Reactive Uptake of Gas-Phase NO₂ by Urban Road Dust in the Dark

Road dust constitutes a prominent source of anthropogenic particulate matter, making its heterogeneous interactions with common atmospheric gas-phase compounds important. Here, we show that three distinct samples of urban road dust-including dust samples collected from city streets in summer and winter, and an urban park in summer-react with NO₂ in the dark, forming NO and surface nitrite. The loss of NO₂ ranged from ∼2 to 13% of its gas-phase concentration and scaled with its concentration as well as with the mass of the road dust sample. The uptake of NO₂ by the winter dust was ∼4 times greater than that seen from summer street dust, which was in turn greater than that by the park dust. The conversion ratio of NO₂ → NO ranged from 0.06 to 0.8 NO produced per NO₂ lost and was greatest for the summer park dust. Exposure of the summer road dust to NO₂ roughly doubles the concentration of inorganic nitrite anion in the dust but does not produce nitrate. The formation of NO and photolabile nitrite products means that heterogeneous NO reactions occurring on the surface of road dust in the dark could have wide implications for the oxidative potential of urban areas.