Trees’ cooling amplified the effect of air purification in Shanxi

Under global climate change and urbanization, the growing frequency of the coupling between urban heat and air pollution has posed significant threats to public health. While increasing tree cover percentage (TCP) holds the potential for both cooling and air purification, these aspects are often examined separately, making it unclear whether TCP's cooling and air purification exhibit synergies or trade-offs, particularly across different TCP ranges. Employing an interpretable machine learning model, we innovatively quantified the nonlinear characteristics and thresholds of TCP's air purification effect in temperate continental climate, as well as its cooling effect. Structural equation modeling further analyzed their interactions under different TCP ranges. The results showed threshold effects, with cooling and air purification stabilizing when TCP reached 7.9% and 12.6%, respectively. More importantly, under high TCP (TCP ≥ 7.9%), TCP-induced cooling amplified air purification through temperature-pollutants interactions. This indirect effect accounted for 61.45% of the total TCP-induced PM₁₀ reduction. However, under low TCP (TCP < 7.9%), both direct and indirect effects exacerbated PM₁₀ pollution, contributing 82.42% and 17.58%, respectively. Clarifying the relationship between TCP's cooling and air purification, provided a basis for the collaborative mitigation of urban heat and air pollution through reasonable urban tree planting.