Consequently, high viscosities and the glassy state could inhibit multiphase reactions within the particle bulk, restricting multiphase chemistry to the particle surface. Knowledge of the viscosity and phase state of BBOA is also needed to predict the formation pathways of PSCs (Fig. ). If BBOA particles are in a non-glassy state, they can take up H2O and HNO3 at low temperatures to form non-glassy PSCs (pathway a). In addition, Ansmann et al. speculated that BBOA particles in a glassy state could heterogeneously nucleate crystalline ice or crystalline nitric acid hydrate (NAT) (pathways c and d). We also speculate that glassy BBOA may take up H2O and HNO3 to form non-glassy PSCs due to the plasticizing effect of small molecules like H2O and HNO3 (pathway b). Researchers previously measured or estimated the viscosity and phase state of BBOA as a function of temperature and relative humidity (RH). They used these results to predict the behavior of BBOA in the troposphere (altitude ≲ 15 km). However, the temperature and RH in the stratosphere differ substantially from those in the troposphere, and these differences can strongly impact viscosity and phase state (as discussed below). Additionally, no studies have quantified the effect of H2SO4 uptake, or the uptake of any inorganic species, on BBOA viscosity and phase state. In a recent modelling study of stratospheric chemistry, researchers assumed that BBOA-H2SO4 particles in the stratosphere exist in a non-glassy state, allowing reactions to occur in the bulk of the particles. However, this assumption has not been verified with experiments or calculations. As a result, the viscosity and phase state of BBOA under stratospheric conditions remain highly uncertain. Several previous studies have quantified the viscosity and phase state of secondary organic aerosol (SOA) in the troposphere. These studies show that SOA, when free of inorganic species, can exist in a glassy state in the upper troposphere. However, these studies did not consider the temperatures and RH in the stratosphere, and most did not consider the uptake of inorganic species, such as H2SO4. In the following, we use laboratory data and a thermodynamic model to predict the viscosity and phase state of BBOA in the stratosphere from pyroCb events. We focus on altitudes from 15 -32 km in the stratosphere, since wildfire plumes from pyroCb events have been observed at these altitudes. We first estimate the viscosity and phase state for unaged BBOA at these heights. Next, we estimate the viscosity and phase state of aged BBOA particles mixed with H2SO4 (aged BBOA-H2SO4 particles) at these heights. The implications of the results for stratospheric chemistry and PSCs are discussed, as well as observational constraints of the viscosity and phase state of BBOA in the stratosphere. We also provide an outlook for future research on the viscosity and phase state of BBOA in the stratosphere.