Quantitative evaluation of uncertainties in satellite retrieval of dust-like aerosols induced by spherical assumption

Abstract

It has been frequently pointed out the necessity to consider non-sphericity of dust-like aerosols in modeling their optical properties. However, most existing aerosol retrieval algorithms still utilize the Mie theory in defining aerosol models and creating look up tables (LUTs) for simplicity. Neglecting non-sphericity can severely influence aerosol optical depth (AOD, τ) retrieval in case of dust-like aerosols—largely due to the difference of derived phase functions under spherical and non-spherical assumptions—whereas this uncertainty has rarely been quantitatively studied. This paper aims at a better understanding of uncertainties on AOD retrieval caused by aerosol non-sphericity. From a dust aerosol model, we adopt different methods to simulate and compare aerosol optical properties on the basis of spherical and non-spherical assumptions respectively. Consequently we generate two LUTs under varieties of observing conditions, aerosol loadings, and surface brightness using radiative transfer (RT) code. From the obtained LUTs we thoroughly evaluate and analyze the differences of TOA reflectance (ΔρTOA) and AOD retrieval errors (Δτ) induced by straightforwardly utilizing the Mie theory in dust-like aerosol retrieval. Errors may be positive or negative, depending on the specific geometry. For low aerosol loading (τ~0.25) and black surface, |ΔρTOA| could be greater than 0.06, with maximum |Δτ| of ~0.12. Moreover, this error can increase more than ten times and become even irregular as aerosol loading gets higher and surface gets brighter. Mean Δτ statistics show that this uncertainty significantly influences derived aerosol climatology as well. Therefore we conclude that the neglect of non-sphericity introduces substantial errors on RT simulation and AOD retrieval, and a representative aspheric aerosol model other than Mie calculation is recommended for inversion algorithms related with dust-like non-spherical aerosols.