Viscosity and Phase State of Biomass-Burning Organic Aerosol

Biomass-burning organic aerosol (BBOA) is a major component of atmospheric particulate matter and is expected to become increasingly important as wildfire activity increases. The viscosity and phase state of BBOA influence its climate and health impacts. This review summarizes current understanding of the viscosity and phase state of BBOA. Measurements show that the viscosity of unaged primary BBOA spans more than ten orders of magnitude, from liquid-like particles to glassy solids, depending on fuel type, combustion conditions, burn stage, and relative humidity. Aging by evaporation, OH oxidation, UV irradiation, and secondary aerosol formation can increase viscosity by several orders of magnitude. Field measurements indicate that atmospheric BBOA is often more viscous than unaged laboratory BBOA. Laboratory measurements further suggest that aged BBOA commonly exists in highly viscous or glassy states in the free troposphere and may exhibit mixing times exceeding one hour. We also highlight future research directions.