Attribution 3.0 License.
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This discussion paper has been under review for the journal Atmospheric Chemistry and Physics (ACP). Please refer to the corresponding final paper in ACP.
Influence of line mixing on the retrievals of atmospheric CO2 from spectra in the 1.6 and 2.1 μm regions
J.-M. Hartmann1, H. Tran1, and G. C. Toon2
1Laboratoire Inter-universitaire des Systèmes Atmosphériques (LISA) UMR CNRS/INSU 7583, Universités Paris VII et Paris XII, 94010 Créteil Cedex, France
2Jet Propulsion Laboratory (JPL), California Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA 91109, USA

Abstract. 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. Secondly, the inclusion of line mixing reduces the dependence of the retrieved CO2 on the airmass and greatly improves the consistency between values obtained independently from spectra in the 1.6 and 2.1 μm bands. These results open very promising prospects for various ground-based and space-borne experiments monitoring the carbon dioxide atmospheric amounts.

Citation: Hartmann, J.-M., Tran, H., and Toon, G. C.: Influence of line mixing on the retrievals of atmospheric CO2 from spectra in the 1.6 and 2.1 μm regions, Atmos. Chem. Phys. Discuss., 9, 4873-4898, doi:10.5194/acpd-9-4873-2009, 2009.
 
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