Date of Award

Spring 2024

Document Type

Restricted Thesis

Degree Name

Bachelor of Arts (BA)

Department

Chemistry

First Advisor

Dr. Juan Navea

Abstract

Clouds and cloud formation are essential for understanding Earth's radiation budget and climate change, with aerosols playing a vital role in the process. Aerosol particles are required for atmospheric water to condense, leading to cloud formation. In the marine atmosphere, this process is driven by sea spray aerosols (SSAs), a type of primary aerosol formed by wave breaking or bubble bursting, resulting in particles that carry the composition of the sea surface microlayer (SSML). Thus, chromophoric compounds and fatty acids found in the marine boundary layer (MBL) are known to partition into SSA, along with ionic and other compounds. The chemistry and physics of SSA and SSML, in particular those related to cloud formation, is driven by the chemical composition of these compounds and their environmental ageing. For instance, large concentrations of fatty acids found in partitioning from SSML to SSA suggest that these particles should behave as hydrophobic compounds, as the SSA dries out and its components concentrate during atmospheric transport. Yet, recent work suggests that fatty-acid-rich SSA is a highly effective cloud formation nuclei. In this thesis, we address this apparent contradiction by examining the photooxidation mechanisms of fatty acids in marine environment. The following chapter develops key concepts in atmospheric chemistry and physics, as well as the chemical environment characteristic of the complex systems that involve SSA and SSML.

Despite the important implications the aging of organic compounds in SSA has, this reaction is poorly understood. Recent studies suggest that organic chromophores present in SSA can induce photosensitized oxidation of fatty acids. However, due to the complexity of these organic chromophores, chemical mimics are necessary to understand this reaction at a molecular level. To better understand the effect of environmental photosensitizers on daytime oxidation of organic compounds, we present here laboratory studies on the photosensitized oxidation of nonanoic acid, a proxy of fatty acids found in SSA, by four different photosensitizers: environmental photosensitizers marine chromophoric dissolved organic matter (mCDOM) and its terrestrial counterpart, humic acid (HA), and 4-benzoylbenzoic acid (4BBA) and 4-imidazolecarboxaldehyde (4-IC) as molecular proxies. A tandem gravimetric and vibrational spectroscopy was used to perform in-situ analysis and liquid chromatography mass spectrometry (LCMS) was used to perform ex-situ analysis. The effect of relevant environmental conditions such as pH was investigated, as well as the speciation of model systems at different pHs, as a range of pH variations exists in the marine environment and pH has been known to affect the photoactivity of organic chromophores. We find that all four photosensitizers initiated the photooxidation of nonanoic acid, although the rate and yield of product formation differed depending on the photosensitizer. The rate of oxidation is dependent on the concentration of photosensitizer and oxygen, with little to no mass change occurring in the absence of oxygen. Photooxidation initiated by deprotonated 4BBA is faster than neutral or protonated 4BBA. We also find that both the aldehyde and 1,1-diol forms of 4-IC are present at low pH and photooxidation initiated by 4-IC is faster at around pH 6, providing insight into how pH affects the efficacy of photosensitizers. Overall, this work shows that organic photosensitizers can oxidize fatty acids to form hydrophilic products and pH has a significant effect on this process.

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