We assumed that BBOA forms a single phase when mixed with H2SO4. Previous studies have shown that when secondary organic aerosol with an average O/C ratio ≳ 0.8 is mixed with ammonium sulfate aerosols, the resulting particles form a single phase. Studies are needed to confirm if a similar threshold applies for BBOA mixed with H2SO4. 3) To calculate the viscosity of BBOA particles from their glass transition temperatures, we assumed a fragility parameter (D) of 10, consistent with values used in previous studies. Measurements are needed to confirm that a fragility of 10 is appropriate for BBOA. 4) More measurements of the H2SO4 content in wildfire aerosols as a function of age in the stratosphere are needed. These measurements could be carried out by high-altitude aircraft or by using infrared spectra of wildfire smoke in the stratosphere from satellite measurements. 13 5) Observational constraints on the viscosity and phase state of BBOA in the stratosphere are essential. Lidar depolarization measurements of wildfire smoke in the stratosphere offer valuable insights into these properties, but additional measurements are needed. Lidar depolarization measurements indicate that wildfire smoke in the stratosphere is often nonspherical, but depolarization is an indirect measurement that does not uniquely determine phase and viscosity. 6) Measurements should also differentiate between ordinary smoke particles slowly lofted into the stratosphere and smoke rapidly injected into the stratosphere through large pyroCb events. PyroCb smoke is processed through a cloud during its ascent. This could result in a different mix of organic molecules than smoke that is not cloud-processed. 7) Our results suggest that aged BBOA and aged BBOA mixed with H2SO4 will be in a glassy state for some conditions in the stratosphere. Previous studies have shown that certain types of organic aerosols in a glassy state can nucleate ice heterogeneously. However, the ice nucleation efficiency of glassy organic aerosols is still an area of active research. Studies are needed to determine whether BBOA in a glassy state can nucleate crystalline PSCs. 8) We considered the uptake of water and H2SO4 by BBOA in the stratosphere. At the coldest temperatures, BBOA particles can also take up nitric acid (HNO3), similar to background H2SO4 particles in the stratosphere, which could further change the composition, and hence, the viscosity of the wildfire smoke particles. Smoke particles can also take up HCl and potentially other chlorine compounds. This uptake should be considered in future studies.