AAAR 36th Annual Conference October 16 - October 20, 2017 Raleigh Convention Center Raleigh, North Carolina, USA
Abstract View
Light Absorption Proprieties of Coated Soot Aggregates with Increasing Fractal Dimension and Comparisons to the Spherical Core-Shell Model
WILLIAM HEINSON, Rajan Chakrabarty, Washington University in St. Louis
Abstract Number: 218 Working Group: Carbonaceous Aerosols in the Atmosphere
Abstract Soot Aggregates (SAs) in the atmosphere significantly influence the earth’s radiation balance, visibility, and public health. They are formed from high-temperature, incomplete combustion of fossil and biomass fuels via diffusion-limited cluster aggregation (DLCA) of spherical monomers. SAs can contain significant amount of surface coatings of organic compounds which may alter their native fractal morphology through capillary and surface tension forces. Depending on the strength of these restructuring forces, the morphologies of SAs can be parameterized with increasing fractal dimension (Df) with values ranging from Df =1.8 to 3.0. We used three aggregation mechanisms–DLCA, Percolation, and Face-centered cubic stacking – to generate aggregates with Df = 1.8, 2.5, and 3, respectively. This range of Df closely mimics the different morphologies of real-world SAs: bare (Df = 1.8); partially collapsed (Df = 2.5); and fully collapsed (Df = 3). Next, we coated these numerical aggregates with non-refractory materials using a custom-made algorithm and calculated their numerically-exact optical properties using the discrete dipole approximation (DDA) algorithm. In many climate models, SAs are approximated by an equivalent-mass core-shell spherical model due to the ease of calculating optical properties using Lorentz-Mie theory. Keeping this in mind, we computed the optical properties of core-shell spheres equivalent in mass to our coated SAs. Comparisons of the core-shell spheres with the coated aggregates showed that the mass absorption cross-sections (MAC) were greatly underestimated in the core-shell approximation with increasing particle size. Due to their porous nature, the monomers of the aggregates are completely illuminated by the incident light while the optical skin depths of the equivalent core-shell spheres prevents the black carbon core from participating in light absorption.