The mixing time of water in the BB-SOA particles, τmix,H2O, was calculated following a method used previously. For all experiments reported here, the calculated τmix,H2O was shorter than the conditioning times (Table ). Consistent with these calculations, the measured viscosity was not sensitive to the conditioning time (Section S3 and Fig. ). Based on these criteria, we assume the water activity in the SOA was at near-equilibrium with the RH of the carrier gas during the viscosity measurement.