Influence of line mixing on the retrievals of atmospheric CO2 from spectra in the 1.6 and 2.1 mu;m regions

摘要

We present the first study of the influence of line mixing among COlines on the remote sensing retrieval of atmospheric carbon dioxide. This isdone in the bands near 1.6 and 2.1 μm which will be used by theGreenhouse Gases Observatory Satellite (GOSAT) instrument and eventualsuccessors of the Orbiting Carbon Observatory (OCO). A purely theoreticalanalysis is first made, based on simulations of atmospheric spectra. Itshows that line mixing cannot be neglected since disregarding this processinduces significant errors in the calculated absorption coefficients,leading to systematic structures in the spectral fit residuals andairmass-dependent biases in the retrieved CO amounts. Thesetheoretical predictions are then confirmed by using atmosphericsolar-absorption spectra measured by a ground-based Fourier transformspectrometer. It is first shown that including line mixing in the forwardmodel used for the inversion leads to a very significant reduction of theresiduals in the 2.1 μm region. Secondly, the inclusion of line mixingreduces the dependence of the retrieved CO on the airmass and greatlyimproves the consistency between values obtained independently from spectrain the 1.6 and 2.1 μm bands. These results open promising prospects forvarious ground-based and space-borne experiments monitoring the carbondioxide atmospheric amounts.