The single scattering properties of soot aggregates with concentric coreshell spherical monomers
Abstract
Anthropogenic soot aerosols are shown as complex, fractallike aggregated structures with high light absorption efficiency. In atmospheric environment, soot monomers may tend to acquire a weakly absorbing coating, such as an organic coating, which introduces further complexity to the optical properties of the aggregates. The single scattering properties of soot aggregates can be significantly influenced by the coated status of these kinds of aerosols. In this article, the monomers of fractal soot aggregates are modelled as semiexternal mixtures (physical contact) with constant radius of soot core and variable sizes of the coating for specific soot volume fractions. The single scattering properties of these coated soot particles, such as phase function, the cross sections of extinction and absorption, single scattering albedo (SSA) and asymmetry parameter (ASY), are calculated using the numerically exact superposition Tmatrix method. The randomorientation averaging results have shown that the single scattering properties of these coated soot aggregates are significantly different from the single volumeequivalent coreshell sphere approximation using the Mie theory and the homogeneous aggregates with uncoated monomers using the effective medium theory, such as MaxwellGarnett and Bruggemann approximations, which overestimate backscattering of coated soot. It is found that the SSA and cross sections of extinction and absorption are increased for soot aggregates with thicker weakly absorbing coating on the monomers. Especially, the SSA values of these simulated aggregates with less soot core volume fractions are remarkably (~50% for core volume fraction of soot aggregates of 0.5, ~100% for a core volume fraction of 0.2, at 0.67 μm) larger than for uncoated soot particles without consideration of coating. Moreover, the cross sections of extinction and absorption are underestimated by the computation of equivalent homogeneous fractal aggregate approximation (within 5% for the Tmatrix method and 1025% for the RayleighDebyeGans approximation due to different soot volume fractions). Further understanding of the optical properties of these coated soot aggregates would be helpful for both environment monitoring and climate studies.
 Publication:

Journal of Quantitative Spectroscopy and Radiative Transfer
 Pub Date:
 March 2014
 DOI:
 10.1016/j.jqsrt.2013.11.009
 Bibcode:
 2014JQSRT.135....9W