Assessing the depolarization capabilities of nonspherical particles in a super-ellipsoidal shape space

Here we use the state-of-the-art invariant imbedding T-matrix method to theoretically assess the backscattering linear depolarization ratio (LDR) of nonspherical particles in a super-ellipsoidal shape space. Super-ellipsoids have inherent flexibility to model the particle aspect ratio, roundness, and concavity, these being salient characteristics of most atmospheric particles (e.g., sea salt and dust aerosols). The complex refractive index of super-ellipsoids was set up with the real part ranging from 1.1 to 2.0 and the imaginary part from 10^-7 to 0.5. To constrain the computational burden, the maximum size parameters for spheroids and super-ellipsoids were set as 100 and 50, respectively. From the LDRs of spheroids, we found that enhanced LDRs (>~60%) are common for optically soft particles. However, as the real part of the refractive index increases (larger than ~1.33), the enhanced LDRs (>~60%) are in high probability observed for nearly-spherical particles, and then disappear as the refractive index exceeds 1.7. To produce the enhanced LDRs, the imaginary part of the refractive index should also be less than ~0.01 such that the backscattered waves from particle-to-air transmission have sizable contributions, as the external reflection of spheroids produces no depolarization. This finding has particular relevance to LiDAR observations of atmospheric particles because the refractive index of most aerosols and hydrometeors at the LiDAR wavelength (e.g., 0.532μm) locates in this region, and aerosols and hydrometeors could have nearly-spherical morphologies. From the LDRs for general super-ellipsoids, we found that the enhanced LDRs (>~60%) exist for nearly-spherical particles with the aspect ratio close to unity, but disappear for super-ellipsoids with an aspect ratio at unity. In addition, the LDRs trend to decrease as the real part of the refractive index increases for convex super-ellipsoids, but show different features for concave super-ellipsoids. Furthermore, super-ellipsoids with different roundness parameters have a distinct dependence on the aspect ratio, which is significantly different from spheroids. The results presented here provide comprehensive references for understanding the LDR change of atmospheric aerosols as the particle shape and refractive index for interpreting LiDAR backscattering signals.