An improved photon path length probability density function-based radiative transfer model for space-based observation of greenhouse gases

We present an improved model to describe the photon path length probability density function (PPDF) that effectively accounts for both aerosol and thin cloud effects for rapid retrieval of greenhouse gas data from space-based high spectral resolution measurements. The reasonably simple PPDF and effective transmittance parameterization permit vertical inhomogeneity of gas absorption and three plane-parallel arbitrarily located layers to account for light-scattering effects due to aerosol and clouds. The basic assumption to construct the PPDF model refers mainly to the presentation of PPDF in terms of weakly correlated aerosol and cloud path length components. The model was validated using Monte Carlo simulations of photon trajectories and tested for a representative set of atmospheric conditions in which both aerosol and clouds modified the path length significantly. This study also focused on the connection of PPDF parameters to atmospheric optical characteristics commonly utilized in the solution of radiative transfer equations. These characteristics were converted into PPDF parameters by PPDF retrieval from a very limited spectral range of simulated radiance including at least one gas absorption line. The results demonstrate that this method can be used effectively for rapid radiative transfer spectral calculations over a rather wide spectral range of radiance at given atmospheric optical characteristics. Another important application of such conversion is to account for a priori knowledge of atmospheric optical characteristics when retrieving amounts of gases within PPDF-based or other methods, such as full physics algorithms that utilize the solutions of radiative transfer equations.