Are Backscattering Ångström Exponents merely indicative of particles size? Advancing beyond by using light polarization

Every day across the globe, dual-wavelength or multi-wavelength lidar instruments assess Backscattering Angstrom Exponents (BAEp), which delineate the wavelength-dependent behavior of particles backscattering coefficients. Traditionally regarded as a proxy for particle size in literature, we challenge this interpretation and propose a reevaluation leveraging light polarization. Specifically, by utilizing light polarization, we can independently extract the Backscattering Ångström Exponents of spherical (s) and non-spherical (ns) particles within a particle mixture (p) = {s, ns}, denoted as BAES and BAEns respectively. We establish and experimentally verify the relationship between BAEp, BAES and BAEns, employing a case study involving mineral dust particles. Remarkably, BAES and BAEns can be simulated numerically using Mie theory for spherical particles and T-matrix numerical code for non-spherical particles, elucidating the influence of particle size and complex refractive index on these parameters. As a result, the range of involved (s) and (ns)-particles size or / and complex refractive index to be considered in lidar inversion algorithms, can be reduced, thus improving the accuracy of such retrievals. We believe this new methodology, published in [1], may then interest the lidar community: to be applied, it indeed only requires a 2/3 + 28 or multi-wavelength polarization lidar instruments, which are operated every day, worldwide.