Light scattering at small angles by atmospheric irregular particles: modelling and laboratory measurements

Abstract : We have investigated the behaviour of light scattering by particulates of various sizes (0.1 µm to 100 µm) at a small scattering angle (below 20 •). It has been previously shown that, for a small angle, the scattered intensities are weakly dependent upon the particulates' composition (Re-nard et al., 2010). Particles found in the atmosphere exhibit roughness that leads to large discrepancies with the classical Mie solution in terms of scattered intensities in the low angular setup. This article focuses on building an effective theoretical tool to predict the behaviour of light scattering by real particulates at a small scattering angle. We present both the classical Mie theory and its adaptation to the case of rough particulates with a fairly simple roughness parameterisation. An experimental device was built, corresponding to the angular setup of interest (low scattering angle and therefore low angular aperture). Measurements are presented that confirm the theoretical results with good agreement. It was found that differences between the classical Mie solution and actual measurements – especially for large particulates – can be attributed to the particulate roughness. It was also found that, in this low angular setup , saturation of the scattered intensities occurs for relatively small values of the roughness parameter. This confirms the low variability in the scattered intensities observed for atmospheric particulates of different kinds. A direct interest of this study is a broadening of the dynamic range of optical counters: using a small angle of aperture for measurements allows greater dynamics in terms of particle size. Thus it allows a single device to observe a broad range of particle sizes whilst utilising the same electronics.