Aerodynamic and Dry Deposition Effects of Roadside Trees on NOx Concentration Changes on Roadways and Sidewalks

This study analyzes changes in NOx concentrations due to the aerodynamic and dry deposition effects of roadside trees in the Jongno area, a central business district of Seoul, Republic of Korea, using a computational fluid dynamics (CFD) model. The simulation results indicate that the on-road NOx concentration was slightly increased (2.09%) due to the aerodynamic effect of roadside trees. However, the dry deposition effect of roadside trees had a greater impact on reducing NOx concentrations (−2.77%) along sidewalks. It was observed that the reduction in NOx concentration due to the dry deposition effect of roadside trees was likely to offset the increase in NOx concentrations due to the aerodynamic effect of roadside trees, resulting in an overall decrease in NOx concentrations. Furthermore, sensitivity tests showed that the increase in NOx concentrations due to the aerodynamic effects of roadside trees was intensified along sidewalks when ambient wind speeds were high, while the decrease in NOx concentration was proportional to the deposition velocity of roadside trees. Therefore, roadside trees should be planted where aerodynamic effects do not significantly increase NOx concentrations in order to improve near-road air quality.