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UBC Theses and Dissertations

The temperature and relative humidity dependence of molecular diffusion and viscosity in organic aerosols and films relevant for kitchens, cities, forests, and fires Kiland, Kristian J.


Predictions of atmospheric chemistry, air quality, and climate depend on molecular diffusion and viscosity within organic aerosol (OA) particles. These predictions have uncertainties due, in part, to a lack of understanding of how viscosity and diffusion depend on ambient parameters such as temperature and relative humidity (RH). This thesis aims to improve the understanding of the temperature and RH-dependency of molecular diffusion of OA through direct experimental observation of diffusivity and viscosity for different types of OA. The viscosity of cooking organic aerosol (COA) films collected in a real kitchen environment was measured with the poke-flow technique. COA’s viscosity was found to be < 7 × 10³ Pa s for RH values from 0 to 69%. This result was many orders of magnitude lower than predicted with a commonly-used parameterization. The parameterization agreed with the measurements after adjusting with a triglycerides-based correction factor. Secondary organic aerosol (SOA) material from biomass burning was generated from the oxidation of phenolic compounds: catechol, guaiacol, and syringol. The SOA viscosity was measured using the poke-flow technique. The viscosity of all systems followed roughly the same trend, with viscosity < 3 × 10³ Pa s at RH > 40% and > 2 × 10⁸ Pa s under dry conditions. A parameterization was developed to extrapolate to relevant tropospheric conditions. Few techniques can measure the temperature dependence of viscosity in OA. A new method based on hot-stage microscopy was developed, validated, and applied to SOA material derived from farnesene photooxidation. The farnesene SOA viscosity ranged from 2.6 × 10⁴ to 3.1 × 10⁶ Pa s when changing the temperature from 67 to 51 °C. The RH and temperature-dependent diffusion of organic molecules were directly measured in sucrose-water, a proxy for SOA, using rectangular fluorescence recovery after photobleaching. Diffusion rates were measured at temperatures ranging from –10 to 40 °C and for RH from 43 to 85%. The corresponding mixing times of organics were often larger than 1 h for conditions found in the free troposphere, meaning that the frequently-made assumption of fast mixing (< 1 h) in chemical transport models may be invalid for these conditions.

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