Viscosity and phase behavior (number and types of phases) influence the role of BBOA in air quality and climate. For example, if BBOA contains two phases, then the equilibrium partitioning of gas phase species into the BBOA is changed compared to the non-phase separated case, which impacts how the particles grow and gain mass -in turn influencing their health and climate impacts. The presence of two non-crystalline phases has also been shown to change cloud condensation nuclei activity. Compared to a well-mixed droplet, a phase-separated droplet with a low polarity organic outer phase has lower surface tension, which lowers the barrier to cloud condensation. If the BBOA particles are sufficiently viscous, the viscosity could limit reactions within the particles by limiting intra-particle diffusion rates. Furthermore, if BBOA particles have a viscosity > 10 Pa s (i.e. the particles are in a glassy state), they may be good nuclei for crystalline ice and influence the properties and frequency of ice clouds. Several studies have directly or indirectly determined the viscosity of laboratory-generated BBOA using the poke-flow technique, volatility measurements, and fluorescence techniques. Several studies have also investigated the phase behavior of laboratory-generated BBOA using optical, fluorescence, and electron microscopy techniques. While the previous laboratory studies have been useful, the composition of BBOA in the real atmosphere may be different than the composition of BBOA generated in the laboratory for several reasons including atmospheric aging of BBOA, higher degrees of dilution, and burning conditions (e.g. fuel type, temperature, humidity, availability of oxygen). These factors could lead to atmospheric BBOA having a very different viscosity than what is assumed based on laboratory results, and phase behaviour may differ as well. Particularly, phase separation has been explained as being driven by differences in polarity, approximated by the oxygen-to-carbon ratio (O:C). Atmospheric aging increases O:C, so atmospheric aging of particles could change their phase behaviour.