Dependence of Soot Optical Properties on Particle Morphology: Measurements and Model Comparisons
James G. Radney, Rian You, Xiaofei Ma, Joseph M. Conny, Michael R. Zachariah, Joseph T. Hodges, Christopher D. Zangmeister
We measure the mass-specific absorption and extinction cross sections for laboratory-generated soot aerosols. Two soot morphologies are investigated, both comprising fractal aggregates of nearly spherical monomers. The first type consists of monomer chains with an aggregate fractal dimension of 1.8, and the second type corresponds to aggregates of fractal dimension 3 which are formed by collapsing the monomer chains while preserving the monomer size. The soot particles are selected by mobility diameter and mass, and the aerosol absorption and extinction coefficients are measured at = 405 nm using photo-acoustic and cavity ring-down spectroscopy techniques, respectively. For both particle morphologies, we find that the optical cross sections are proportional to particle mass, with mass absorption coefficients between 5.7 m2 g-1 and 6 m2 g-1, mass extinction coefficients between 12 m2 g-1 and 16 m2 g-1 and single-scattering albedos from 0.5 to 0.6. Results are also compared to theoretical calculations of light absorption and scattering from the simulated particle agglomerates. The observed absorption is relatively well modeled, with the minimum differences between the calculated and measured mass absorption coefficients ranging from 5% (lacey soot) to 14% (compact soot). The model, however, was unable to satisfactorily reproduce the measured extinction, underestimating the single-scattering albedo for both particle morphologies. These discrepancies between calculations and measurements underscore the need for validation and refinement of existing models of light scattering and absorption by soot agglomerates.