Laboratory Evaluation of the Lidar Particle Depolarization Ratio (PDR) of Sulfates, Soot, and Mineral Dust at 180.0° Lidar Backscattering Angle

Abstract

In polarization lidar field experiments, the particles depolarization ratio (PDR) is key for accurate retrievals of particles backscattering vertical profiles specific to nonspherical particles, such as mineral dust. Precise values of the intrinsic depolarization of mineral dust are however difficult to obtain, due to the complexity of mineral dust in size, shape, and complex refractive index, which prevents from analytical solutions to the Maxwell’s equations. In this contribution, a new approach is proposed, based on a laboratory experiment, to provide accurate evaluations of the lidar PDR of mineral dust and soot particles in laboratory (Miffre et al., J Quant Spectrosc Radiat Transf 169:79–90, 2016). This laboratory aerosol Pi-polarimeter takes benefit from the scattering matrix formalism to provide accurate evaluations of the lidar PDR in laboratory in the exact lidar backscattering direction of 180.0°, which is a world first (Miffre et al., J Quant Spectrosc Radiat Transf 169:79–90, 2016). After detailing the principle of the Pi-polarimeter, case studies are considered to reveal the intrinsic PDR of several mineral dust samples differing in size and mineralogy, allowing for the first time to our knowledge, to experimentally investigate the dependence of the lidar PDR with size and complex refractive index. The case study of soot particles (lidar PDR in the 10% range) is also studied (Paulien et al., J Quant Spectrosc Radiat Transf 260:107451, 2021), together with that of core-shell organic sulfates (Dubois et al., Phys Chem Chem Phys 23:5927–5935, 2021). We believe these laboratory findings, which provide accurate evaluations of lidar PDR at 180.0° lidar angle, may be used by the lidar community to interpret the complexity and the wealth of polarization lidar signals, which are every day worldwide acquired.