Two-phase morphology and drastic viscosity changes in biomass burning organic aerosol after hydroxyl radical aging

Abstract

Understanding the impact of wildfire-derived biomass burning organic aerosol (BBOA) on air quality, climate, and atmospheric chemistry requires knowledge of its phase behavior and viscosity – properties that remain poorly characterized after atmospheric aging. We investigated how hydroxyl radical (OH) aging affects these properties in BBOA generated from smoldering pine wood. Samples were aged in an oxidative flow reactor with equivalent atmospheric aging times of 1.3, 5.2, and 8.6 days. Phase behavior was assessed using optical microscopy, and viscosity was measured using the poke-flow technique. Across all aging times and relative humidities (0–90% RH), particles consisted of a hydrophilic core and a hydrophobic shell. Under dry conditions, viscosity increased by 4–5 orders of magnitude with aging, and the most aged particles became glass-like. Viscosity was strongly RH-dependent. From these measurements, we estimated mixing times and glass formation in 200 nm particles throughout the troposphere. Aged BBOA is predicted to remain well mixed in the boundary layer, but in the free troposphere (∼1–12 km), mixing times often exceed 1 hour and particles are frequently in a glassy state. These findings have implications for particle growth, evaporation, and ice nucleation, and suggest that OH aging alone cannot fully explain tar ball formation in the atmosphere.