Photochemical Reaction Products of Polycyclic Aromatic Hydrocarbons Adsorbed at an Air-Water Interface

Abstract

Polycyclic aromatic hydrocarbons (PAH) are ubiquitous pollutants resulting from incomplete combustion and hence normally unavoidable. The kinetic aspects of the environmental degradation process have received considerable attention; however, not much is known about the degradation products themselves. Aqueous solutions of naphthalene, acenaphthene, 9H-fluorene, phenanthrene and pyrene were subjected to UVB light in a bulk-phase and a thin-film reactor to assess the identity of the formed photodegradation products. To aide in the challenging analysis of these mixtures, a novel analytical technique based on high performance liquid chromatography (HPLC) coupled to UV diode-array detection and dopant-assisted atmospheric pressure photoionization mass spectrometry (APPI-MS) was developed, including the optimization of the chromatography, the introduction of an online-concentration system for increasing sensitivity and the deployment of a novel gas-phase dopant delivery system for APPI-MS. The dopant-assisted APPI-MS (DAPPI-MS) was further optimized, to allow for qualitative analysis of oxygenated polycyclic aromatic compounds (OPAC). The application of the developed analytical technique allowed for the identification of 2-hdyroxy-9,10-phenanthrenequinone and 6H-dibenzo[b,d]pyran-6-ol as degradation products of phenanthrene and 9H-fluorene for the first time. Most studies address PAH degradation in the bulk-water phase, neglecting the fact that the most significant contribution to their environmental degradation takes place in aerosols, especially for volatile PAH (2-4 aromatic rings). PAH can accumulate on the air-water interface, which provides a unique venue for their photochemical degradation. The interfacial photodegradation of 9H-fluorene was evaluated by measuring the formation rate of its photooxidation products in films of different thickness in the thin-film reactor. By increasing the surface-to-volume ratio of a thin film by a factor of five, the formation rate of 9H-fluorene-9-ol, 9H-fluorene-9-one and 6H-dibenzo[b,d]pyran-6-ol was significantly enhanced, highlighting the importance of interfacial degradation of organic pollutants on hydrometeors with large surface-to-volume ratio, such as fog.