Interactions between polycyclic aromatic hydrocarbons (PAHs) and titanium dioxide (TiO<sub>2</sub>) nanoparticles (NPs) can produce unforeseen photoproducts in the aqueous phase. Both PAHs and TiO<sub>2</sub>-NPs are well-studied and highly persistent environmental pollutants, but the consequences of PAH-TiO<sub>2</sub>-NP interactions are rarely explored. We investigated PAH photoproduct formation over time for benzo[a]pyrene (BaP), fluoranthene (FLT), and pyrene (PYR) in the presence of ultraviolet A (UVA) using a combination of analytical and computational methods including, identification of PAH photoproducts, assessment of expression profiles for gene indicators of PAH metabolism, and computational evaluation of the reaction mechanisms through which certain photoproducts might be formed. Chemical analyses identified diverse photoproducts, but all PAHs shared a primary photoproduct, 9,10-phenanthraquinone (9,10-PQ), regardless of TiO<sub>2</sub>-NP presence. The computed reaction mechanisms revealed the roles photodissociation and singlet oxygen chemistry likely play in PAH mediated photochemical processes that result in the congruent production of 9,10-PQ within this study. Our investigation of PAH photoproduct formation has provided substantial evidence of the many, diverse and congruent, photoproducts formed from physicochemically distinct PAHs and how TiO<sub>2</sub>-NPs influence bioavailability and time-related formation of PAH photoproducts.
