Estimating the seasonal and spatial variation of urban vegetation's PM_2.5 removal capacity

Fine particulate matter (PM_2.5) is one of the most severe factors contributing to urban air pollution, posing significant risks to human health and environmental quality. Urban vegetation, acting as a natural method for pollution mitigation, can effectively reduce harmful air particle concentrations through processes like adsorption and deposition. While much research has quantified urban vegetation's role in PM_2.5 removal, the spatial variability and seasonal fluctuations of this process in urban environments remain poorly understood. Furthermore, few studies have quantitatively explored the environmental factors that influence this capability. Using Shanghai as a case study, this research estimates the PM_2.5 reduction by urban vegetation in 2022, integrating the i-Tree Eco model with Local Climate Zones (LCZs) classification. The results indicate that vegetation plays a significant role in PM_2.5 removal, with a total annual removal of 835 tons and an average removal rate of 0.51 g⋅m^-2⋅year^-1 per unit leaf area. The maximum annual air quality improvement reached 21.7%, with an average of 4.09%. The removal flux exhibited a clear "double peak" pattern throughout the year, with peaks occurring in late spring and late summer. Significant spatial variations in PM_2.5 removal capacity were observed across different LCZs, ranked as follows: Dense Trees > Open Lowrise > Large Lowrise > Bush/Shrub > Scattered Trees > Others. Notably, Open Lowrise areas demonstrated considerable potential in both removal flux and total removal. The 38-42 mm evapotranspiration range was found to be the most effective for PM_2.5 removal. However, when evapotranspiration exceeded 50 mm, removal efficiency showed a clear diminishing marginal effect, closely linked to the regulation of leaf stomatal opening and closing. The findings of this study underscore the importance of vegetation in improving air quality and provide valuable insights for urban planning and environmental policy.