High viscosity and two phases observed over a range of relative humidities in biomass burning organic aerosol from Canadian wildfires

Biomass burning organic aerosol (BBOA) is a major contributor to organic aerosol in the atmosphere. The impacts of BBOA on climate and health depend strongly on their physicochemical properties, including viscosity and phase behaviour (number and types of phases); these properties are not yet fully characterized. We collected BBOA field samples during the 2021 British Columbia wildfire season to constrain the viscosity and phase behaviour at a range of relative humidities, and compared them to previous studies on BBOA. Particles from all samples exhibited two-phased behaviour with a polar hydrophilic phase and a non-polar hydrophobic phase. We used the poke-flow viscosity technique to estimate the viscosity of the particles. Both phases of the BBOA had viscosities >108 Pa s at relative humidities up to 50%. Such high viscosities correspond to mixing times within 200 nm BBOA particles of >5 h. Two phases and high viscosity have implications for how BBOA should be treated in atmospheric models.