where dp is the BB-SOA particle diameter and Dorg(RH, T) is the diffusion coefficient of organics as a function of RH and temperature. To calculate Dorg(RH, T), the values of 𝜂(RH, 𝑇) were substituted into the Stokes-Einstein equation, which is valid when the size of the diffusing molecule is about the same or larger than the size of the matrix molecules that it diffuses through. We estimated the hydrodynamic radius of the molecules in our catechol + OH SOA by assuming a spherical geometry and using the average molecular weight and density from Nakao et al. (2011) (139 g mol -1 and 1.4 g L -1 , respectively). For dp, we used the approximate aerosol diameter of an accumulation mode particle in the troposphere, 200 nm. The mixing times of BB-SOA calculated with Eq. 1 can exceed 1 h at altitudes > 3-7 km (Fig. ). Fast mixing of approximately 0.5-1 h, i.e., at time scales smaller than the typical modeling time step, is often assumed in chemical transport models. Our results challenge this assumption and show significantly longer times are expected for phenolic BB-SOA in many regions of the troposphere. This has important ramifications for the transport of pollutants, such as polycyclic aromatic hydrocarbons (PAHs), which are emitted from biomass burning. PAHs can partition into the condensed phase during the formation of phenolic BB-SOA, allowing them to become trapped inside SOA particles (Fig. ). Once trapped inside, the long mixing times calculated here suggest that these PAHs may be protected from oxidation by tropospheric oxidants (i.e. OH, O3, and NO3). As a result, unoxidized PAHs may be transported to remote regions in the atmosphere and impact remote ecosystems such as the Arctic.