where D Org (RH, T) is the RH-and temperature-dependent diffusion coefficient of the organic molecules within SOA, k is the Boltzmann constant, η(RH, T) is the RH-and temperaturedependent viscosity of the SOA, and R diff is the radius of the diffusing molecule. For R diff , we assumed a value of 0.4 nm, as done previously, which is generally consistent with the expected sizes of SOA molecules. The Stokes-Einstein equation gives values consistent with experimentally measured diffusion coefficients of organic molecules when the radius of the diffusing molecules is equal to or larger than the radius of the molecules composing the organic matrix. The calculated annual average mixing time of organics within guaiacol-NO 3 and guaiacol-OH SOA particles are shown as a function of latitude and altitude in Fig. . For guaiacol-NO 3 SOA (Fig. ), mixing times are > 1 h for the vast majority of the troposphere. Mixing times are only < 1 h for some regions of the planetary boundary layer (-70° to -50°, -20° to 20°, and 60° to 90°). In contrast, the mixing times within guaiacol-OH SOA (Fig. ) in the planetary boundary layer are always < 1 h. The short mixing times for guaiacol-OH SOA within the planetary boundary layer are due to the relatively warm temperatures and high RH values in this region of the atmosphere and because the viscosity of guaiacol-OH SOA decreases significantly as RH increases.