Benzene photolysis on ice: implications for the fate of organic contaminants in the winter.
The members of the important class of organic pollutants known as BTEX (benzene, toluene, ethylbenzene, and xylenes) do not undergo direct photolysis in natural waters, because their absorption spectra do not overlap that of the solar radiation which reaches the Earth's surface. Recent work has shown that aromatic compounds undergo significant red-shifts in their absorption spectra when they are present at air-ice interfaces, suggesting that BTEX components could undergo direct photolysis at ice surfaces. Using glancing-angle laser-induced fluorescence (LIF) as a probe, we measured benzene photodegradation at lambda > 295 nm having a rate constant of (3 +/- 1 x 10(-4) s(-1)) under our experimental conditions. We predict that the photolysis rate at environmental ice surfaces will be similar, based on the photon flux dependence we measured. This study presents the first report of direct benzene photolysis under environmentally relevant conditions. The results suggest that direct photolysis could be an important removal pathway for organic pollutants such as BTEX in snow-covered regions, for example, in polar or urban areas contaminated by oil spills or leaks.