Predicting Secondary Organic Aerosol Enhancement in the Presence of Atmospherically Relevant Organic Particles

Secondary organic aerosol (SOA) produced from atmospheric oxidation of organic vapors comprises a large portion of ambient particulate matter. Currently, SOA models typically assume that all organic species form a well-mixed phase as a simplification, which follows that SOA formation is enhanced in the presence of pre-existing organic aerosol (OA) according to Raoult's Law. In this work, we show through experiments with atmospherically relevant OA that not all organic species are equally miscible, and the thermodynamics of mixing are composition dependent. SOA formation from alpha-pinene ozonolysis was investigated in the presence of OA that was collected from Toronto ambient air and other sources including biomass burning, meat-cooking emissions, and diesel exhaust. Compared to experiments with ammonium sulfate seed particles, enhanced SOA yields were observed with particles from biomass burning and meat cooking but not with diesel exhaust and concentrated ambient particles. We demonstrate that both kinetic (bulk diffusion-limitation) and thermodynamic (miscibility-limitation) factors are important in determining atmospheric organic aerosol partitioning. We develop parametrization methods using bulk elemental ratios (H/C and O/C) and functional group abundance (R-OH and R-COOH) to estimate average intermolecular interactions, which allow us to use Hansen Solubility Framework we had previously developed to predict atmospheric organic aerosol miscibility and SOA yield enhancements in these complex mixtures. The framework has also been utilized to better understand the liquid-liquid phase separation between organic aerosol and inorganic salts. Our results show that a molecular description of thermodynamic forces is needed to describe aerosol mixing in the atmosphere and accurately parametrize SOA formation.