Fractal scaling of coated soot aggregates

Recent field observations have shown soot aggregates (SAs) to contain significant amounts of surface coatings of organic compounds which obfuscate their native fractal morphology and make them visually appear as “near-spherical.” Morphologies of these aggregates are currently parameterized using fractal dimension (D_f) values greater than the universal 1.8. This is done to account for the supposedly morphological restructuring of an aggregate to a more compact form upon condensation of organic materials. Using multiple-angle light scattering analysis, it has been experimentally shown that restructuring of SA morphology only takes place during the evaporation process, not condensation. Based on this seminal finding, here we formulate the correct parameterizations to describe the morphology of surface coated aggregates. We perform detailed three-dimensional morphological characterization of computer simulated coated aggregates that mimic atmospheric SAs and show that their D_f remains invariant at 1.8 with increasing coating mass by as much as 18 fold. We find coating to affect only the fractal prefactor k₀, an understudied parameter which controls the aggregate shape anisotropy and local packing fraction of monomers. Specifically, k₀ was observed to scale with the ratio of aggregate's total (coating + bare) mass M_total to bare mass M_bare as k₀ = 1.34*(M_total/M_bare)^0.56.