Photochemical Production of Singlet Oxygen by Toronto Road Dust

Road dust, which consists of brake and tire wear, pavement particles, crustal material, semivolatile vehicle exhaust components, and natural organic matter, can contribute to both airborne particulate matter and urban runoff. To date, research has mainly focused on the health impact of road dust, but little work has been conducted to characterize its role as a reactive surface in the environment. Our group has previously shown that illuminated road dust is a source of singlet oxygen, an important environmental oxidant. Here, we report the singlet oxygen steady state concentration ([¹O₂]_ss) of illuminated aqueous suspensions and aqueous extracts of road dust samples collected from three different road types (local, arterial, and expressway) in Toronto, Canada. We find that the [¹O₂]_ss generated by aqueous extracts of road dust samples spans less than an order of magnitude and there is no clear trend in the [¹O₂]_ss with road type, but that road dust [¹O₂]_ss depends on organic content and light absorbance. The median singlet oxygen apparent quantum yield of the road dust extracts is 5.4% (UV-A), which is more than double the median reported values (UV-A) for river, lake, soil and wastewater samples. Comparisons of road dust aqueous extracts with aqueous road dust suspensions reveal that suspended road dust has a much higher [¹O₂]_ss, emphasizing the need to examine singlet oxygen production by the insoluble components in particulate matter. Overall, our results highlight the potential of road dust to transform pollutants in the atmosphere and in urban runoff.