Vibrational Sum Frequency Spectroscopic Investigations of Sulfur Dioxide Adsorption to Atmospherically Relevant Aqueous Surfaces

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Title: Vibrational Sum Frequency Spectroscopic Investigations of Sulfur Dioxide Adsorption to Atmospherically Relevant Aqueous Surfaces
Author: Ota, Stephanie Tomoko, 1978-
Abstract: Aqueous aerosol surfaces are an important platform for chemical reactions through which gases are transported in the atmosphere. The chemical complexity of aqueous aerosols is well-established, but many questions remain about the molecular nature of their surfaces, particularly with respect to the uptake of gases. The pollutant sulfur dioxide, SO<sub>2</sub>, has been implicated in environmental phenomena such as acid rain, climate change, and cloud formation. SO<sub>2</sub> is fundamentally interesting because it forms spectroscopically identifiable complexes with water at aqueous surfaces. This dissertation aims to understand how temperature and aqueous composition impact the formation of surface complexes between water and SO<sub>2</sub>. Vibrational sum frequency spectroscopy (VSFS), a surface specific technique, is used to probe the vibrational modes of water and small organic molecules, investigating changes to the overall orientation, bonding environment, and structure of interfaces when aqueous surfaces are exposed to SO<sub>2</sub>. SO<sub>2</sub> adsorption to water at tropospherically relevant temperatures (0--23 °C) is examined first. The results show enhanced SO<sub>2</sub> surface affinity at colder temperatures, with most of the topmost water molecules showing evidence of binding to SO<sub>2</sub> at 0 °C compared to a much lower fraction at room temperature. Surface adsorption results in significant changes in water orientation at the surface but is reversible at the temperatures examined. The surface and vibrational specificity of these studies can be used to distinguish between the effects of surface adsorption compared to bulk accommodation. This distinction is utilized to demonstrate that SO<sub>2</sub> complexation is independent of solution acidity, confirming that bulk absorption is unnecessary for surface adsorption to occur. Finally, the impact of the organic species succinic acid and formaldehyde on the formation of surface SO<sub>2</sub> complexes is examined. These experiments indicate that SO<sub>2</sub> surface complexation occurs primarily with water but that surface active organic species may interact with gases under certain circumstances, namely when the organic species are more chemically reactive towards the gas. These studies have important implications for atmospheric chemistry and the uptake of gases, particularly in the complex aqueous environments expected in the troposphere.
Description: xv, 108 p. : ill. (chiefly col.)
Date: 2011-06

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