Abstract View
Phase Shift Parameter Controls Light Absorption Enhancement for Coated Fractal Aggregates
PAYTON BEELER, William Heinson, Rajan K. Chakrabarty, Washington University in St. Louis
Abstract Number: 289
Working Group: Aerosol Physics
Abstract
The mass absorption cross-section (MAC) of black carbon (BC) aggregates is an important parameter in many climate models. Failure to account for the complex morphology and mixing state of BC aggregates will result in significant underestimation of their MAC, and ultimately errors in global radiative forcing calculations. Consensus holds that organic compounds can coat the surface of BC aggregates and alter their morphology towards an increase in phase shift parameter. However, the detailed evolution pathway of MAC during the coating-induced BC restructuring remains understudied till date. Here, we perform computer simulation to comprehensively probe the MAC enhancement factor (EMAC,BC) for coated BC aggregates with changing phase shift parameter and amount of coating. Three aggregation models: diffusion-limited cluster-cluster aggregation, percolation, and simple cubic lattice stacking, were adopted to generate aggregates mimicking real-world BC in the freshly-emitted, partially-collapsed, and fully-collapsed states, respectively. The simulated aggregates were subsequently coated with non-refractory organics, and their optical properties were calculated with the Amsterdam discrete dipole approximation. Our results show that the amount of coating determines the magnitude of EMAC, BC per a universal scaling relationship. Finally, we find that the phase shift parameter determines whether EMAC, BC is positive or negative (i.e. increased or decreased absorption) over the atmospheric lifetime of BC aggregates, and determines whether core-shell model overestimates or underestimates light absorption by coated BC aggregates.