In polarization lidar experiments, the lidar particle depolarization ratio (PDR) is a key quantity, that typically hovers around 30 % for mineral dust, particularly in proximity to the source areas. To explore how the lidar PDR relates to the size and complex refractive index (CRI) of particles, a groundbreaking laboratory experiment (world first) was conducted for the first time at lidar exact backscattering angle of 180.0°, as reported by Miffre et al. in 2023 1 at (355, 532) nm lidar wavelengths. Four dust samples differing in their CRI were investigated: silica (SiO2), a major component of mineral dust, hematite, as the main light absorbent in mineral dust, then Arizona and Asian dust, two heterogeneous mixtures of the above two oxides in various proportions. At 355 nm, the lidar-measured PDR for silica was found equal to (33 ± 1) %. However, for hematite, the observed PDR was much lower, at only (10 ± 1) %. We hence demonstrate that when hematite is present, the complex refractive index governs the dust lidar PDR. In turn, Arizona dust exhibits higher depolarization than Asian dust, due to the higher proportion in hematite in the latter. Conversely, when hematite is less prevalent or absent, the dust lidar PDR is mostly size-dependent and increases with larger particle sizes, although the particle shape may also contribute to this trend. Moreover, similar laboratory findings were observed regarding six pollen types 2, namely ragweed, birch, pine, ash, spruce and cypress, and also soot 3 aerosols, which also exhibit light absorption properties at the 355 nm wavelength. We believe these laboratory findings, emphasizing the key role play the imaginary part of the complex refractive index, may help the lidar community to better interpret lidar inversions based on mineral dust, pollen and soot aerosols.
Miffre et al. (Thu,) studied this question.