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When simulating the formation and life cycle of secondary organicaerosol (SOA) with chemical transport models, it is often assumedthat organic molecules are well mixed within SOA particles on thetimescale of 1 h. While this assumption has been debatedvigorously in the literature, the issue remains unresolved in partdue to a lack of information on the mixing times within SOAparticles as a function of both temperature and relativehumidity. Using laboratory data, meteorological fields, anda chemical transport model, we estimated how often mixing timesare  < 1 h within SOA in the planetary boundary layer(PBL), the region of the atmosphere where SOA concentrations are onaverage the highest. First, a parameterization for viscosity asa function of temperature and RH was developed for α-pineneSOA using room-temperature and low-temperature viscosity data forα-pinene SOA generated in the laboratory using massconcentrations of  ∼ 1000 µg m−3. Based on thisparameterization, the mixing times within α-pinene SOAare  < 1 h for 98.5 % and 99.9 % of theoccurrences in the PBL during January and July, respectively, whenconcentrations are significant (total organic aerosol concentrationsare  > 0.5 µg m−3 at the surface). Next, asa starting point to quantify how often mixing times of organicmolecules are  < 1 h within α-pinene SOA generatedusing low, atmospherically relevant mass concentrations, wedeveloped a temperature-independent parameterization for viscosityusing the room-temperature viscosity data for α-pinene SOAgenerated in the laboratory using a mass concentration of  ∼ 70 µg m−3. Based on this temperature-independentparameterization, mixing times within α-pinene SOA are  < 1 h for 27 and 19.5 % of the occurrences in the PBLduring January and July, respectively, when concentrations aresignificant. However, associated with these conclusions are severalcaveats, and due to these caveats we are unable to make strongconclusions about how often mixing times of organic moleculesare  < 1 h within α-pinene SOA generated using low,atmospherically relevant mass concentrations. Finally,a parameterization for viscosity of anthropogenic SOA as a functionof temperature and RH was developed using sucrose–water data. Basedon this parameterization, and assuming sucrose is a good proxy foranthropogenic SOA, 70 and 83 % of the mixing times withinanthropogenic SOA in the PBL are  < 1 h for January andJuly, respectively, when concentrations are significant. Thesepercentages are likely lower limits due to the assumptions used tocalculate mixing times

Mixing times of organic molecules within secondary organic aerosol particles: a global planetary boundary layer perspective