Depolarization of nearly spherical particles: The Debye series approach

Backscattering depolarization of nonspherical particles plays a critical role in active Light Detection And Ranging (LiDAR) retrievals of cloud or aerosol parameters, as well as in particle characterization techniques. However, the interpretation of backscattering light from particles is a challenging research subject. This paper addresses the depolarization of nearly spherical particles by using the Debye series approach. Specifically, the T matrix is represented as an infinite sum of terms; the terms in the expansion are correspondingly associated with diffraction and reflection (p = 0), and multiple transmissions (p > 0) from the particle to the medium as waves undergo internal reflections. We found that the enhanced depolarization for optically soft particles stems from multiple transmissions. However, this is mostly from the transmission after one internal reflection (p = 2), when the refractive index is larger than 1.3. Moreover, the interference among multiple transmissions was found to play an essential role in suppressing the depolarization ratio as the refractive index approaches unity. These findings have implications in interpreting the backscattering optical properties of atmospheric aerosols and hydrosols in water.