AAAR 32nd Annual Conference
September 30 - October 4, 2013
Oregon Convention Center
Portland, Oregon, USA
Abstract View
Effects of Multiple Scattering on the Radiative Properties of Fractal Soot Aggregates
FENGSHAN LIU, Gregory Smallwood, National Research Council Canada
Abstract Number: 118 Working Group: Aerosol Physics
Abstract Optical diagnostics play important role in our understanding of black carbon (soot) formation in combustion and in obtaining quantitative information on the morphology of black carbon emitted from various combustion devices. The Rayleigh-Debye-Gans theory for fractal aggregates (RDG-FA) has often been used to interpret the detected scattering signals. One of the important assumptions in the development of RDG-FA is that the multiple scattering within aggregates is negligible. The validity of this assumption has been investigated previously and it has been commonly believed that multiple scattering within aggregates affects aggregate scattering more than absorption.
This study critically examines the effect of multiple scattering on the absorption and scattering properties of fractal soot aggregates. The numerical method used in this study is the generalized Mie-solution method (GMM) applied to numerically generated fractal aggregates of prescribed fractal dimension of Df = 1.4, 2.1 and prefactor kf = 2.3 for aggregate sizes up to Np = 800. GMM allows calculations with and without multiple scattering and the difference between the two results is attributed to multiple scattering. Calculations were conducted to three different incident light wavelengths of 532, 1064, and 2118 nm, while the primary particle diameter and remained fixed at dp = 30 nm, resulting in three different primary particle size parameters of xp = dp/ = 0.177, 0.0886, and 0.0445. Numerical results show that multiple scattering effect increases with increasing primary particle size parameter xp. Multiple scattering has a stronger influence on aggregate absorption than aggregate scattering. Even for aggregates of fairly open structure of Df = 1.4, multiple scattering on aggregate absorption is significant. The so-called ‘shielding effect’ in aggregate absorption is fundamentally due to multiple scattering.