Accounting for multiple scattering effects for the evaluation of absorption and scattering cross sections

Aerosols such as soot are a matter of concern in our daily life. Due to their impact on human healthbut also on global warming, their characterization is of extremely high relevance. In this context,extensive work has been done to characterize soot particles by optical techniques because suchapproaches enable an in-situ determination. These techniques essentially rely on the attenuationof the light (LOSA), on the scattering properties (Angular Light Scattering) or on their thermalbehaviour after laser heating (LII). In any cases, the signals are interpreted based on modelsrequiring precise light-particles theories in order to express the absorption and scattering crosssections. For nano-aerosols as soot, the Rayleigh-Debye-Gans (RDG) approximation for fractalaggregates (FA) is frequently used essentially due to its simplicity since it provides a simpleanalytical expression of the scattering and absorption cross sections. However, this approximationrelies on two strong assumptions. First, the monomers composing the aggregate behave asRayleigh spheres, i.e., they are small enough compared to the wavelength and transparentenough to make the phase shift negligible. Second, it is supposed that there is no electromagneticcoupling between the monomers (internal multiple-scattering), i.e., each one sees only the incidentlight and not the light scattered by its neighbours.Many studies have compared rigorous solutions with RDG-FA ones and observed importantdiscrepancies. Indeed, RDG-FA can lead to errors up to 45% on the predicted forward scattering[1, 2] and optical determination of the fractal dimension can be overestimated by a factor up to10% [3]. Also, impact on LII measurements has been shown to be potentially important [4].Nevertheless, the precise underlying mechanisms leading to these discrepancies were not well understood and thus, until now, no models where proposed to improve RDG-FA theory regarding the internal coupling effects.The present communication aims to show to the LII and soot communities the recent advances made on this topic which were achieved by the use of the phasor approach which enables a comprehensive description of the internal electric field and its link with corrections to bring to RDGFA. The internal coupling effects will be first illustrated for a pair of monomers [5] followed by fractal aggregates [6]. Correction factors will be proposed for absorption and forward scattering. Recent advances concerning the impact of internal coupling on scattering structure factor will be also presented.