Extension of RDG-FA for Scattering Prediction of Aggregates of Soot Taking into Account Interactions of Large Monomers

• Published in: Particle & particle systems characterization 2008-04, Vol.25 (1), p.54-67
• Main Authors: Yon, Jérôme, Rozé, Claude, Girasole, Thierry, Coppalle, Alexis, Méès, Loïc
• Format: Article
• Language: English
• Subjects: DDSCAT; fractal dimension; multiscattering; RDG-FA; soot aggregates
• ISSN: 0934-0866; 1521-4117
• DOI: 10.1002/ppsc.200700011

The characterization of soot fractal aggregate, i.e., morphology, size, number and volume fraction, by optical diagnostics is very widespread since it is a sensitive and nonintrusive technique. In general, light absorption or scattering by fractal aggregates is quantitatively interpreted by the Rayleigh‐Debye‐Gans for Fractal Aggregates (RDG‐FA) theory due to its simplicity. Nevertheless, important conditions have to be obeyed for the correct use of this theory. Among them, aggregates and primary particles have to be very small in comparison to the wavelength. In addition, internal scattering must be negligible. Moreover, the form factor functions that allow the fractal morphology to be taken into account have been established for a polydispersed population. In contrast to previous studies that evaluated the valid RDG‐FA range by comparing the difference between rigorous calculations and the RDG‐FA approach, the aim of the present study is to evaluate possible corrections that could be applied to the classical RDG‐FA formula in order to take into account of aggregates and primary particles internal multiscattering. Therefore, new generalized form factors are proposed allowing the evaluation of the optical properties of one aggregate with a primary particle diameter of up to 90 nm, without any assumptions concerning aggregate complex internal interactions. The presented results are based on the assumption that DDSCAT (Discrete Dipole Approximation) provides accurate results, with a wavelength, λ = 632 nm, a soot fractal dimension of 1.8 and Dalzel and Sarofim optical index, m = 1.57 – i0.56. The characterization of soot fractal aggregates by optical diagnostics is a sensitive and nonintrusive technique. However, important conditions have to be obeyed for the correct use of the common Rayleigh‐Debye‐Gans theory for Fractal Aggregates (RDG‐FA). The aim of the present study is to evaluate possible corrections in order to take into account the presence of aggregates and the internal multiscattering of primary particles.