High Viscosity and\nTwo Phases Observed over a Range\nof Relative Humidities in Biomass Burning Organic Aerosol from Canadian\nWildfires

Biomass burning organic aerosol (BBOA) is a major contributor\nto\norganic aerosol in the atmosphere. The impacts of BBOA on climate\nand health depend strongly upon their physicochemical properties,\nincluding viscosity and phase behavior (number and types of phases);\nthese properties are not yet fully characterized. We collected BBOA\nfield samples during the 2021 British Columbia wildfire season to\nconstrain the viscosity and phase behavior at a range of relative\nhumidities and compared them to previous studies on BBOA. Particles\nfrom all samples exhibited two-phased behavior with a polar hydrophilic\nphase and a nonpolar hydrophobic phase. We used the poke-flow viscosity\ntechnique to estimate the viscosity of the particles. Both phases\nof the BBOA had viscosities of >10<sup>8</sup> Pa s at relative\nhumidities\nup to 50%. Such high viscosities correspond to mixing times within\n200 nm BBOA particles of >5 h. Two phases and high viscosity have\nimplications for how BBOA should be treated in atmospheric models.