UCLA

In this study, large eddy simulation (LES) is adopted to evaluate the aerodynamic and deposition effects of street trees on fine particulate matter (PM2.5) concentration within street canyons. The Extended Nonperiodic Domain LES for Scalar Transport (ENDLESS) is used to allow exploration of vegetation effects at a neighborhood scale (up to 100 canyons) while maintaining a reasonable resolution required to resolve flow patterns within each canyon. We investigate three emission scenarios: (i) only local traffic emissions within the canyon (the first canyon in the urban environment); (ii) only background pollution originating from the upwind canyons; and (iii) a combination of scenarios (i) and (ii). Numerical results show that the presence of trees has different effects on the PM2.5 level within canyons in different emission scenarios and at different spatial scales. At the street scale with only local traffic emissions, aerodynamic effect of trees results in an increase in the concentration near leeward walls and a decrease in the concentration near windward walls, which overwhelms the deposition effect. On the other hand, trees have a negligible impact on the transport of background pollution into the canyon or its distribution within the canyons. The deposition has beneficial effects that only manifest in a considerable way at the neighborhood scale. Finally, the effect of trees on a simple operational urban pollution model (OUPM) is investigated. After modifications for aerodynamic and deposition effect of trees, the predictions of the OUPM show good agreement with LES results.