Investigation into Photoinduced Auto-Oxidation of Polycyclic Aromatic Hydrocarbons Resulting in Brown Carbon Production.

Brown carbon (BrC) is a collection of oxidized atmospheric aromatic compounds detected worldwide with broad functionality. This multifunctional nature allows BrC to be water-soluble and bioavailable and demonstrate light absorption at multiple wavelengths. Polycyclic aromatic hydrocarbons (PAH) are major primary products of combustion emissions and have long been known to oxidize in the environment as components of secondary organic aerosols. In this study, we have exposed aqueous PAH suspensions to simulated sunlight to investigate oxidized PAH as BrC precursors. Illuminated samples of naphthalene and anthracene demonstrated growth of several new products with absorptions and oxidation consistent with humic-like substances (HULIS). Reactions of aqueous naphthalene, anthracene, and their oxidized derivatives were found to produce chromatographic and spectroscopic evidence of HULIS formation when exposed to sunlight. The association of oxyradicals with HULIS has implications on human health via lung tissue damage; and its absorption character may add to radiative forcing processes in the atmosphere. The overall product characterizations from naphthalene and anthracene indicate reaction mechanism pathways that use oxidized alcohol and quinone as intermediate species.