Radiative transfer simulation of dust-like aerosol: uncertainties from particle shape and refractive index

The composition, particle shape, number concentration, size distribution, and spatial and temporal distribution of dust aerosols cause significant uncertainties in relevant radiative transfer simulations. It has been found that the spherical particle approximation introduces errors in radiative transfer calculation involving dust. Although several previous studies have been conducted to quantify the effects of spherical and nonspherical particles, no consensus has been reached as to whether the nonspherical effect of dust aerosols is significant for flux calculation. In this study, we employ a newly developed database of the single-scattering properties of tri-axial ellipsoidal mineral dust-like aerosols from ultra-violet to far-infrared spectral region. This database was developed using a combination of four methods (Lorenz-Mie theory, the T-matrix method, the discrete dipole approximation and the improved geometric optics method) to compute the phase matrix, extinction efficiency and single-scattering albedo of ellipsoids with various aspect ratios and sizes. The optical properties of dust-like aerosols from this database are used as the input to the latest version of libRadtran radiative transfer package. The differences in radiance and flux between the spherical model and the ellipsoidal model are obtained under different choices of refractive index and particle size distribution. The errors due to using the spherical model and the uncertainty of refractive index are thus quantified. The results show that tri-axial ellipsoidal model particle is a better-suited particle model than the spherical model that will reduce the error related with particle morphology and can substantially improve the radiance and flux modeling. The error caused by the uncertainty in the real/imaginary part of the refractive index is considered to be of crucial importance. Further efforts have been made to evaluate the non-spherical effect of aerosol in estimating dust aerosol radiative forcing in GCM.