PPDF-based method to account for atmospheric light scattering in observations of carbon dioxide from space

We present an original method that accounts for thin clouds in carbon dioxide retrievals from space-based reflected sunlight observations in near-infrared regions. This approach involves a reasonable, simple parameterization of effective transmittance using a set of parameters that describe the path-length modification caused by clouds. The complete retrieval scheme included the following: estimation of cloud parameters from the 0.76-mu m O-2 A-band and from the H2O-saturated absorption area of the 2.0-mu m band; a necessary correction to utilize these parameters at the target CO2 1.58-mu m band using estimated ground surface albedo outside of gas absorption lines in this band; and retrieval of CO2 amount at the 1.58-mu m band using a maximum a posteriori method of inversion. The primary retrieved parameters refer to the CO2 volume mixing ratio vertical profile that is then transformed to an averaged- column amount under a pre-defined increment of pressure. A set of numerical simulations with synthetic radiance spectra particular to Greenhouse Gases Observing Satellite (GOSAT) observations showed that the proposed method provides acceptably accurate CO2 retrievals from an atmosphere that includes thin cirrus clouds. Efficiency of the aerosol and cloud corrections was demonstrated by comparing it with a modified iterative maximum a posteriori-DOAS (IMAP-DOAS) that neglects path length modifications due to the scattering effects.