Phase state and diffusion within atmospheric particles

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Biological sources (for example vegetation) and anthropogenic sources (for example transportation) emit copious quantities of volatile organic compounds. In the atmosphere, these volatile organic compounds are oxidized by a complex series of reactions to form semivolatile organic compounds, followed by condensation of the low volatility products and formation of microscopic particles. We refer to these microscopic particles as secondary organic material (SOM) particles. The term “secondary” indicates the material is formed in the atmosphere rather than emitted directly into the atmosphere. SOM particles can affect Earth's climate by scattering and absorbing solar radiation and by acting as seeds (i.e. nuclei) for clouds. SOM particles can also negatively affect air quality and human health. Currently the scientific understanding of SOM particles is low, which limits our ability to predict their impact on climate, air quality and health.******Recent studies have shown that predictions of the mass, size, reactivity, optical properties and cloud nucleating properties of SOM particles are sensitive to the following properties: 1) molecular diffusion of organics within SOM particles, 2) glass formation within SOM particles and 3) liquid-liquid phase separation within SOM particles. This proposal focuses on these three key properties. We will directly measure molecular diffusion rates of organics within SOM particles. The results will be used to quantify the accuracy of the Stokes-Einstein equation, which is often used to estimate diffusion rates of organics in SOM particles. In addition, we will carry out the first direct measurements of diffusion rates of organics within SOM at temperatures less than 293 K, a temperature range relevant to a large part of the atmosphere. We will also determine conditions at which individual SOM particles in the atmosphere become a glass (the same physical state as a glass window pane). Finally, we will determine conditions at which SOM particles undergo liquid-liquid phase separation to form two liquids (a water-rich liquid and an organic-rich liquid) within individual particles. We expect these results will be used by others when predicting the mass, size, reactivity, optical properties and cloud nucleating properties of SOM particles in the atmosphere. In short, these results will be important to many future modelling studies of atmospheric chemistry and climate.