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-abstraction 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.