Detailed investigations of molecular collisions at the gas-surface interface provide insight into the dynamics and mechanisms of important interfacial reactions. A thorough understanding of the fundamental interactions between a gas and surface is crucial to the study of heterogeneous chemistry of atmospheric organic aerosols. In addition to changing the chemical and physical properties of the particle, reactions with oxidizing gases may alter aerosol optical properties, with implications for the regional radiation budget and climate. Molecular beams of CO₂, NO₂ and O₃ were scattered from long-chain methyl (CH₃-), hydroxyl (OH-), vinyl (H₂C=CH-) and perfluorinated (CF₃(CF₂)₈-, or F-) ω-functionalized alkanethiol self-assembled monolayers (SAMs) on gold, to explore the reaction dynamics of atmospherically important triatomics on proxies for organic aerosols. Energy exchange and thermal accommodation during the gas-surface collision, the first step of most interfacial reactions, was probed by time-of-flight techniques. The final energy distribution of the scattered molecules was measured under specular scattering conditions (θi = θf = 30°). Overall, extent of energy transfer and accommodation was found to depend on the terminal functional group of the SAM, incident energy of the triatomics, and gas-surface intermolecular forces. Reaction dynamics studies of O3 scattering from H2C=CH-SAMs revealed that oxidation of the double bond depend significantly on O₃ translational energy. Our results indicate that the room-temperature reaction follows the Langmuir-Hinshelwood mechanism, requiring accommodation prior to reaction. The measurements also show that the dynamics transition to a direct reaction for higher translational energies. Possible environmental impacts of heterogeneous reactions were probed by evaluating the change in the optical properties of laboratory-generated benzo[a]pyrene (BaP)-coated aerosols, after exposure to NO₃ and NO₂, at 532 nm and 355 nm by three aerosol analysis techniques: cavity ring-down aerosol spectroscopy (CRD-AS) at 355 nm and 532 nm, photoacoustic spectroscopy (PAS) at 532 nm, and an aerosol mass spectrometer (AMS). Heterogeneous reactions may lead to the nitration of organic-coated aerosols, which may account for atmospheric absorbance over urban areas. Developing a detailed understanding of heterogeneous reactions on atmospheric organic aerosols will help researchers to predict the fate, lifetime, and environmental impact of atmospherically important triatomics and the particles with which they collide. / Ph. D.

Phase transitions and other intrinsic properties (shape, size, architecture) of molecularly structured aerosol particles are important for understanding their role in planetary atmospheres and for technical applications. By combining bath gas cooling with time resolved mid-infrared spectroscopy and modeling, information is obtained on dynamic processes and intrinsic properties of fluoroform and ethane aerosol particles. The distinct infrared spectral features of fluoroform aerosol particles make it a particularly suitable model system. Homogeneous crystallization rates of the sub-micron sized aerosol particles are determined (JV = 10⁸ - 10¹⁰ cm-³s-¹ or JS = 10³ – 10⁵ cm-²s-¹ at a temperature of T = 78 K), and the controversial question regarding volume versus surface nucleation in freezing aerosols is addressed. It is demonstrated that current state of the art measurements of droplet ensembles cannot distinguish between the two mechanisms due to inherent experimental uncertainties. The evolution of particle shape from spherical supercooled droplets to cube-like crystalline particles and eventually to elongated crystalline particles is recorded and analyzed in detail with the help of vibrational exciton model calculations. Phase behaviour of pure ethane aerosols and ethane aerosols formed in the presence of other ice nuclei under conditions mimicking Titan’s atmosphere provide evidence for the formation of supercooled liquid ethane aerosol droplets, which subsequently crystallize. The observed homogeneous freezing rates (JV = 10⁷ – 10⁹ cm-³s-¹) imply that supercooled ethane could play a similar role in ethane rich regions of Titan’s atmosphere as supercooled water does in the Earth’s atmosphere.

Phase transitions and other intrinsic properties (shape, size, architecture) of molecularly structured aerosol particles are important for understanding their role in planetary atmospheres and for technical applications. By combining bath gas cooling with time resolved mid-infrared spectroscopy and modeling, information is obtained on dynamic processes and intrinsic properties of fluoroform and ethane aerosol particles. The distinct infrared spectral features of fluoroform aerosol particles make it a particularly suitable model system. Homogeneous crystallization rates of the sub-micron sized aerosol particles are determined (JV = 10⁸ - 10¹⁰ cm-³s-¹ or JS = 10³ – 10⁵ cm-²s-¹ at a temperature of T = 78 K), and the controversial question regarding volume versus surface nucleation in freezing aerosols is addressed. It is demonstrated that current state of the art measurements of droplet ensembles cannot distinguish between the two mechanisms due to inherent experimental uncertainties. The evolution of particle shape from spherical supercooled droplets to cube-like crystalline particles and eventually to elongated crystalline particles is recorded and analyzed in detail with the help of vibrational exciton model calculations. Phase behaviour of pure ethane aerosols and ethane aerosols formed in the presence of other ice nuclei under conditions mimicking Titan’s atmosphere provide evidence for the formation of supercooled liquid ethane aerosol droplets, which subsequently crystallize. The observed homogeneous freezing rates (JV = 10⁷ – 10⁹ cm-³s-¹) imply that supercooled ethane could play a similar role in ethane rich regions of Titan’s atmosphere as supercooled water does in the Earth’s atmosphere.

Phase transitions and other intrinsic properties (shape, size, architecture) of molecularly structured aerosol particles are important for understanding their role in planetary atmospheres and for technical applications. By combining bath gas cooling with time resolved mid-infrared spectroscopy and modeling, information is obtained on dynamic processes and intrinsic properties of fluoroform and ethane aerosol particles. The distinct infrared spectral features of fluoroform aerosol particles make it a particularly suitable model system. Homogeneous crystallization rates of the sub-micron sized aerosol particles are determined (JV = 10⁸ - 10¹⁰ cm-³s-¹ or JS = 10³ – 10⁵ cm-²s-¹ at a temperature of T = 78 K), and the controversial question regarding volume versus surface nucleation in freezing aerosols is addressed. It is demonstrated that current state of the art measurements of droplet ensembles cannot distinguish between the two mechanisms due to inherent experimental uncertainties. The evolution of particle shape from spherical supercooled droplets to cube-like crystalline particles and eventually to elongated crystalline particles is recorded and analyzed in detail with the help of vibrational exciton model calculations. Phase behaviour of pure ethane aerosols and ethane aerosols formed in the presence of other ice nuclei under conditions mimicking Titan’s atmosphere provide evidence for the formation of supercooled liquid ethane aerosol droplets, which subsequently crystallize. The observed homogeneous freezing rates (JV = 10⁷ – 10⁹ cm-³s-¹) imply that supercooled ethane could play a similar role in ethane rich regions of Titan’s atmosphere as supercooled water does in the Earth’s atmosphere. / Science, Faculty of / Chemistry, Department of / Graduate

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