Oxidation\nof Polycyclic\nAromatic Hydrocarbons (PAHs)\nTriggered by a Photochemical Synergistic Effect between High- and\nLow-Molecular-Weight PAHs

Photooxidation\nof polycyclic aromatic hydrocarbons (PAHs),\nwhich\nare widely observed in atmospheric particulate matter (PM), largely\ndetermines their atmospheric fate. In the environment, PAHs are highly\ncomplex in chemical composition, and a great variety of PAHs tend\nto co-occur. Despite extensive investigation on the photochemical\nbehavior of individual PAH molecules, the photochemical interaction\namong these coexisting PAHs is still not well understood. Here, we\nshow that during photooxidation, there is a strong photochemical synergistic\neffect among PAHs extracted from soot particles. We find that neither\nsmall PAHs with low molecular weights of 200–350 Da and 4–8\naromatic rings (named PAH<sub>small</sub>) nor large PAHs with high\nmolecular weights of 350–600 Da and 8–14 aromatic rings\n(named PAH<sub>large</sub>) undergo photooxidation under red-light\nirradiation (λ = 648 nm), even though PAH<sub>large</sub> can\nabsorb light with this wavelength. Interestingly, when PAH<sub>large</sub> is mixed with PAH<sub>small</sub>, substantial photooxidation is\nobserved for both PAH<sub>large</sub> and PAH<sub>small</sub>. Comparisons\nof in situ infrared (IR), high-resolution mass spectrometry, and electron\nparamagnetic resonance analysis indicate that the presence of PAH<sub>small</sub> inhibits the light quenching effect arising from the\nπ–π stacking of PAH<sub>large</sub>. This leads\nto the formation of singlet oxygen (<sup>1</sup>O<sub>2</sub>), which\ninitiates the photooxidation. Our findings reveal a new mechanism\nfor the photooxidation of PAHs and suggest that complex atmospheric\nPAHs exhibit distinct photoreactivity from simple systems.