|Title||Humidity-Dependent Viscosity of Secondary Organic Aerosol from Ozonolysis of β-Caryophyllene: Measurements, Predictions, and Implications|
|Publication Type||Journal Article|
|Year of Publication||2021|
|Authors||Maclean, AM, Smith, NR, Li, Y, Huang, Y, Hettiyadura, APS, Crescenzo, GV, Shiraiwa, M, Laskin, A, Nizkorodov, SA, Bertram, AK|
|Journal||ACS EARTH AND SPACE CHEMISTRY|
|Date Published||JAN 2021|
To predict important secondary organic aerosol (SOA) properties, information on viscosity or diffusion rates within SOA is needed. Ozonolysis of β-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 β-caryophyllene using the poke-flow technique. At an RH of 0 and 48%, the viscosity was between 6.9 × 105 and 2.4 × 107 Pa s, and between 1.3 × 103 and 5.6 × 104 Pa s, respectively. Based on these viscosities and the fractional Stokes–Einstein equation, characteristic mixing timescales of organics within 200 nm β-caryophyllene SOA particles range from ∼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 β-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.