Impact of solid road barriers on reactive pollutant dispersion in an idealized urban canyon: A large-eddy simulation coupled with chemistry

This study conducts chemistry-coupled large-eddy simulations on reactive gaseous and particulate pollutant dispersions in an idealized street canyon. Four road-barrier configurations are considered: barrier-free, side barriers, center barrier, and the combination of side and center barriers. Regarding the formation of secondary aerosols, the center barrier reduces the canyon-averaged mass concentration of particulate matter (PM) but increases the inorganic particle concentration. This is because nitric acid (HNO₃) is limited in the formation of ammonium nitrate due to the long residence time in the street, and the center barrier increases the HNO₃ inflow from the background air. The side barriers increase the canyon-averaged PM₁₀ mass concentration but reduce PM number concentration due to sufficient time for coagulation. Using combined barriers reduces the most PM₁₀ mass concentration and number concentration from the barrier-free case. Regarding the formation of secondary gases, the side barriers enhance the NO₂ formation due to the worse ventilation. In contrast, the center barrier largely reduces the canyon-averaged NO concentration but slightly reduces the NO₂ concentration from the barrier-free case, because the center barrier increases the O₃ inflow from the background air favoring NO₂ formation. Additionally, the combined barriers show smaller canyon-averaged NO₂ and O₃ concentrations than the center barrier. From the view of controlling reactive pollutants, urban planners are recommended to apply the combined barriers to enhance better air quality in street canyons, rather than using either side barriers or a center barrier alone.