The main goal of this study has been to advance the understanding of the effect of the presence of trees on the ventilation of pollutants in an urban geometry. This has been pursued by an experimental campaign conduced in the wind tunnel of the Laboratoire de Mécanique des Fluides et d’Acoustique at ECL. The urban geometry considered consists in a street canyon placed perpendicular to the wind direction within the wind tunnel. Three main configurations of trees were analysed, namely: no trees, two rows of scattered trees and two dense rows of trees. A linear source emitting a mixture of ethane and air has been used to simulate the pollutant.