Microclimate simulation and lichen-based validation analyzing street trees' impact on atmospheric pollutant dispersion at the urban canyon scale

This study investigates the impact of street trees on air pollutant concentrations, specifically NO₂ and PM10, in urban environments using computational fluid dynamics (CFD) simulations with ENVI-met software. The study explores how different levels of tree cover influence the dispersion of atmospheric pollutants, focusing on three scenarios: current tree cover, complete removal of street trees, and a 50 % reduction in tree cover. Avenida da Liberdade in Lisbon, known for its high tree density, serves as the study site. To ensure the accuracy of the simulations, the method was validated using air quality data from a local monitoring station, supplemented by an analysis of lichen diversity on 80 trees, a common biomonitor for pollution. The results indicate that both NO₂ and PM10 concentrations are higher under tree canopies, with the greatest increase observed on the windward side of the avenue. Specifically, PM10 levels rose by up to 2.97 %, and NO₂ by up to 25.84 % in the scenario with the highest tree cover. Moreover, the study highlights that street trees have a more significant effect on NO₂ concentrations compared to PM10. The findings suggest that, in this specific case—where there is a high density of trees and low wind speed— reducing tree coverage and improve permeability to the wind, could improve pollution dispersion. This study provides key findings into the complex role of urban trees in air quality and offers a foundation for future research into the modelling of additional pollutants, such as PM2.5 and ozone, to gain a more comprehensive understanding of their impacts on urban air quality. • The presence of street trees, particularly large ones, can increase the concentration of atmospheric pollutants such as NO₂ and PM10 beneath their canopies in high density areas. • This simulation has been validated using biomonitor, specifically through the analysis of lichens. • Concentrations of NO₂ and PM10 are higher on the downwind side of Avenida da Liberdade. • Trees have a more pronounced effect on NO₂ concentrations compared to PM10.