Molecular Corridors, Volatility and Particle Phase State in Secondary Organic Aerosols

Secondary organic aerosols (SOA) derived from the multigenerational oxidation of gaseous precursors are major components in atmospheric aerosols. Chemical evolution of SOA from a variety of volatile organic compounds (VOC) adheres to characteristic “molecular corridors” with an inverse correlation between volatility and molar mass. Parameterizations were developed to predict the saturation mass concentration of organic compounds containing oxygen, nitrogen, and sulfur from the elemental composition that can be measured by soft-ionization high-resolution mass spectrometry. The chemical nature of organic compounds observed in field measurements and laboratory experiments was characterized by mapping them into molecular corridors. SOA can occur in amorphous solid or semi-solid phase states depending on chemical composition, relative humidity (RH), and temperature. The phase transition between amorphous solid and semi-solid states occurs at the glass transition temperature (Tg). A method was developed to estimate Tg of pure compounds containing carbon, hydrogen, and oxygen atoms based on their elemental composition. Viscosity can be predicted using the Tg-scaled Arrhenius plot of fragility by accounting for hygroscopic growth of SOA and applying the Gordon-Taylor mixing rule. Applying this method in an air quality model, recent global simulations have shown that SOA particles should be mostly liquid or semi-solid in the planetary boundary layer and glassy solid in the middle and upper troposphere.