AAAR 35th Annual Conference October 17 - October 21, 2016 Oregon Convention Center Portland, Oregon, USA
Abstract View
Fractal Scaling of Coated Soot Aggregates
WILLIAM HEINSON, Rajan Chakrabarty, Washington University in St. Louis
Abstract Number: 184 Working Group: Carbonaceous Aerosols in the Atmosphere
Abstract Soot Aggregates (SAs) in the atmosphere significantly influence the earth’s radiation balance and climate, visibility, and public health. They are formed from high-temperature, incomplete combustion of fossil and biomass fuels via diffusion-limited aggregation of spherical monomers. Real-world combustion sources, such as wildfires and engines, operate in fuel-rich conditions and co-emit large amounts of gas-phase organic compounds along with SAs. These compounds, either upon cooling or after undergoing atmospheric processing, get deposited on the aggregate surfaces as layers of external coating. Recent studies have shown SAs can contain significant amounts of surface coatings of organic compounds which obfuscate their native fractal morphology and make them visually appear as “near-spherical”. Depending on the amount of coating mass, the morphologies of SAs are currently parameterized using mass fractal dimension (D$_f) values in the range of 1.8 ≤ D$_f ≤3.0. Here we perform detailed three-dimensional morphological characterization of simulated surface coated aggregates that mimic atmospheric SAs. We show that D$_f remains invariant at 1.8 with increasing coating mass. We find coating to affect only the fractal prefactor, an understudied parameter which controls the aggregate shape anisotropy and local packing fraction of monomers. We provide revised scaling relationships to enable better representation of SA morphologies in climate models.