How can trees protect us from air pollution and urban heat? Associations and pathways at the neighborhood scale

With rapidly increasing urbanization, cities frequently suffer from the development of urban pollution and heat islands, the co-occurrence of which increases the vulnerability of cities and threatens public health. Greening practices, particularly the plantation of trees, have become an effective strategy for enhancing the quality of urban environments. However, extensive studies examining the effects of urban greening tend to focus on the singular contribution of trees to mitigating either urban air pollution or heat. In this study, the metropolis of Shanghai, China was selected as a study case to explore associations between particulate matters with aerodynamic diameters ≤10 μm (PM10) and heat mitigation by urban trees. During the summer months, we obtained hourly PM10 concentration and meteorological data from environmental monitoring stations in Shanghai, and land cover data were derived from 0.8-m-resolution GF-2 satellite images. Regression analysis indicated a negative non-linear logarithmic pattern between canopy cover and PM10 or heat indicators within neighborhoods. Mediation analysis confirmed the significant mediatory role of heat mitigation pathways, particularly extreme heat mitigation on PM10 reduction from urban trees. The proportion of indirect PM10mean mitigation effects of urban trees associated with reductions in average temperature, maximum temperature, extreme heat days, and extreme heat hours amounted to 27.11%, 27.59%, 30.61%, and 39.76%, respectively. Different results were obtained when using PM10max as the dependent variable, with only extreme heat events (EHEs) pathways being significant, whereas the proportion of indirect effects associated with extreme heat days and extreme heat hours accounted for 25.89% and 37.08%, respectively. Overall, mitigating extreme heat events makes a larger contribution to air purification than temperature regulation. Our findings enrich and extend the current understanding of associations between the air purification and cooling effects of urban trees, and highlight the potential co-benefits for effectively dealing with air pollution and urban heat in high-density cities.