AAAR 34th Annual Conference
October 12 - October 16, 2015
Hyatt Regency
Minneapolis, Minnesota, USA
Abstract View
Phase Equilibration Timescales of Engine Exhaust SOA Generated in a Photo-oxidation Reactor
Mariam Fawaz, Mohamad Baassiri, Nareg Karaoghlanian, ALAN SHIHADEH, American University of Beirut
Abstract Number: 435 Working Group: Aerosol Chemistry
Abstract Secondary organic aerosols (SOA) constitute a major fraction of particle pollutants in the atmosphere, and they exert important influences on human health and global climate. When predicting atmospheric SOA concentrations, regional air quality models commonly assume that gas-particle partitioning is rapid relative to atmospheric timescales, and that semi-volatile species therefore follow thermodynamic equilibrium partitioning between the condensed and vapor phases. Based on recent evidence from single-particle studies that SOA particles exist in a glassy, amorphous state for which mass transfer is intrinsically slow compared to atmospheric time scales, the assumption that SOA is well-described by equilibrium thermodynamics has been called into question. In this work, we studied the evaporation kinetics of SOA when an ensemble of nanoparticles was perturbed from an initial equilibrium state via isothermal dilution in a 20˚C and 12-15% RH Teflon chamber. Changes in particle size were observed as the SOA system was allowed to return to phase equilibrium by evaporation. SOA was generated in a well stirred photo-oxidation reactor using diluted engine exhaust (5000:1) from a single-cylinder gasoline engine, and then injected into the Teflon chamber. Initial SOA mass loading in the chamber was 10 micrograms/m$^3, and initial condensation sink diameter was approximately 100 nm. In repeated measurements, it was found that SOA particles approached equilibrium with a characteristic e-folding time of 11-14 min. Wall-corrected particle size versus time was well-fit by a theoretical model of particle evaporation treating the aerosol as a single lumped species, where the effective evaporation coefficient ranged from 0.05 to 0.1 depending on the assumed ranges of molecular mass and effective binary diffusion coefficient. These results support the notion that mass transfer in fresh engine exhaust derived SOA is not greatly inhibited by intra-particle diffusion, and that its atmospheric phase partitioning behavior can be treated using thermodynamic equilibrium.