The effects of trees on micrometeorology in a real street canyon: Consequences for local air quality

This study analyses the effects of trees on local meteorology of a Mediterranean City (Lecce, IT) using field measurements and computational fluid dynamics simulations. Measurements were collected for 51 days in a street canyon with trees to cover different meteorological and foliage conditions. Building façades and ground temperatures were estimated from infrared images, flow and turbulence measured by ultrasonic anemometers. In the case of approaching wind parallel to the street axis, trees induce large wind direction fluctuations below tree crowns and velocities up to about 80% lower than those at roof top. This, combined with the obstruction by tree crown, lead to lower ventilation in the bottom part of the street, especially during nocturnal hours, and to in-canyon volume-averaged pollutant concentration about 20% larger than in the tree-free case. Ignoring trapping effects of trees, as typically done in many air quality models, may lead to underestimation of ground level concentrations.