Unsaturated Fatty Acids Enhance Aqueous Atmospheric Oxidation Ability by Producing Oxygen‐Containing Radicals in Fog Droplets

The surface of atmospheric aqueous aerosol is covered with an organic film. However, there have been limited studies about the photochemical process between the organic coating and aqueous samples such as fogwater, which contains light absorbing brown carbon (BrC). Here, the interactional aging process between unsaturated fatty acids and aqueous samples was performed by laboratory studies and field observations. On the one hand, glycine and alanine were selected as organic nitrogen‐containing compounds to form BrC with carbonyl compounds like glyoxal or methylglyoxal. Oleic acid was induced to form organic peroxy radicals through H‐ion by the excited triplet BrC or hydroxyl radical (OH). On the other hand, one type of aqueous formation pathway of Criegee intermediates (CIs) was proposed through the oxidation of oleic acid. CIs may be formed by OH addition to C=C bonds and scavenged by interfacial reactions. Results from ultra‐high resolution Fourier transform ion cyclotron resonance mass spectrometry show that the synergistic effect of oleic acid and OH may have a higher oxidative capacity than OH. Furthermore, our study demonstrates that oleic acid can improve the aqueous oxidation ability by producing oxygen‐containing radicals. These findings highlight that the formation of free radicals is greatly influenced by photochemical reactions, which further reveal the complexities of fog organic chemistry. Plain Language Summary Little is known about how organic coating drives the molecular variance of atmospheric water droplets such as fogwater. Here we report the interactional photochemical process between unsaturated fatty acids and aqueous samples by laboratory studies and field observations. We found that unsaturated fatty acids can enhance the oxidation ability by forming oxygen‐containing radicals to participate in the aging process of aqueous organics. Our results provide unique insight toward a molecular level understanding of the origin, formation, and transformation of reactive intermediates at the air‐water interface, which further reveal the complexities of aqueous atmospheric chemistry. Key Points Oleic acid is induced to form organic peroxy radicals by the excited triplet brown carbon or OH Criegee intermediates may be formed by OH addition to C=C bonds and scavenged by interfacial reactions Synergistic effect of oleic acid and OH may have a higher oxidative capacity