Diffusion Coefficients and Mixing Times of Organic Molecules in β-Caryophyllene Secondary Organic Aerosol (SOA) and Biomass Burning Organic Aerosol (BBOA)

Information on the diffusion rates of organic molecules within secondary organic aerosol (SOA) and biomass burning organic aerosol (BBOA) is needed to predict the impact of these aerosols on atmospheric chemistry, air quality, and climate. Nevertheless, no studies have measured diffusion rates of organics within SOA generated from β-caryophyllene or within BBOA. Here, we measured diffusion rates of organic molecules in laboratory-generated SOA and BBOA as a function of water activity (a_w) using fluorescence recovery after photobleaching. The SOA was generated by the ozonolysis of β-caryophyllene, and the BBOA was generated by the pyrolysis of pine wood. Only the water-soluble component of the BBOA was studied. The measured diffusion coefficients of organic molecules in β-caryophyllene range from 1.1 × 10^–16 to 1.3 × 10^–14 m² s^–1 for a_w values ranging from 0.23 to 0.86. For BBOA, the diffusion coefficients range from 7.3 × 10^–17 to 6.6 × 10^–16 m² s^–1 for aw values ranging from 0.23 to 0.43. Based on these values, the mixing times of organic molecules within a 200 nm SOA or BBOA are less than 1 min for a_w values >0.23. Since a_w values are often greater than 0.23 in the planetary boundary layer and temperatures in the planetary boundary are often within 5 K of our experimental temperatures, mixing times are likely often short in that part of the atmosphere for the types of aerosols studied here. For β-caryophyllene SOA, we compared the measured diffusion coefficients with predictions based on the Stokes–Einstein relation and the fractional Stokes–Einstein relation. For both the Stokes–Einstein and the fractional Stokes–Einstein relations, the measured diffusion coefficients agree with the predicted diffusion coefficients. This work illustrates that when the radius of the diffusing molecules is greater than the average radius of the matrix molecules, the Stokes–Einstein equation is able to predict diffusion coefficients in β-caryophyllene SOA with reasonable accuracy.