is the unit viscosity (1 Pa s); and f1 and f2 are the mass fractions of the two subsystems at the given RH and temperature. Studies by Song et al. and Klein et al. show that the ZSR approach predicts viscosities within 1 -2 orders of magnitude for mixtures of organic and inorganic material. To predict the viscosity of the aged BBOA-H2SO4 particles using Equation 10, the H2SO4-to-BBOA (H2SO4:BBOA) mass ratio is needed.An upper limit to the H2SO4:BBOA mass ratio for pyroCb smoke in the stratosphere can be estimated based on limited measurements. In 2017, the NASA Atmospheric Tomography (ATom) mission sampled pryoCb smoke in the northern hemisphere and lowest part of the stratosphere from the large-scale wildfires in the Pacific Northwest of North America. Single particle measurements from this mission indicate that the average H2SO4:BBOA mass ratio was ~0.37 and ~0.79 for 2 months and 9 months, respectively, after the initial pryoCb injection into the stratosphere (Section S1 and Fig. ). The H2SO4:BBOA mass ratio was even smaller for the largest particles (≳ 0.5 µm) (Fig. ). Based on the size distributions of the typical smoke in the stratosphere and smoke from pryoCb events, the largest particles were likely mostly from pyroCb events, whereas the smaller particles were likely aged smoke that is widespread in the upper troposphere and lower stratosphere. Based on these limited measurements, we suggest that H2SO4:BBOA mass ratios of 0.37 and 0.79 are likely an upper limit for pyroCb smoke in the lower stratosphere with an aging time of 2 months and 9 months, respectively. To be consistent with the ATom measurements, in the following, we consider H2SO4:BBOA mass ratios of 0.37, 0.5, and 0.79.