Temperature and RH span wide ranges in the troposphere. To extrapolate our room-temperature viscosity results to other temperatures and RH values, we developed a parameterization to predict BB-SOA viscosity as a function of temperature and RH based on a previous approach (details are provided in Section S7). The resulting viscosity parameterization for catechol + OH SOA as a function of RH and temperature is shown in Fig. . Using the data from catechol + O3 or syringol