Photoinduced Transformation in Glyoxal- and Methylglyoxal-Ammonium Solutions: Role of Photolysis and Photosensitization

Brown carbon (BrC) species originating from direct combustion emissions or secondary formation processes can contribute significantly to secondary organic aerosol mass and global radiative forcing. While the secondary BrC formation via dark reactions involving ammonium and glyoxal/methylglyoxal (NH₄⁺-G/MG) has been extensively documented, their photochemical behaviors remain poorly understood. This study investigates the photoinduced chemical changes of G, MG, and BrC species during irradiation. The dominant BrC species in NH₄⁺-G systems and NH₄⁺-MG systems are imidazole-2-carboxaldehyde (2-IC) and 1-(5-methyl-1<i>H</i>-imidazol-2-yl) ethanone (M-IE), respectively, which are identified as potent photosensitizers experimentally, with support from density functional theory (DFT) simulations. Reactive oxygen species derived from photolysis and photosensitization contribute to compositional diversity by yielding small organic acids and ring-opening products (e.g., amides). Interestingly, in NH₄⁺-MG systems, both M-IE and MG can initiate the transformation of the dominant nonlight-absorbing product (i.e., 5-methylimidazole, 5-MIM) into imidazole-5-carboxaldehyde (5-IC), which is a BrC species and a potential photosensitizer. This work elucidates the photoinduced transformations in NH₄⁺-G/MG systems at a molecular level and highlights that MG and M-IE are sources of oxidants to form new BrC species via photosensitization. These findings provide new insights into the behaviors of atmospheric photosensitizers, the oxidative potential from dicarbonyls, and the photochemical evolution of BrC in aerosols.