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

Abstract

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 _x reactions occurring on the surface of road dust in the dark could have wide implications for the oxidative potential of urban areas.

Figure 1

NO₂ (orange; left axis) and NO (blue; right axis) concentrations when 1.0 g of road dust was exposed to a 50 ppb NO₂ gas at 50% RH. An uptake of NO₂ and a simultaneous release of NO were observed when road dust was put into contact with the gas flow. Yellow-highlighted area indicates the time at which the sample was in contact with NO₂, while gray-highlighted area indicates no contact. Note that the NO and NO₂ scales are different on their respective axes.

Figure 2

Changes in NO₂ (orange, left axis) and NO (blue, right axis) concentrations when 1.0 g of HS road dust was exposed to 35–150 ppb NO₂ at 50% RH.

Figure 3

Changes in NO₂ (orange, left axis) and NO (blue, right axis) concentrations when 0.5–3.0 g of HS road dust samples was exposed to 50 ppb NO₂ at 50% RH.

Figure 4

Changes in NO₂ (orange) and NO (blue) concentrations when 0.5 g of SD road dust was exposed to 50 ppb NO₂ at 50% RH. A clear increase in the magnitude of the NO changes with increasing exposure of NO₂ to the dust can be seen, while the magnitude of the NO₂ changes remains constant. Yellow-highlighted areas indicate the times at which the sample was in contact with NO₂, while gray-highlighted areas indicate no contact.

Figure 5

Changes in NO₂ (orange, left axis) and NO (blue, right axis) concentrations when 1.0 g of HP road dust was exposed to 50 ppb NO₂ at 50% RH. The NO concentration initially spikes when the gas is first diverted into the chamber but quickly stabilizes. Yellow-highlighted area indicates the time at which the sample was in contact with NO₂, while gray-highlighted area indicates no contact.