Oxidation of Polycyclic Aromatic Hydrocarbons (PAHs) Triggered by a Photochemical Synergistic Effect between High- and Low-Molecular-Weight PAHs

Photooxidation of polycyclic aromatic hydrocarbons (PAHs), which are widely observed in atmospheric particulate matter (PM), largely determines their atmospheric fate. In the environment, PAHs are highly complex in chemical composition, and a great variety of PAHs tend to co-occur. Despite extensive investigation on the photochemical behavior of individual PAH molecules, the photochemical interaction among these coexisting PAHs is still not well understood. Here, we show that during photooxidation, there is a strong photochemical synergistic effect among PAHs extracted from soot particles. We find that neither small PAHs with low molecular weights of 200-350 Da and 4-8 aromatic rings (named PAH_small) nor large PAHs with high molecular weights of 350-600 Da and 8-14 aromatic rings (named PAH_large) undergo photooxidation under red-light irradiation (λ = 648 nm), even though PAH_large can absorb light with this wavelength. Interestingly, when PAH_large is mixed with PAH_small, substantial photooxidation is observed for both PAH_large and PAH_small. Comparisons of in situ infrared (IR), high-resolution mass spectrometry, and electron paramagnetic resonance analysis indicate that the presence of PAH_small inhibits the light quenching effect arising from the π-π stacking of PAH_large. This leads to the formation of singlet oxygen (¹O₂), which initiates the photooxidation. Our findings reveal a new mechanism for the photooxidation of PAHs and suggest that complex atmospheric PAHs exhibit distinct photoreactivity from simple systems.