Surface Chemistry of Organic Molecules in Atmospheric and Indoor Environments Investigated by Sum Frequency Generation Spectroscopy


This work examines important heterogeneous processes of organic molecules on surfaces, in the contexts of atmospheric and indoor environments. In large forest ecosystems, biogenic secondary organic aerosols (SOAs) constitute a dominant fraction of organic particulate matter in the atmosphere. The formation of SOAs starts from the emission of volatile organic compounds (VOCs), among which monoterpenes are a common class, and the adsorption of VOCs and their oxidation products to aerosol seeds or particles. The first part of the thesis presents the experimental measurements of the adsorption thermodynamics and reversibility of a suite of monoterpenes, compared to non-terpene hydrocarbons, for insights into the formation and evolution of biogenic SOAs. The high surface-to-volume ratio of indoor environments makes surface chemistry particularly important to the study of indoor air quality. However, surface-mediated transformations in indoor environments are not adequately understood. The second part of the thesis provides an overview of the research opportunities in indoor surface chemistry and then dives into a specific study on squalene, a major ozone-active constituent in human skin oil. We present a combined spectroscopic and atomistic modeling approach to elucidate the conformational and orientational preferences of squalene at the air/squalene interface and their implications for reactions with ozone. Beyond idealized model surfaces, we initiated a field campaign, Stay-at-HomeChem, that aims at collecting authentic home-derived samples under the stay-at-home orders during the COVID-19 pandemic to study the impact of increased human occupancy on indoor air chemistry. In the last section, we demonstrate the efforts on developing a sum frequency scattering setup that can potentially probe suspended gas-phase aerosols for the first time. This capability will expand the research objects from macroscopically flat surfaces to submicron aerosol surfaces, which would largely benefit both the atmospheric chemistry and indoor air chemistry communities.

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