Reactive Uptake of Isoprene Epoxydiols Increases the Viscosity of the Core of Phase-Separated Aerosol Particles

Atmospheric oxidation of volatile organic compounds, such as isoprene, and subsequent condensation or heterogeneous reactions lead to the formation of secondary organic aerosol (SOA), a ubiquitous component of submicron aerosol. Liquid-liquid phase-separated organic-inorganic aerosol particles have been observed in the laboratory and field; however, the impacts of multiphase reactions on aerosol viscosity are not well understood for phase-separated aerosol particles. In this study, phase-separated aerosol particles were reacted with gaseous isoprene epoxydiol (IEPOX), an abundant isoprene oxidation product. Acidic sulfate particles (H₂SO₄ + (NH₄)₂SO₄ at pH = 1.4) were coated with laboratory-generated biogenic SOA (α-pinene + O₃) and anthropogenic SOA (toluene + OH), resulting in a core-shell morphology. After reaction with IEPOX, the phase-separated aerosol particles no longer displayed characteristics of a liquid core. Instead, they became irregularly shaped, taller after impaction onto substrates, and had decreased spreading ratios for both types of SOA, implying an increase in particle viscosity. As the SOA from α-pinene and toluene was already viscous, this is indicative of a change in phase state for the core from liquid to viscous state. An example reaction that may be facilitating this phase change is IEPOX reaction with inorganic sulfate to produce organosulfates, especially after IEPOX diffuses through the organic coating. The modification of the aerosol physicochemical properties suggests that phase state is dynamic over the atmospheric lifetime of SOA-containing particles, with multiphase chemistry between aerosol particles and gaseous species leading to more viscous aerosol after uptake of isoprene oxidation products (e.g., IEPOX).