Light scattering matrix for soot aerosol: Comparisons between experimental measurements and numerical simulations

Abstract

Fractal aggregate model has shown significant superiority in simulating integral optical properties (like optical cross sections etc.) of soot aerosol. In this study, angular distributions of scattering matrices for soot samples were measured at 532 nm wavelength. Experiments were conducted over scattering angles from 5° to 175° using an improved and validated apparatus. Soot particles emitted from burning of toluene and n-heptane were collected and re-aerosolized for matrix measurements. Electrical mobility diameter distributions of soot aggregates were measured and converted to volume equivalent diameter distributions according to various conversion factors. Size distributions of soot monomers and fractal parameters were obtained through statistical analyses of transmission electron microscope (TEM) images. Fractal aggregate models were generated by a tunable particle-cluster algorithm, using measured size distributions of aggregates and statistically obtained morphological parameters. Multiple sphere T-matrix (MSTM) method was employed for the calculation of individual particles, and bulk scattering matrices were integrated from particles with same morphology parameters. Results indicated that measured scattering matrices for these two soot samples show similar angular behaviors. Compared with fresh soot particles, the collection and re-aerosolization treatments lead to larger mobility diameters and more compact particle structures. Fractal parameter, conversion factor, refractive index and polydisperse monomer all have various effects on different matrix elements. Inter-comparisons between simulations and measurements focus more on the performance of ideal fractal model consisting of point-contact monomers in simulating measured scattering matrices, and there is no doubt that ideal fractal aggregates cannot simultaneously reproduce measured angular distributions of all matrix elements.