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

When simulating the formation and life cycle of secondary organic aerosol (SOA) with chemical transport models, it is often assumed that organic molecules are well mixed within SOA particles on the timescale of 1 h. While this assumption has been debated vigorously in the literature, the issue remains unresolved in part due to a lack of information on the mixing times within SOA particles as a function of both temperature and relative humidity. Using laboratory data, meteorological fields, and a chemical transport model, we estimated how often mixing times are α -pinene SOA using room-temperature and low-temperature viscosity data for α -pinene SOA generated in the laboratory using mass concentrations of  ∼ 1000 µg m −3 . Based on this parameterization, the mixing times within α -pinene SOA are  0.5 µg m −3 at the surface). Next, as a starting point to quantify how often mixing times of organic molecules are α -pinene SOA generated using low, atmospherically relevant mass concentrations, we developed a temperature-independent parameterization for viscosity using the room-temperature viscosity data for α -pinene SOA generated in the laboratory using a mass concentration of  ∼ 70 µg m −3 . Based on this temperature-independent parameterization, mixing times within α -pinene SOA are α -pinene SOA generated using low, atmospherically relevant mass concentrations. Finally, a parameterization for viscosity of anthropogenic SOA as a function of temperature and RH was developed using sucrose–water data. Based on this parameterization, and assuming sucrose is a good proxy for anthropogenic SOA, 70 and 83 % of the mixing times within anthropogenic SOA in the PBL are  < 1 h for January and July, respectively, when concentrations are significant. These percentages are likely lower limits due to the assumptions used to calculate mixing times.