(54b) UV Degradation of Polycyclic Aromatic Hydrocarbons in Thin Water Films

Polycyclic aromatic hydrocarbons (PAH) are ubiquitous pollutants resulting from incomplete combustion and are unavoidable in our combustion-driven society. Several of them exhibit carcinogenic properties and are therefore of environmental concern. Hence, their environmental photo-degradation, predominantly the kinetic aspects, have received considerable attention, however not much is known about the degradation pathway. Most studies address the degradation in the bulk-water phase, neglecting the fact that the most significant contribution to their environmental degradation takes place in aerosols, especially for the fairly volatile PAH (2-4 aromatic rings). It has been shown by Valsaraj et al.¹ that PAH can accumulate on the water / air interface, a different environment from the usually studied bulk phases, with different reaction pathways and kinetic parameters.

The photolysis of two volatile PAH (acenaphthene, 9H-fluorene) is assessed in a thin-film reactor simulating stagnant films as they are encountered in the atmosphere (fog, clouds). The products are extracted via a small solid-phase extraction column, which allows direct injection onto a liquid chromatographic system, enabling the determination of compounds at sub-ppb level. The analysis of the photo-degradation products of the PAH is conducted with high performance liquid chromatography coupled to an ultraviolet diode-array detector and a dopant assisted atmospheric-pressure photoionization mass spectrometer. The abundance of obtained information (UV spectrum, positive ion and negative ion mass spectra) allows the targeted synthesis of candidate compounds suspected to be the degradation products and the subsequent assessment of the degradation pathway.

The thin-film environment also results in changed degradation reaction kinetics that are enhanced,² compared to those of bulk-phase reactions. The influence of surface reactions on the product composition and the degradation rate is evaluated by performing experiments with different film thicknesses.

1 Valsaraj KT, Thoma GJ, Reible DD, Thibodeaux LJ. Atmos. Environ. 1993, 27A (2), 203?210

2 Chen J, Ehrenhauser FS, Valsaraj KT, Wornat MJ. J. Phys. Chem. A 2006, 110(29), 9161-9168