Effects of multiple scattering on radiative properties of soot fractal aggregates

Influence of soot aggregate size and internal multiple scattering on LII signal and the absorption function variation with wavelength determined by the TEW-LII method

“…The first assumption has been found reasonably valid for soot aggregates in visible range [25]. Limitations of RDG-FA associated with the second and the third assumptions have been studied by comparing RDG-FA with more accurate numerical evaluations of the aggregate-light interaction [8,9,26] and the error of RDG-FA has been considered similar to experimental uncertainties [10,[27][28][29]. In essence, RDG-FA is the first-order theory with high-order effects neglected [10].…”

“…The fundamental reason for the dependence of aggregate absorption on morphology is the multiple scattering within the fractal aggregate. Recently, a dimensionless correction factor h has been determined to correct the aggregate absorption cross section of the RDG-FA theory based on the accurate results of DDA [8]. DDA is a numerical method consisting in decomposing the particle by a number of spatially uniformly distributed discrete dipoles.…”

“…As such, the LII signal is proportional to N agg × N p × D p 3 × E(m)/λ that is directly proportional to the soot volume fraction. However, several studies, [8,9], have shown that the aggregate absorption cross section is not exactly proportional to the product of N p and σ p . Indeed, the complex phenomena, namely the internal multiple scattering effects induced by the fractallike morphology of the particle and for certain wavelengths non-strictly Rayleigh regime, can either reduce or enhance the absorption cross section from the one evaluated by the aforementioned method, depending on the aggregate size and wavelength.…”

“…The recent study [8] introduced a correction factor for the evaluation of the aggregate absorption cross section, which was determined by the discrete-dipole approximation (DDA) calculations for a wide range of aggregate size between N p = 1 and 1006 (number of primary particles) and wavelength between 266 and 1064 nm and was found to fall between 0.6 and 1.3. The departure of the correction factor from unity reflects the effect of multiple scattering within the aggregate and is dependent on aggregate size and wavelength.…”

“…Nevertheless, similar to the theoretical expression for the LII signal, the theory of TEW-LII formulated so far has also neglected the multiple scattering effects on aggregate absorption/emission. Therefore, the second objective of the present paper is to re-analyze the TEW-LII results of Bejaoui et al [1] by taking into account the multiple scattering effects on aggregate absorption cross section using the correction factor of Yon et al [8].…”