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Abstract
A new and computationally profitable theoretical approach based on the first order Born dipole approximation is here applied to evaluate the scattering/extinction properties of randomly oriented filamentary soot agglomerates composed by Rayleigh particles (primary spherules). The influence on the scattering pattern of both the scattering contribution from other spherules in the aggregate and of the self-interaction mechanism is represented. Four main morphologies, i.e., the straight and the zigzag chain, the random and the fractal cluster are considered. Major points evidenced are: (a) the Dissymmetry Ratios (i.e., the ratios of scattered light at two different angles) show very low (< 1%) sensitiveness to the soot refractive index uncertainties as well as to the self-interaction effect, and, vice versa, a pronounced dependence on aggregrate morphology and monomer diameter; (b) the combined measurements of at least two Dissymmetry Ratios are expected to be useful to assess the prevailing aggregate morphology; (c) zigzag chain as compared to straight chain scattering patterns result lo be appreciably different only at scattering angles more than 40"-6O°; (d) the influence of the multiple scattering (i.e., of the scattering contributes from aggregate spherules in the far-field other than the considered primary spherule in the same aggregate) on the extinction factor is not negligible and it is quantifiable below 10% for primary diameters not more 40 nm
As working example, we face the problem of recognizing the time evolving characteristics of soot aggregates produced in a shock tube. We delineate a procedure to retrieve first the unknown morphology and then the number density of aggregates, the mean diameter and number of monomers per aggregate and the complex refractive index. The results match quite well both the extinction and the Dissymmetry Ratio R(30°/150°) measurements.