Adsorption and photochemical transformation of polycyclic aromatic hydrocarbons on atmospheric water films

Abstract

Atmospheric water films affect the processing of chemicals in the atmosphere and have potential effects on human health and the environment. In this work, adsorption and photochemical transformations of gas-phase PAHs were studied in a flow-tube photo-reactor with a view to understanding the behavior of gas-phase PAHs occurring in thin water films such as those of aerosols and fog. Naphthalene and phenanthrene were chosen as model PAHs for this study. Bulk water-air and air-to-interface partition constants of naphthalene and phenanthrene were estimated from the experiments based on the dependence of the equilibrium uptake on the water film thickness. Theoretical computations of molecular dynamics (MD) of PAHs showed a deep free energy minimum at the air-water interface for PAHs entering water phase from the air. The MD-simulated hydration free energy was in agreement with experimental data. Suwannee River fulvic acid (SRFA) was chosen as a surrogate for the surface active substances present in atmospheric water films. The effect of SRFA in the aqueous phase on the equilibrium partitioning of PAHs to the air-water interface were investigated. To compare with SRFA, the effect of a conventional surfactant, sodium dodecyl sulfate, was also studied. Several photooxidation products of naphthalene and phenanthrene were identified in the water films and the mechanism of photooxidation was assessed. The effect of singlet oxygen on PAH photooxidation was ascertained. The photooxidation of PAHs can be very complex, especially for PAHs with several aromatic rings. It was proposed that phenanthrene photodegraded via three different pathways: through radical cation intermediates, via reaction with singlet oxygen, and via reaction with hydroxyl radical. The reaction rate constants were also determined and were substantially higher in the thin water film as compared to the bulk phase reaction. Effects of SRFA on the photooxidation of naphthalene and phenanthrene were investigated to give insight into the photooxidation process of PAHs in fog droplets in which surface active compounds are present. The presence of SRFA in the water led to multiple effects on the rate of reaction. These were characterized via a dual mechanism of self-sensitized and surfactant-sensitized pathways for reaction.