Title: Surfactant Behavior in Atmospheric Aerosols
Description: Atmospheric aerosols are very important in the Earth climate system due to their role in cloud formation and the global radiation budget. However, there are still many unanswered questions about how the composition of the aerosol varies and how this composition affects the climate system. While aerosols contain a mix of organic and inorganic material, a sub-fraction of the organic material in atmospheric aerosols is surface active, arranging itself into organic films at the gas-aerosol interface. These films can inhibit trace gas uptake, affecting atmospheric chemistry and composition, and they can also impact water uptake, influencing cloud formation properties. Additionally, these films can depress surface tension of atmospheric aerosols, leading to enhanced cloud nuclei. Organic film behavior strongly depends on aerosol pH as well as ionic content, and given the complexity of atmospheric chemistry, hundreds of possible surfactants could exist at a given time in atmospheric aerosols.

Therefore, it is imperative to study and understand the formation of organic films and their behavior at atmospherically relevant conditions. In this work, we focus on three main questions about surfactant systems: 1. Do organic films form at all atmospherically relevant conditions? 2. How can complex reactive systems be modeled in terms of surface tension and light absorbing reaction products? and 3. What are the different effects that oxidation of organic films can have on cloud condensation nuclei activity? We studied systems of long chain fatty acids and α-dicarbonyls in aqueous aerosol mimics by using pendant drop tensiometry to measure surface tension, UV-VIS to measure the formation of light-absorbing products, Aerosol chemical ionization mass spectrometry (Aerosol-CIMS) to characterize the reaction products, and a continuous flow streamwise thermal gradient cloud condensation nuclei counter (CFSTGC) to measure the CCN activity. We found that organic films of oleic acid and stearic acid formed at all atmospherically relevant conditions (high ionic content and pH 0-8), though the efficacy of the surface film at depressing surface tension changed as the ionization state of the organic changed.

Reactive systems of methylglyoxal and glyoxal showed the formation of some cross-reaction products that added to the total product mass formed; however, most of the products formed were from self-reaction. The formation of light absorbing products as well as the surface tension could be described solely by the effects of the isolated organics combined in parallel, rather than including any terms about cross-reaction species. The oxidation of mixed inorganic-organic aerosols with a sodium oleate film showed little change in CCN activity as compared to pure inorganic aerosols, but the same oxidation with an oleic acid film showed depressed CCN activity. This led to the idea that oxidative aging in the atmosphere might not always increase the hygroscopicity of aerosols. Overall, the results of this thesis demonstrate how variable aerosol properties are due to the organics present within complex aerosol compositions. This work will help direct future laboratory studies on atmospherically relevant systems in order to help elucidate an understanding of surfactant behavior in atmospheric aerosols.