Vibrational sum-frequency spectroscopic investigations of small tropospheric aerosol molecules at vapor/water interfaces

The molecular level nature of surfaces plays a much larger role in chemical and physical processes than that anticipated even 20 years ago. Chemical processes in the atmosphere are particularly dependent on the surface properties of aerosols. The research presented in this dissertation is an investigation of environmentally significant molecules found in the troposphere including prominent compounds in the sulfur cycle, acids, bases, and various salts at vapor/water interfaces. The adsorption and reaction of these tropospheric molecules and their effects on the surface properties of aqueous aerosols are probed using vibrational sum-frequency spectroscopy (VSFS) and surface tension measurements. VSFS selectively probes vibrational modes of molecules in the interfacial region, and can be used to elucidate the orientation, bonding environment, and interfacial structure of these molecules.; First, investigations of individual organic compounds in the sulfur cycle (DMS, DMSO, DMSO2, and DMSO3) at the vapor/water interface are probed. These studies reveal significantly different adsorbate-adsorbate and water-adsorbate interactions depending on the degree of oxidation.; Secondly, investigations of SO2 and CO2 gas and their reactive uptake products at the vapor/water interface show that these gases can react in the interfacial region. A SO2:H2O surface complex is observed and remains even in the presence of surface organics. Since the uptake of both gases lead to acidic aerosols, the effects of aerosol pH on the vapor/water interfacial region was examined; additional insights were obtained from isotopic dilution studies that simplify spectral interpretation by reducing the number of vibrational modes contributing to the spectra and constraining the fitting parameters.; Finally, the effects of "structure-making" salts (Na2SO 4, NaHSO4, Na2CO3, NaHCO3, Na2SO3, NaHSO3, NaF) and "structure-breaking" salts (NaCl, NaBr, and NaI) on the vapor/water interface were examined to understand the interfacial structure of aerosols containing more than one chemical species. Additionally, DMS and DMSO3 are examined in the presence of several of the salts to represent a more realistic model aerosol. These results show that the structure of the surface is specific to the particular organic compound and salt.