The polarimetric characteristics of dust with irregular shapes: Evaluation of the spheroid model

In the atmosphere, the dust shapes are various and a single model is difficult to represent the complex shapes of dust. We proposed a tunable model to represent dust with various shapes. Two tunable parameters were used to represent the effects of the erosion degree and binding forces from the mass center, respectively. Thus, the model can represent various dust shapes by adjusting the tunable parameters. To evaluate the applicability of the spheroid model in calculating the optical properties, the aspect ratios of spheroids were retrieved by best fitting the phase function of dust with irregular shapes. Our findings show that the dust shapes have a substantial impact on the scattering matrix, and sometimes the sign of elements of the scattering matrix can be modified by changing the tunable parameters. The applicability of the spheroid model is significantly affected by the erosion degree and binding forces, and substantial deviations could be observed when the dust size is relatively large. Besides, the sign of F12/F11 and F34/F11 can be modified from negative to oppositive at some scattering angles if substituting the irregular dust with best-fitted spheroids. As the binding force is small, the deviation of extinction/scattering cross-section generally increases with the erosion degree, and the relative difference can reach approximately 30 % when the erosion degree is large, while the deviation is mitigated with the binding force increasing. Besides, with the binding force increasing, the retrieved aspect ratio is more close to 1 : 1. The deviations of the spheroid model on estimating the polarized light were also investigated using the successive-order-of-scattering (SOS) vector radiative transfer (VRT) model. With a diameter (dp) of 0.2 μm, the relative difference of normalized radiance does not exceed 3 %, and the absolute values of the deviation of the polarized bidirectional reflectance factor (PBRF) and the ratio of radiance to polarized intensity (DoLP) are below 0.005 and 0.02, respectively. However, with the particle size increasing, the difference becomes much more substantial. The relative difference of the normalized radiance can exceed 10 %, and the deviation of the PBRF and DoLP can vary in the range of −0.015 −0.025 and the range of −0.05 to 0.15. Thus, the use of the spheroid model in the component retrievals based on the polarized light should consider the effects of more complex dust shapes.