Finding trade-offs between heat stress and air quality in street tree planting scenarios.

While the benefits of trees for thermal comfort in urban street canyons are well established, their impact on air quality remains uncertain, particularly regarding pollutants emitted by heavy traffic at the pedestrian level. High-resolution microscale models of the urban boundary layer, such as Large Eddy Simulation (LES)-based models, provide valuable insights into street-level processes, allowing for precise simulations of air quality and bio-meteorological variables.

To explore this complex interaction, this study employed the LES model PALM to simulate different tree coverage scenarios in two different urban streets under varying atmospheric stratifications. The simulations incorporated real geographic conditions and quasi-real meteorology to ensure realistic representation. Special attention was given to selecting boundary conditions that closely matched the real temperature stratification while allowing for a meaningful comparison between cases. The results revealed significant spatio-temporal variability in thermal comfort and particulate matter concentrations at the pedestrian level, highlighting the intricate relationship between urban greenery, airflow dynamics, and pollution dispersion. As expected, trees notably improved thermal comfort by reducing the biometeorological index UTCI  in shaded areas during maximum irradiance and contributing to cooling effects after sunset (in the case of optimal tree conditions). Oppositely, in narrow street canyons, tree-induced changes in airflow led to a substantial increase in PM₁₀ concentrations by more than 100% compared to tree-free scenarios. This effect was primarily attributed to the slowdown and vertical displacement of the primary vortex, which hindered pollutant dispersion.

The study highlights the need for careful and  responsible urban planning when implementing greening strategies in cities. For example, our results indicate that when the number of trees in the street is halved, the impact on thermal comfort remains comparable to the scenario with the full amount of trees, but the air pollution aggravation is notably lower