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 < 1 h within SOA in the planetary boundary layer (PBL), the region of the atmosphere where SOA concentrations are on average the highest. First, a parameterization for viscosity as a function of temperature and RH was developed for alpha-pinene SOA using room-temperature and low-temperature viscosity data for alpha-pinene SOA generated in the laboratory using mass concentrations of similar to 1000 mu g m(-3). Based on this parameterization, the mixing times within alpha-pinene SOA are < 1 h for 98.5% and 99.9% of the occurrences in the PBL during January and July, respectively, when concentrations are significant (total organic aerosol concentrations are > 0 : 5 mu g m(-3) at the surface). Next, as a starting point to quantify how often mixing times of organic molecules are < 1 h within alpha-pinene SOA generated using low, atmospherically relevant mass concentrations, we developed a temperature-independent parameterization for viscosity using the room-temperature viscosity data for alpha-pinene SOA generated in the laboratory using a mass concentration of similar to 70 mu gm(-3). Based on this temperature-independent parameterization, mixing times within alpha-pinene SOA are < 1 h for 27 and 19.5% of the occurrences in the PBL during January and July, respectively, when concentrations are significant. However, associated with these conclusions are several caveats, and due to these caveats we are unable to make strong conclusions about how often mixing times of organic molecules are < 1 h within alpha-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.