Humidity-Dependent Viscosity of Secondary Organic Aerosol from Ozonolysis of β-Caryophyllene: Measurements, Predictions, and Implications

To predict important secondary organic aerosol (SOA) properties, information on viscosity or diffusion rates within SOA is needed. Ozonolysis of beta-caryophyllene is an important SOA source; however, very few viscosity or diffusion rate measurements have been performed for this SOA type and none as a function of relative humidity (RH). In this study, we measured viscosity as a function of RH for SOA generated from the ozonolysis of beta-caryophyllene using the poke-flow technique. At an RH of 0 and 48%, the viscosity was between 6.9 x 10(5) and 2.4 x 10(7) Pa s, and between 1.3 x 10(3) and 5.6 x 10(4) Pa s, respectively. Based on these viscosities and the fractional Stokes-Einstein equation, characteristic mixing timescales of organics within 200 nm beta-caryophyllene SOA particles range from similar to 0.2 h at 0% RH to <3 s at 48% RH, suggesting that these particles should be well-mixed under most conditions in the lower atmosphere. The chemical composition of the SOA was also determined using nano-desorption electrospray ionization mass spectrometry. The measured chemical composition and the method of DeRieux et al. (ACP, 2018) were used to predict the viscosity of beta-caryophyllene SOA. If the mass spectra peak abundances were adjusted to account for the sensitivity of the electrospray ionization to larger molecular weight components, the predicted viscosity overlapped with the measured viscosity at 0% RH, while the predicted viscosities at 15-48% RH were slightly higher than the measured viscosities. The measured viscosities also overlapped with viscosity predictions based on a simple mole-fraction based Arrhenius mixing rule.

Automated summary

This study measured the viscosity of SOA generated from the ozonolysis of beta-caryophyllene as a function of relative humidity, showing significant differences in mixing timescales at different RH. Chemical composition of the SOA was determined using nano-desorption electrospray ionization mass spectrometry, and predicted viscosities overlapped with measured viscosities. The findings suggest that beta-caryophyllene SOA particles should be well-mixed under most conditions in the lower atmosphere.