Single-Particle Viscosity of Isoprene-Derived Secondary Organic Aerosol Formed across a Range of Relative Humidities and Aging Conditions Reveals Morphology-Dependent Effects

Secondary organic aerosol (SOA) is a major component of fine particulate matter (PM_2.5) globally, which impacts public health and the Earth's climate. The viscosity of SOA-containing PM_2.5 varies dramatically, from liquid to semisolid to solid, changing SOA formation kinetics and multiphase reactions on and within particles. Isoprene, the biogenic volatile organic compound (BVOC) with the highest emissions globally, is oxidized to isoprene epoxydiols that subsequently undergo multiphase chemistry with acidic aerosols to form isoprene-derived SOA (iSOA) and is typically assumed to be liquid in models. Herein, we show that iSOA formed at different relative humidities (RH) on acidic, sulfate-containing seed particles ranges from liquid to semisolid through a series of chamber experiments. A recently developed method was used to directly measure individual particle melting temperatures (<i>T</i>_m) that were converted to glass transition temperatures (<i>T</i>_g), viscosities, and mixing time scales. We also show that heterogeneous hydroxyl radical (^•OH) oxidation of iSOA increases viscosity relative to predictions for SOA formed at high RH, but it decreases relative to predictions under low RH conditions. Existing models struggle to predict these trends, demonstrating the importance of RH and degree of ^•OH exposure in predicting the extent of iSOA formation and overall PM_2.5 concentrations.