Biomass burning organic aerosol (BBOA) from wildfires has multiple phases and is more viscous than laboratory generated BBOA

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, and their relationships to BBOA chemistry, 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 laboratory generated BBOA made from smoldering pine wood. Particles from all samples exhibited two-phased behaviour with a higher polarity hydrophilic core and a lower polarity hydrophobic shell. We used the poke-flow viscosity technique to estimate the viscosity of the particles. We found that both phases of the field samples had viscosities >10⁸ Pa s at relative humidities up to 50%, which is more viscous than any laboratory generated BBOA or BBOA proxies previously measured. Aerosol mass spectrometry showed that the field samples were more oxidized than those generated in the lab, which is a likely explanation for the higher viscosity. The two phases and high viscosity have implications for how BBOA should be treated in atmospheric models