Determination of the evaporation coefficient of secondary organic aerosols from gasoline engine exhaust -

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 (OA) 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 OA is well-described by equilibrium thermodynamics has been called into question. In this study, the evaporation kinetics of an ensemble of SOA nanoparticles is observed when they are heated to 40 ˚C in a constant temperature, atmospheric pressure flow tube. In particular, particle volume changes are tracked in time, and the observations are fitted to a theoretical model of particle evaporation in order to obtain the effective evaporation coefficient. The evaporation coefficient describes the rate of evaporation relative to the maximum theoretical rate of evaporation defined by kinetic theory of gases. 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 µg-m³ to 40µg-m³. It was found that particle evaporation kinetics were well described by Maxwell’s equation, with effective evaporation coefficients approaching unity. These results indicate that in the atmosphere anthropogenic SOA attain phase equilibrium on time scales approaching minutes or tens of minutes in extreme cases, and can generally be treated as continuously in thermodynamic equilibrium for regional air quality models.