Surface Crust Formation in SOA Leads to Reduced and Nearly Size-independent Evaporation
MEREDITH SCHERVISH, Jacqueline Wilson, ManishKumar Shrivastava, Alla Zelenyuk, Manabu Shiraiwa, University of California, Irvine
Abstract Number: 345
Working Group: Aerosol Chemistry
Abstract
The measurements of room-temperature evaporation kinetics of size-selected α-pinene SOA has shown to exhibit nearly size-independent evaporation behavior characterized by a rapid initial evaporation stage in a few hours, followed by a slow evaporation stage. Similar evaporation behavior was observed for aerosols particles containing high molecular weight polyethylene glycol (PEG) mixtures and other polymer-containing mixtures. It is hypothesized that this size-independent evaporation can be explained by the accumulation of low-volatility, high molecular weight, highly viscous species at the surface of the particle during evaporation. In the α-pinene system, where particle-phase oligomers are present, decomposition of these oligomers has also been shown to contribute to nearly size-independent evaporation. We applied the kinetic multilayer model of gas-particle interactions (KM-GAP) to simulate the evaporation in the PEG and α-pinene systems. A composition-dependent diffusivity is implemented in each particle bulk layer. Additionally, in the α-pinene SOA system oligomer decomposition is implemented. KM-GAP is also used to simulate condensation onto these evaporated particles to explore the effect of surface crust formation on growth. We show that surface crust formation occurs during evaporation when the diffusivity of the initially homogeneous bulk reaches below 10-14 cm2 s-1. As evaporation continues, the diffusivity of the outer bulk layers will continue to decrease at a faster rate than the internal layers, forming the surface crust. The formation of this crust significantly reduces the size dependence of evaporation in both systems by limiting further evaporation. In the α-pinene system, oligomer decomposition additionally reduces the size-dependence of evaporation and reproduces the two-stage evaporation from experiments.