3.3 Viscosity of aged BBOA-H2SO4 particles Figure also includes calculated viscosities for aged BBOA-H2SO4 particles with with H2SO4:BBOA mass ratios of 0.37, 0.5, and 0.79. Measurements from the ATom mission suggest that H2SO4:BBOA mass ratios of 0.37 and 0.79 are likely upper limit for pyroCb smoke in the lower stratosphere with aging times of 2 months and 9 months, respectively (see Methods). The H2SO4:BBOA mass ratio of 0.5 is provided as an intermediate value between the ratios of 0.37 and 0.79. To calculate the viscosities of aged BBOA-H2SO4 particles, we used the viscosities of unaged BBOA as proxies for aged BBOA. Consequently, the calculated viscosities should be considered as lower limits to the viscosities of aged BBOA-H2SO4 particles for several reasons that are outlined in the Methods section. For H2SO4:BBOA = 0.37 (Fig. ) at temperatures between 200 -250 K, the viscosity of the aged BBOA-H2SO4 particles is approximately 3 orders of magnitude lower, or more, compared to the unaged case (compare Fig. and). This decrease is attributed to the low viscosity of the H2SO4 and its associated water compared to BBOA at the same RH and temperature. Despite the reduction in viscosity, for the temperature range of 200 -250 K, all the BBOA types have viscosities of approximately 10 4 Pa s or greater. In addition, 6 out of the 10 systems still exhibited viscosities greater than 10 12 Pa s for the full temperature range of 200 -250 K. For the remaining 4 systems, the particles still reached 10 12 Pa s, although over a narrower temperature range. Overall, these results suggest that aged BBOA with a H2SO4:BBOA mass ratio of 0.37 will have a viscosity of approximately 10 4 Pa or greater and will often be in a glassy state for the temperature range of 200 -250K. For H2SO4:BBOA = 0.37 (Fig. ) at temperatures ≲ 200 K, all of the aged BBOA-H2SO4 types transition from a glassy solid state to a semisolid or liquid state due to the increased RH in this low-temperature regime. At an altitude of 15 km, the transition from a glassy state to a semisolid/liquid state occurs at about 195 -200 K, while at an altitude of 32 km, the transition occurs at about 180 -185 K. For H2SO4:BBOA = 0.79 (Fig. ) at temperatures between 200 and 250 K, the viscosity decreases by 1 order of magnitude, or more, compared to H2SO4:BBOA = 0.37. Again, this reduction is attributed to the low viscosity of the H2SO4 and its associated RH compared to BBOA at the same RH and temperature. For the temperature range of 200 -250 K, all the BBOA types have viscosities of approximately 10 3 Pa s or greater. In addition, 3 out of the 10 systems exhibited viscosities greater than 10 12 Pa s for the full temperature range of 200 -250 K. For the remaining 7 systems, the particles still reach 10 12 Pa s, although over a more narrow temperature range Overall, these results suggest that aged BBOA with a H2SO4:BBOA mass ratio 0.79 will have viscosities of approximately 10 3 Pa s or greater for the temperature range of 200 to 250 K. In addition, the glass state will form in these aged BBOA for some stratospheric conditions.