Laboratory evaluation of the scattering matrix elements of mineral dust particles from 176.0° up to 180.0°-exact backscattering angle

Abstract

In this paper, the scattering matrix elements of an ensemble of mineral dust particles are for the first time evaluated in laboratory for scattering angles ranging from 176.0° to the π-backscattering angle of 180.0° with a high angular resolution of 0.4° and compared with the outputs of T-matrix numerical code. Elastic light scattering is addressed at near and exact backscattering angles with a newly-built laboratory polarimeter, validated on spherical particles following the Lorenz–Mie theory. The ratios fij(θ) = Fij(θ)/F11(θ) of the scattering matrix elements of mineral dust particles are then precisely evaluated in laboratory from 176.0° up to 180.0° with a 0.4° angular resolution (even 0.2° between 179.2° and 180.0°), which is new. When approaching the π-backscattering angle, the slopes of the scattering matrix elements are almost zero, as theoretically predicted by Hovenier and Guirado [17]. Moreover, our laboratory findings are found in good agreement with the outputs of the T-matrix numerical code, showing the ability of the spheroidal model to describe light-scattering by mineral dust also from near to exact backscattering. Atmospheric implications for polarization lidar retrievals are then discussed in terms of linear and circular depolarization ratios for mineral dust. These results, which complement other existing light scattering experiments, may be used to extrapolate light scattering by mineral dust particles up the π-backscattering angle, which is useful in radiative transfer and climatology, in which backscattering is involved.