In the analysis above, we used a CH2O value of 15 ppmv, consistent with measurements in plumes that recently entered the stratosphere via pyroCb events. Over time, CH2O levels decrease due to dilution, eventually reaching background values of ~5 ppmv. To evaluate how a reduction in CH2O affects viscosity and phase state, we recalculated viscosities using a CH2O value of 5 ppmv (Figure ). The lower CH2O level increases viscosity. For example, for the hydrophobic phase studied by Gregson et al., H2SO4:BBOA =0.79, CH2O = 15 ppmv, and an altitude of 15 km, the highest observed viscosity was ∼10 12 Pa s. In contrast, under the same conditions but with CH2O = 5 ppmv, the highest observed viscosity was ∼10 Pa s. To calculate the viscosities of aged BBOA-H₂SO₄ particles, we used the ZSR method. Previous studies have demonstrated that the ZSR method can estimate the viscosity of mixed organic and inorganic particles within 1 -2 orders of magnitude. Figure presents the predicted viscosities of aged BBOA-H₂SO₄ particles (CH2O = 15 ppmv and H₂SO₄:BBOA ratio = 0.37, 0.5, and 0.79) with all of the predicted viscosities reduced by two orders of magnitude. Even after this reduction, the aged BBOA with a H2SO4:BBOA mass ratio of 0.37 are still predicted to be in a glassy state often for the 200 -250 K temperature range. In addition, the aged BBOA with a H2SO4:BBOA mass ratio of 0.79 will still form the glass state for some stratospheric conditions and BBOA types. 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. For individual organic molecules, D values typically range from 5 to 20. To assess the impact of this range, we recalculated viscosities using D values of 5 and 20, with a water vapour mixing ratios of 15 ppmv and H₂SO₄:BBOA mass ratios of 0, 0.37, 0.5, and 0.79 (Figure and). The largest reductions in viscosity were observed for a D value of 20. Nevertheless, even for D = 20, the aged BBOA with a H2SO4:BBOA mass ratio of 0.37 are still predicted to be in a glassy state often for the 200 -250 K temperature range. In addition, the aged BBOA with a H2SO4:BBOA mass ratio of 0.79 are still predicted to form the glass state for some stratospheric conditions and BBOA types.