Comparison of mineral dust scattering properties simulated by the spheroid and irregular-hexahedral models

Monitoring of the vertical structure of airborne mineral dust, the most abundant aerosol species in the atmosphere, is a significant task for lidar measurements. However, due to the complex morphology and large range of particle size, precise modeling of scattering properties of dust aerosol, particularly for the backward direction, is needed to link lidar measurements to particle microphysical properties. In this study, we investigate two scattering models for non-spherical dust aerosol simulation: the Spheroid model (Dubovik et al., 2006) and the Irregular-Hexahedral model (Saito et al., 2021). The Spheroid model characterizes non-spherical particles as a mixture of spheroids, while the latter utilizes an ensemble of 20 irregular hexahedral particles. Previous studies have proved their feasibilities of simulating the scattering properties of coarse non-spherical particles. Nevertheless, there is a lack of direct comparison between the two models, especially the capability of simulating backward lidar measurements.

In this regard, firstly, a comprehensive sensitivity study was conducted to compare the sensitivities of particle scattering properties produced respectively by the two models to the change of particle microphysical properties. The particle microphysical properties are characterized by bimodal lognormal size distributions and wavelength independent refractive index (RI) to mimic mineral dust aerosols. Preliminary results show the two models produce same variation tendencies of scattering properties as RI and the fine-mode volume fraction (FVF) change. However, discrepancy between the two models increases with the increase of FVF. Particularly, the spectral depolarization ratio produced by the Irregular-Hexahedral model is evidently larger than that by the Spheroid model. Furthermore, backscattering properties produced by the Irregular-Hexahedral model show larger sensitivity to particle imaginary part of the RI. In the second step, we are going to investigate how these differences influence the retrieval of dust aerosol microphysical properties from the measurements of multi-wavelength Mie-Raman-polarization lidars by incorporating the models into BOREAL (Basic algOrithm for REtrieval of Aerosol with Lidar) (Chang et al., 2022). Scenarios of different types of dust aerosols (pure, polluted, fresh, transported, etc.) will be identified and used for the retrieval and a better understanding of the retrieval differences will be gained based on both specific case studies and statistical analysis.