AAAR 33rd Annual Conference
October 20 - October 24, 2014
Rosen Shingle Creek
Orlando, Florida, USA
Abstract View
Radiative Properties of Soot Fractal Aggregates Formed by Polydisperse Point-Touch Spherical Particles
FENGSHAN LIU, Jerome Yon, National Research Council Canada
Abstract Number: 410 Working Group: Aerosol Physics
Abstract Optical diagnostics play an important role in our understanding of black carbon (BC), also known as soot formation in combustion and in obtaining quantitative information on the morphology of black carbon emitted from various combustion devices. It is highly desirable to predict the radiative properties of BC particles in order to accurately estimate their role in the Earth’s radiation budget. The Rayleigh-Debye-Gans theory for fractal aggregates (RDG-FA) has often been used to interpret the detected scattering signals and predict the radiative properties of soot fractal aggregates. RDG-FA has been formulated for aggregates formed by point-touch monodisperse primary particles. In reality, however, BC particles in general comprise polydisperse primary particles with a certain degree of overlapping or bridging.
In this study fractal aggregates formed by polydisperse primary particles are numerically generated using a diffusion limited cluster-cluster aggregation (DLCA) algorithm. The mean and standard deviation of primary particles are 34.2 nm and 6.85 nm, respectively. The aggregate size range considered is 1 to 1006. The fractal dimension and the prefactor are 1.78 and 1.44. Radiative properties of these fractal aggregates, including absorption cross section, scattering cross section, and the asymmetry factor, were calculated for m = 1.6 + 0.6i, a typical value of soot in the visible spectrum, and at wavelengths of 532 and 1064 nm using RDG-FA and the generalized Mie-solution method (GMM). Through a direct comparison between the GMM and RDG-FA results, the accuracy of the RDG-FA theory can be assessed. To evaluate the effect of polydispersity in primary particle size on the radiative properties of soot fractal aggregates, additional calculations were calculated for aggregates formed by monodisperse primary particles whose sizes are such that the aggregate volume is conserved.