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Viscosity of secondary organic material and related atmospheric implications Grayson, James W.

Abstract

Aerosols are ubiquitous throughout the Earth’s atmosphere, and secondary organic material (SOM), which is produced from the oxidation of volatile organic compounds, is estimated to constitute a significant fraction of atmospheric aerosol mass. Furthermore, particles containing SOM can cause negative health outcomes, and affect Earth’s climate, both directly by scattering solar radiation, and indirectly by acting as nuclei for cloud droplets. Despite the importance of particles containing SOM, their physical properties, such as viscosity, are poorly constrained. To address this knowledge deficit, a technique to measure the viscosity of small samples of material, similar to that produced in environmental simulation chambers, was developed and validated. This technique was then used to measure the viscosity of SOM produced via the ozonolysis of α-pinene in an environmental simulation chamber. The viscosity of this material was found to depend strongly on the relative humidity (RH) used when measuring viscosity and the concentration of SOM mass at which the SOM was produced. A difference between the viscosity of the water-soluble component of SOM and the total SOM (water-soluble and water-insoluble components) was also observed. The viscosity of saccharides and a tetraol were subsequently measured, with these compounds serving as proxies of highly oxidized components of SOM found in the atmosphere. For saccharides, viscosity was determined to increase by at least four orders of magnitude as molar mass doubled. In addition, the tetraol was determined to have a viscosity at least two orders of magnitude lower than that of SOM produced via the oxidation of isoprene, in which the tetraol has been identified. Finally, literature viscosity data for organic compounds was used to demonstrate that saturation vapour concentration, the mass based equivalent of saturation vapour pressure, is a useful parameter for predicting viscosity, and better than elemental oxygen-to-carbon ratio or molar mass, at least for organic compounds containing only one or two functional groups. The results presented in this dissertation increase our knowledge of the viscosity of SOM, and its dependence on RH, the SOM mass concentration at which the SOM is produced, number of hydroxyl functional groups in the organic molecule, and molar mass.

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Attribution-NonCommercial-NoDerivatives 4.0 International

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