Preprints
https://doi.org/10.5194/amt-2022-192
https://doi.org/10.5194/amt-2022-192
 
13 Sep 2022
13 Sep 2022
Status: this preprint is currently under review for the journal AMT.

Investigating the dependence of mineral dust depolarization on complex refractive index and size with a laboratory polarimeter at 180.0° lidar backscattering angle

Alain Miffre, Danaël Cholleton, Clément Noël, and Patrick Rairoux Alain Miffre et al.
  • University of Lyon, Université Claude Bernard Lyon 1, CNRS, Institut Lumière Matière, F-69622, Villeurbanne, France

Abstract. In this paper, the dependence of the particles depolarization ratio (PDR) of mineral dust on the complex refractive index and size is for the first time investigated through a laboratory π-polarimeter operating at 180.0° backscattering angle and at (355, 532) nm wavelengths for lidar purposes. The dust PDR is indeed an important input parameter in polarization lidar experiments involving mineral dust. Our π-polarimeter provides sixteen accurate values of the dust lidar PDR at 180.0° corresponding to four different complex refractive indices, studied at two size distributions (fine, coarse) and at (355, 532) nm wavelengths, while accounting for the highly irregular shape of mineral dust, which is difficult to model numerically. At 355 nm, the lidar PDR of coarser silica, the main oxide in mineral dust, is equal to (33±1) % while that of coarser hematite, the main light absorbent in mineral dust, is (10±1) %. This huge difference is here explained by accounting for the high imaginary part of the hematite complex refractive index. In turn, Arizona dust exhibits higher depolarization than Asian dust, due to the higher proportion in hematite in the latter. As a result, when the strong light absorbent hematite is involved, the dust lidar PDR primarily depends on the particles complex refractive index and its variations with size are less pronounced. When hematite is less or not involved, the dust lidar PDR increases with increasing sizes and the (355, 532) nm wavelength dependence of the dust lidar PDR then allows discussing on the involved particle sizes, thus highlighting the importance of dual-wavelength (or more) polarization lidar instruments. We believe these laboratory findings will help improving our understanding of the challenging dependence of the dust lidar PDR with complex refractive index and size to help interpret the complexity and the wealth of polarization lidar signals.

Alain Miffre et al.

Status: open (until 24 Oct 2022)

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Alain Miffre et al.

Alain Miffre et al.

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Short summary
The depolarization ratio of hematite, silica, Arizona and Asian dust is evaluated in laboratory with a π-polarimeter operating at 180.0° and at (355, 532) nm wavelengths. The hematite depolarization equals (10±1) % at 355 nm for coarser particles while that of silica is (33±1) %. This huge difference is here explained by accounting for the high imaginary part of the hematite complex refractive index, thus revealing the key role played by light absorption in mineral dust lidar depolarization.