Atmospheric formation of 9,10-phenanthraquinone in the Los Angeles air basin

Quinones are highly reactive and toxic compounds, capable of increasing cellular oxidative stress by raising concentrations of intracellular hydrogen peroxide. It has been suggested that these compounds, although also directly emitted by vehicular exhaust, are mostly formed in the atmosphere by photochemical reactions of their parent polycyclic aromatic hydrocarbons (PAHs). However, because of the low concentrations and instability of some quinones, relatively little is known about their levels in ambient air samples. The tricyclic compound 9,10-phenanthraquinone (PQ), one of several quinones found in the particle-phase of ambient air, is a compound that can irreversibly inactivate key cellular proteins. In this study, we report concentration changes of PQ in atmospheric samples as the air moves across the Los Angeles basin by the prevailing wind trajectory. The trajectory is, for the most part, from the coastal areas to the Inland Empire in the eastern end of the Los Angeles basin; the samples were collected during the photoactive season of early summer. For this study, five sites along the predominant wind trajectory were selected and samples were collected as the air parcel passed over each site. Particle-phase PQ concentrations ranged from 100 to 2000 pg m−3. A significant increase in PQ concentration was observed as the air parcel moved inland from Long Beach (source area) to Riverside, the last sampling site in the wind trajectory and considered as the final receptor area. Our results suggest that ca. 90% of the PQ in the receptor site was photochemically formed during atmospheric transport.