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We describe an original methodology for CO2 retrievals using space-based measurements of reflected sunlight spectra. The effects of optical-path modification by aerosols were considered in terms of photon path-length statistics. First, the general approach was verified using a representative set of photon trajectories produced by the Monte Carlo technique. This method enabled accurate consideration of optical-path modification by aerosols and was effective in CO2 retrievals if aerosol optical properties were assumed. The next approach involved a limited number of parameters that describe the photon path-length distribution function (PPDF) and which were retrieved simultaneously with the CO2 amount. This approach was efficient under conditions of strong path modification by desert dust aerosol. The retrieval procedure included the following: estimation of PPDF parameters from radiance spectra in the O-2 A-band; the necessary correction to use these estimated parameters in the 1.6-mu m band; and, finally, CO2 retrievals from the 1.6-mu m band. The procedure was verified by numerical simulations using an independent radiative transfer approach to produce radiance spectra expected for the Greenhouse Gases Observing Satellite (GOSAT) sensor. (C) 2008 Elsevier Ltd. All rights reserved.