AAAR 33rd Annual Conference
October 20 - October 24, 2014
Rosen Shingle Creek
Orlando, Florida, USA
Abstract View
Evaporation Kinetics of Secndary Organic Aerosols Derived from Engine Exhaust Precursors
Mohammad Baasiri, ALAN SHIHADEH, American University of Beirut
Abstract Number: 579 Working Group: Health Related Aerosols
Abstract Organic aerosols constitute a major fraction of particle pollutants in the atmosphere, and they exert important influences on human health and global climate. When predicting concentrations of organic aerosols in the atmosphere, regional air quality models commonly assume that gas-particle partitioning is rapid, and that therefore semi-volatile species closely follow thermodynamic equilibrium partitioning between the condensed and vapor phases. Based on recent evidence from single-particle studies that secondary organic aerosols (SOA) 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 study, the evaporation kinetics of an ensemble of SOA nanoparticles are observed when they are heated to 40 ˚C in a constant temperature, atmospheric pressure flow tube. In particular, particle volume changes were tracked in time, and the observations fitted to a theoretical model of particle evaporation in order to obtain the effective evaporation coefficient. SOA was generated by photo-oxidizing diluted (5000:1) exhaust from a single-cylinder gasoline engine. Investigated particle mass loadings spanned a range from 18 micrograms/m$^3 to 40 micrograms/m$^3. It was found that particle evaporation was well described by Maxwell's equation, modified for non-continuum effects, with effective evaporation coefficients of order 0.1. These results indicate that at least for engine exhaust SOA, mass transfer rate is not constrained by intra-particle diffusion processes. More importantly, these results suggest that atmosphere anthropogenic SOA attain phase equilibrium on time scales approaching minutes or tens of minutes in extreme cases, and that contrary to recent literature, treatment of SOA partitioning using thermodynamic equilibrium indeed may be valid.