The mediating effect of microclimate in the impacts of roadside vegetation barriers on air pollution in pedestrian spaces

As urbanization accelerates, the combined effects of urban heat island and traffic-related pollution have worsened the deterioration of thermal environments and the accumulation of airborne pollutants in pedestrian spaces, significantly threatening public health. While vegetation barriers have shown promise as a cost-effective intervention, the unclear mechanisms of their interaction in pollutant removal and microclimate regulation currently limit the optimization of their ecological benefits. We selected ten sampling points along five pedestrian-heavy roads in Nanjing to monitor air temperature, air humidity, and concentrations of PM_2.5, PM₁₀, and NO₂, while measuring vegetation structural parameters (Sky View Factor and shelterbelt porosity at different heights). Our results indicated that vegetation barriers with low Sky View Factor, low shelterbelt porosity at 8 ∼ 16 m, and high shelterbelt porosity at 0 ∼ 4 m were more effective in reducing pollutant and enhancing localized cooling and humidifying effects. The influence of vegetation configurations on pollutant reduction and microclimate regulation varied significantly across different vertical layers. Structural Equation Modeling further revealed that the reduction of PM_2.5 primarily achieved through direct effects of the vegetation barrier, the reduction of PM₁₀ relying entirely on indirect effects, and the reduction of NO₂ being influenced by both direct and indirect effects, where cooling positively facilitates NO₂ reduction but negatively impacts PM₁₀ reduction. This study highlights the importance of optimizing shelterbelt porosity and considering microclimate effects in urban greening designs to enhance roadside vegetation barrier performance and provide guidance for sustainable urban green practices.