Correction of atmospheric scattering effects in space-based observations of carbon dioxide: Model study of desert dust aerosol

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 O₂ A-band; the necessary correction to use these estimated parameters in the 1.6-μm band; and, finally, CO₂ retrievals from the 1.6-μm 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.