Calculating observationally consistent multi-spectral dust depolarisation ratios with spheroids

Due to their high versatility, spheroids are commonly employed to model optical properties of mineral dust. However, they are usually considered to be of limited use when studying multispectral depolarisation ratios of mineral dust. An earlier study, in which a spheroidal model was fitted against measurements of the linear depolarisation ratio in near-backscattering direction from 131 dust samples [1], served as a starting point for single scattering calculations using the T-matrix method.

Calculations were performed for different log-normal size distributions at wavelengths of 355nm, 532nm, 1064nm, and 910.6nm. The former three wavelengths are commonly used in lidar remote sensing, while the fourth wavelengths is used in depolarisation-capable ceilometers. In addition, aspect ratios between 0.5 and 2.0 with linearly equidistant steps of 0.01 were considered. Multiple aspect ratios were identified, which yield the linear depolarisation ratios in backscattering direction at 355nm, 532nm, 1064nm consistent with reported lidar field observations and laboratory experiments.

When additionally considering ceilometer observations of dust plumes over the tropical Atlantic during February and March 2025 at a wavelength of 910.6nm, the number of aspect ratios yielding observationally consistent depolarisation ratios is reduced. These aspect ratios encompass both prolate and oblate spheroids. One of these aspects ratios (ε=1.46) corresponds to the median aspect ratio obtained from earlier electron microscopy analysis of freshly emitted dust in the Moroccan Sahara [2].

 

 

[1] M. Kahnert, F. Kanngießer, E. Järvinen, and M. Schnaiter, “Aerosol-optics model for the backscatter depolarisation ratio of mineral dust particles,” J. Quant. Spectrosc. Radiat. Transf. 254, 107177 (2020).

[2] A. Panta, K. Kandler, A. Alastuey, C. González-Flórez, A. González-Romero, M. Klose, X. Querol, C. Reche, J. Yus-Díez, and C. Pérez García-Pando, “Insights into the single-particle composition, size, mixing state, and aspect ratio of freshly emitted mineral dust from field measurements in the Moroccan Sahara using electron microscopy,” Atmos. Chem. Phys. 23, 3861–3885 (2023).