Morphological Scaling and Direct Forcing Implications of Coated Soot Aggregates (Invited Presentation)

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, 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. We performed detailed three-dimensional morphological characterization of simulated surface coated aggregates that mimic atmospheric SAs. Our results show that D_fremains invariant at 1.8 with increasing coating mass. We also found coating to affect only the fractal prefactor, an understudied parameter which controls the aggregate shape anisotropy and local packing fraction of monomers. Based on our results, we provide revised scaling relationships to enable better representation of SA morphologies in climate models.

There has been a recent surge in interest to investigate the role of coating on enhancement of light absorption by SAs. Several recent field and laboratory studies have reported an absorption enhancement of up to a factor of three due to the presence of organic coating on SAs.  To accurately determine this phenomenon, we calculated the optical properties of our simulated coated SAs using the discrete dipole approximation (DDA) algorithm. DDA is optimal for coated SAs since it can easily model irregular geometries with no extra cost in computational time or accuracy. Concurrent with previous experimental findings, our results show an enhancement in light absorption by coated SAs up to a factor of three. More surprisingly, however, is that the aerosol scattering cross-section showed an increase of an order of magnitude compared to the absorption cross-section. This enhancement leads to an increased single scattering albedo for coated SAs in comparison to those of equivalent-size nascent SAs. We infer that coated SAs could lead to a net negative forcing beyond a threshold coating mass.