The viscosity was calculated from Tg,BBOA and κ using the same approach used by Li et al. to calculate viscosity of SOA in the troposphere. First, we calculated the water content of the BBOA at a given RH using κ and the following equations: 26 , (5) and , (6) where wBBOA and wH2O represent the mass fraction of BBOA and H2O in a BBOA-water mixture, respectively. In Equation , represents the density of water, and represents the density of BBOA. For , we used 1 g/cm , and for , we used 1.3 g/cm , consistent with previous assumptions and measurements. Subsequently, the glass transition temperature of the BBOA and water mixture (Tg,mix) was calculated using the Gordon-Taylor equation and the glass transition temperature of dry BBOA (Tg,BBOA): 18 (7) where represents the Gordon-Taylor constant, suggested to be 2.5, and Tg,w is the glass transition temperature of water, set to 136 K. Viscosity at a given temperature (T) was then calculated using the Vogel-Fulcher-Tammann (VFT) equation: is the viscosity at infinite temperature (set to 10 -5 Pa s), D is the fragility parameter, assumed to be 10 as done previously, and T0 is the Vogel temperature calculated using the following equation: