Previous studies have investigated the phase behavior of some types of atmospheric aerosol particles, such as mixtures of secondary organic aerosol with inorganic salts and mixtures of secondary organic aerosol with hydrocarbon-like organic aerosol . However, the phase behavior of primary BBOA remains understudied and unresolved. Models simulating the microphysics, chemical evolution, concentration or radiative properties of wildfire smoke typically assume that BBOA comprises only a single phase . On the other hand, Jahn et al. identified that some sampled BBOA particles exhibited an organic shell coating an organic core using transmission electron microscopy images , with the caveat that the vacuum conditions required for this type of microscopy may affect the particle morphology. In addition, bulk-phase liquid tar condensates generated from high-temperature wood distillation were reported to have separated into a water-soluble and non-soluble oily phase , although it is not known whether phase separation would occur in particles containing this material after equilibration with ambient RH. To our knowledge no study has yet explicitly shown liquid-liquid phase separation (LLPS) in BBOA particles for troposphere temperatures and RH conditions. Studies of BBOA phase behavior as a function of RH are vital, since the RH can vary considerably throughout the atmosphere and the RH controls the water content in particles, which can influence the phase behavior .