Assessment of the Moderate-Resolution Imaging Spectroradiometer algorithm for retrieval of aerosol parameters over the ocean

The Moderate Resolution Imaging Spectroradiometer aerosol algorithm over the ocean derives spectral aerosol optical depth and aerosol size parameters from satellite measured radiances at the top of the atmosphere (TOA). It is based on the adding of apparent optical properties (AOPs): TOA reflectance is approximated as a linear combination of reflectances resulting from a small particle mode and a large particle mode. The weighting parameter eta is defined as the fraction of the optical depth at 550 nm due to the small mode. The AOP approach is correct only in the single scattering limit. For a physically correct TOA reflectance simulation, we create linear combinations of the inherent optical properties (IOPs) of small and large particle modes, in which the weighting parameter f is defined as the fraction of the number density attributed to the small particle mode. We use these IOPs as inputs to an accurate multiple scattering radiative transfer model. We find that reflectance errors incurred with the AOP method are as high as 30% for an aerosol optical depth of 2 at 550 nm. The retrieved optical depth has a relative error of up to 8%, and the retrieved fraction eta has an absolute error of approximately 6%. We show that the use of accurate radiative transfer simulations and a bimodal fraction f yields accurate values for the retrieved optical depth and the fraction f.