Abstract View
Mass Accommodation and Gas-Particle Partitioning of Semivolatile Compounds in Organic Aerosols: Diffusivity, Viscosity, and Penetration Depth Dependence
MANABU SHIRAIWA, Ulrich Pöschl, University of California, Irvine
Abstract Number: 80
Working Group: Aerosol Chemistry
Abstract
Mass accommodation is an essential process for gas-particle partitioning of semi-volatile organic compounds into organic aerosols. The mass accommodation coefficient is commonly used to represent a probability of a molecule collided to the surface to be incorporated into the particle phase. It is often applied, however, without specifying if and how deep a molecule has to penetrate beneath the surface to have entered the condensed phase (adsorption vs. absorption). While this aspect may usually not be critical for liquid particles with rapid surface-bulk exchange, it can be critical to distinguish and resolve the kinetics of accommodation at the surface, transfer across the gas-particle interface, and further transport into the particle bulk for viscous semisolid or glassy solid particles. In this study, we suggest a novel concept of penetration-depth-dependent mass accommodation coefficient, which is a function of surface accommodation coefficient, volatility, bulk diffusivity, and particle-phase reaction rate coefficient. Its application in traditional Fuchs-Sutugin gas-phase diffusion model yields consistent results with detailed kinetic multilayer modeling for partitioning of secondary organic aerosols (SOA). We reveal that for highly viscous particles mass accommodation coefficient of low-volatile compounds would remain unity, but it can be reduced by several orders of magnitude for semivolatile compounds which need to be properly accounted to accurately simulate kinetics of particle growth. These results challenge traditional approach of SOA models and provide a practical method to properly account for effects of viscosity on SOA processes in regional and global air quality models.