Photochemical Production of Singlet Oxygen by Urban Road Dust

Road dust resuspension is a major source of particulate matter in many urban centers, especially those in which traction materials are applied to roadways in winter. Although many studies have investigated the composition and toxicity of road dust, nothing is currently known regarding its photochemical reactivity. Here, we show for the first time that road dust is photochemically active: in particular, we use a molecular probe technique to show that the illumination of aqueous road dust suspensions leads to the production of singlet oxygen (1O2), an important environmental oxidant. In experiments conducted using size-fractionated road dust, we found that the surface area-normalized steady-state 1O2 concentration ([1O2]ss) increased with decreasing particle size. We also observed correlations between [1O2]ss and the dissolved organic carbon content and ultraviolet absorbance properties of dust extracts, which suggests the involvement of chromophoric water-soluble organic carbon in the observed photochemistry. Interestingly, [1O2]ss in aqueous road dust extracts was lower than in the corresponding particle-containing samples, which implies that the particle surface itself also participated in 1O2 production. This work provides evidence that road dust photochemistry may influence the lifetime of urban pollutants that react via 1O2-mediated pathways.