Photooxidation of Nonanoic Acid by Molecular and Complex Environmental Photosensitizers

Abstract

Photochemical aging and photooxidation of atmospheric particles play a crucial role in both the chemical and physical processes occurring in the troposphere. In particular, the presence of organic chromophores within atmospheric aerosols can trigger photosensitized oxidation that drives the atmospheric processes in these interfaces. However, the light-induced oxidation of the surface of atmospheric aerosols, especially those enriched with organic components, remains poorly understood. Herein, we present a gravimetric and vibrational spectroscopy study aimed to investigate the photosensitized oxidation of nonanoic acid (NA), a model system of fatty acids within organic aerosols, in the presence of complex organic photosensitizers and molecular proxies. Specifically, this study shows a comparative analysis of the photosensitized reactions of thin films containing nonanoic acid and four different organic photosensitizers, namely marine dissolved organic matter (m-DOM) and humic acids (HA) as environmental photosensitizers, and 4-imidazolecarboxaldehyde (4IC) and 4-benzoylbenzoic acid (4BBA) as molecular proxies. All reactions show predominant photooxidation of nonanoic acid, with important differences in the rate and yield of product formation depending on the photosensitizer. Limited changes were observed in the organic photosensitizer itself. Results show that, among the photosensitizers examined, 4BBA is the most effective in photooxidizing nonanoic acid. Overall, this work underscores the role of chromophores in the photooxidation of organic thin films and the relevance of such reactions on the surface of aerosols in the marine environment.

Figure 1

Schematic diagram of the QCM and HATR-FTIR flow system. The experimental apparatus is divided into three segments: (A) Gas control manifold, (B) HATR flow cell with broadband light source in a purged spectrophotometer compartment, and (C) QCM flow chamber with broadband light source in a purged enclosure.

Figure 2

Percentage of mass increase attributed to photoinduced oxidation of thin films with varying mass ratios of photosensitizer to NA (photosensitizer/NA). Two different photosensitizers were used: (A) 4BBA, (B) 4IC. The change in mass labeled “No O₂” indicates the mass analysis of a 1:5 mixture under anaerobic conditions. Shade represents standard deviation of triplicate experiments. Only 0.1% of data is plotted for clarity.

Figure 3

Selected spectra of the ATR–FTIR, referenced to the initial spectrum of NA, in the presence of (A) 4BBA, (B) 4IC. Spectra presented with 10 min intervals for at least 40 min of irradiation. Lines become increasingly light with increased time. No significant absorption features are observed in the region between 2200–2700 cm^–1.

Scheme 1. Proposed Mechanism for the Photosensitized Oxidation of NA

Based on mechanism proposed by Tinel et al., adapted for thin films and the absence of water. The subscript “ox” refers to the reaction products listed in Figure 4.

Figure 4

LC-MS relative intensities of products. NA+P represent the dimerization between nonanoic acid and the photosensitizer (either 4BBA or 4IC).

Figure 5

Percentage of mass change in thin films with varying mass ratios of photosensitizer to NA (photosensitizer:NA). Two different environmental photosensitizers were used: (A) humic acid (HA), (B) marine dissolved organic matter (m-DOM). Shade represents standard deviation of triplicate experiments. Only 0.1% of data is plotted for clarity.

Figure 6

Net mass gain in m-DOM:NA thin film, calculated using eq 10. Shade represents standard deviation of triplicate experiments. Only 0.1% of data is plotted for clarity.

Figure 7

Selected spectra of the ATR–FTIR, referenced to the initial spectrum, of NA in the presence of (A) HA, (B) m-CDOM. Spectra presented with 10 min intervals. Lines become increasingly light with increased time.