Influence of urban forest size and form on PM2.5 and O3 concentrations: A perspective of size threshold

Urban forest (UF) is an effective means of mitigating air pollution. At the stage where PM_2.5-O₃ composite pollution is prevalent and UF expansion reaches a bottleneck, optimizing UF form based on its area size becomes a superior management strategy. However, existing research has not fully explored the threshold effects of UF size. Using the panel threshold regression model, this study examines relationships between air pollution and UF (size and form) in 305 counties within the Yangtze River Delta (YRD) from 2014 to 2022. The results show (1) UF size exhibits a double threshold effect in three relationships: between patch density (PD) and PM_2.5, between splitting index (SPLIT) and PM_2.5, and between SPLIT and O₃. Meanwhile, a single threshold effect of UF size is observed in relationships between other UF form metrics and pollutant concentrations. (2) When UF size plays the threshold effect in relationships between UF form and pollutant concentrations, its threshold values usually lie around 3%, 14%, and 45%. UF form metrics’ correlations with PM_2.5 are generally opposite to those with O₃. (3) When UF size is below 45%, PM_2.5 reduction favors complex-shaped and dispersed UF patches, while O₃ reduction benefits from more regular and concentrated UF patches. Once UF size exceeds 45%, a continuous regional UF network can potentially address PM_2.5-O₃ composite pollution. This study aims to provide insights into atmospheric governance and UF planning.