Scattering matrices of mineral dust aerosols: a refinement of the refractive index impact

Dust, as one of the most important aerosols, plays a crucial role in the atmosphere by directly scattering and absorbing solar and infrared radiation, while there are significant uncertainties in determining dust optical properties to quantify radiative effects and to retrieve their properties. Both laboratory and in situ measurements show variations in dust refractive indices (RIs), and different RIs have been applied in different numerical studies of model developments, aerosol retrievals, and radiative forcing simulations. This study reveals the importance of the dust RI for the model development of its optical properties. The Koch-fractal polyhedron is used as the modeled geometry, and the pseudo-spectral time domain method and improved geometric-optics method are combined to cover optical property simulations over the entire size range. Our results indicate that the scattering matrix elements of different kinds of dust particles can be reasonably reproduced by choosing appropriate RIs even using a fixed particle geometry. The uncertainty of the RI would greatly affect the determination of the geometric model, as a change in the RI, even in the widely accepted RI range, strongly affects the appropriate shape parameters to reproduce the measured dust phase matrix elements. A further comparison shows that the RI influences the scattering matrix elements differently from geometric factors, and, more specifically, the P11, P12, and P22 elements seem more sensitive to dust RI. In summary, more efforts should be devoted to account for the uncertainties on the dust RI in modeling its optical properties, and the development of corresponding optical models can potentially be simplified by considering only variations over different RIs. Considerably more research, especially from direct measurements, should be carried out to better constrain the uncertainties related to the dust aerosol RIs.