On the use of light polarization to investigate the size, shape, and refractive index dependence of backscattering Ångström exponents.

In this Letter, we exploit the polarization property of light to investigate the Ångström exponent describing the wavelength dependence of optical backscatter between two wavelengths. Where previous interpretation of Ångström exponent was that of a particle size indicator, the use of light polarization makes it possible to investigate the Ångström exponent dependence on the particle shape by separately retrieving the backscattering Ångström exponent of the spherical (s) and non-spherical (ns) particles contained in an atmospheric particle mixture $(p) = {s, {\rm ns}}$(p)={s,ns}. As an output, analytical solutions of the Maxwell's equations (Lorenz-Mie theory, spheroidal model) can then be applied to investigate the Ångström exponent dependence on the particle size and complex refractive index for each assigned shape. Interestingly, lidar-retrieved vertical profiles of backscattering Ångström exponents specific to $s$s- and ns-particles can be used by the optical community to evaluate a range of involved particle sizes and complex refractive indices for both particle shapes, $s$s and ns, as we remotely demonstrate on a case study dedicated to a dust nucleation event.