To calculate the viscosity of aged BBOA-H2SO4 particles at a given RH and temperature, we first determined the viscosity of aged BBOA particles and H2SO4 particles separately at the given RH and temperature, as outlined below. To determine the viscosity of aged BBOA particles at a given RH and temperature, we followed the same method described in Section 2.3. The viscosities determined in Section 2.3 correspond to unaged BBOA. We assume that these viscosities represent lower limits for aged BBOA due to the following factors: a) aging through OH and O3 reactions has been shown to increase BBOA viscosity at room temperature and on the timescale of days, b) UV exposure at room temperature also increases BBOA viscosity on the timescale of days, and c) dilution also increases BBOA viscosity. Nevertheless, additional measurements are needed to confirm this assumption for stratospheric temperatures, relative humidities, and residence times. We hope our work will motivate further studies on this topic. In addition, it is important to keep in mind that unaged BBOA in the stratosphere will often be in a glassy state (see below), and hence aging reactions with OH and O3 should be limited for initial conditions in the stratosphere. To determine the viscosity of H2SO4 particles at a given RH and temperature, we used the AIOMFAC-VISC thermodynamic model. This model applies a semi-empirical approach based on the Eyring's absolute rate theory to describe the viscosity of aqueous electrolyte solutions. The model reproduces viscosity data often within 10% for a wide range of aqueous electrolyte systems. After determining the viscosity of the aged BBOA particles and H2SO4 particles separately at a given RH and temperature, we used the Zdanovskii-Stokes-Robinson (ZSR) approach to calculate the viscosity of aged BBOA-H2SO4 particles at the given RH and temperature. The ZSR approach is expressed mathematically as the following: , (10) Where , , and represents the viscosity of the BBOA-H2SO4 particles, BBOA particles, and H2SO4 particles, respectively, all at the same RH and temperature.