The oxidation of atmospheric organic particles is an important aging mechanism that affects the physical and chemical properties of aerosols. These aerosol particles contain complex organic compounds, such as fatty acids and chromophoric compounds that absorb light within the solar spectrum and can initiate photochemistry. Yet, the light-induced reactions of these organic particles are not well understood. In this study, we used a dual state-of-the-art gravimetric and vibrational spectroscopy system to probe the viability the photosensitized oxidation reaction of nonanoic acid, a proxy of fatty acids within organic aerosols, in the presence of complex organic photosensitizers. We irradiated thin films to simulate light-induced reactions between nonanoic acid and humic acids as terrestrial photosensitizers. Similarly, marine dissolved organic matter (m-DOM) from the NSF-SeaSCAPE 2019 mesocosm campaign was used to probe sea spray aerosols (SSA). Considerable differences were found in the photosensitizing capability of humic acids and m-CDOM, including the breakdown of the organic fractions into volatile organic compounds. To understand the mechanism involved in these photooxidation reactions, two molecular photosensitizers were also investigated: 4-benzoylbenzoic acid (4-BBA) and a nitrogen-containing photosensitizer, 4-imidazolecarboxaldehyde, to simulate the nitrogen rich m-DOM. Ex-situ analysis via liquid chromatography-mass spectrometry (LC-MS) shows primarily photooxidation of nonanoic acid induced by the photosensitizer. We have found that the rate of the photooxidation reaction is proportional to the concentration of photosensitizer and the partial pressure of oxygen, and independent on the concentration of fatty acid present in the organic thin film. From the photosensitizers examined, 4BBA produced the least fractionation. Overall, this study shows a molecular level mechanism of photooxidation of organic thin films and the relevance of such reactions in the marine environment.

Photooxidation of fatty acids within environmentally-relevant organic thin films: A study of model systems for marine and terrestrial photosensitizers

ACS Spring 2022

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.