Recent studies have suggested that reactions occur on or in BBOA in the stratosphere, leading to unexpected depletion of the UV-blocking stratospheric ozone layer. In modelling studies, researchers have assumed that the BBOA-H2SO4 particles in the stratosphere are in a non-glassy state and that reactions occur in the bulk of the BBOA-H2SO4 particles. These assumptions have generally aligned with model-measurement comparisons. Our results suggest that wildfire smoke particles will have viscosities ≳ 10 3 Pa s, when the H2SO4:BBOA mass ratio is ≲ 0.79 and the temperature is 200 -250 K. These high viscosities can influence the characteristic reaction diffusive length scale and reactive uptake rates. For instance, reactive uptake rates often scale with the square root of 1/viscosity, meaning a two-order-of-magnitude increase in viscosity can reduce the reactive uptake rate by one order of magnitude. Consequently, these elevated viscosities may need to be considered to accurately describe the stratospheric chemistry of wildfire smoke particles. Our results also suggest that wildfire smoke particles can be in a glassy state (viscosity ≥10 Pa s) under certain temperatures and RHs in the stratosphere when the H2SO4:BBOA mass ratio is ≲ 0.79. In such a state, bulk reactions will be inhibited, and multiphase chemistry may be restricted to the surface of the particles. Therefore, surface reactions may need to be considered for some conditions in the stratosphere. As mentioned above, BBOA may influence the formation mechanisms of PSCs via several pathways (Fig. ). In the current study, we address the possibility of pathway c in Figure . In Figure , we show the RH and temperature conditions where vapor becomes supersaturated with respect to ice, indicated by vertical black lines. The solid black line corresponds to conditions at an altitude of 15 km, while the dashed black line corresponds to an altitude of 32 km. If viscosity is ≥ 10 12 Pa s to the left of the vertical black lines (i.e., colder temperatures) at the respective altitudes, BBOA can exist in a glassy state while the vapor is supersaturated with respect to ice. This glassy state may provide a surface for heterogeneous ice nucleation, leading to the formation of ice-containing PSCs (pathway c, Fig. ). For H2SO4:BBOA ratios of 0, 0.37, 0.5, and 0.79 at altitudes of 15 km and 32 km (Fig. ), the viscosity is ≥ 10 12 Pa s to the left of the corresponding vertical black lines. This indicates that under certain stratospheric conditions, the particles are in a glassy state, the vapor is supersaturated with respect to ice, and the particles can act as potential surfaces for heterogeneous ice nucleation, possibly leading to the formation of ice-containing PSCs (pathway c, Fig. ).