The introduction of trees in cities is often encouraged to regulate the urban heat island and to improve thermal comfort and air quality, in a context of global warming. However, in certain configurations, trees can also reduce the ventilation in street-districts, preventing the dispersion of pollutants emitted in the streets and altering the thermal comfort. These uncertainties about tree benefits in cities on microclimate and pollutant dispersion need clarification in order to propose relevant urban greening strategies. One of the challenges is to consider the multiscale influence of trees on turbulent exchanges of mass and energy within and at the top of the urban canopy. The originality of the CITRY project is to jointly evaluate the impact of trees on the urban microclimate and on the dispersion of submicron to several tens microns particulate pollutants by considering (1) the multiscale character of the processes at the origin of ventilation in the street, from the local turbulent flow to the exchanges with the atmospheric boundary layer, and (2) the modifications induced by trees on the turbulence of mechanical and thermal origins and on the atmospheric thermal stratification. To that purpose, the project is based (1) on the development, evaluation and use, in real configurations and through vegetation scenarios, of an innovative approach of multiscale (from city to street scales) and unsteady (explicit representation of turbulent motions) simulations of the urban atmosphere in presence of trees and the dispersion of particles of different sizes, and (2) on the realization of a measurement campaign in the city of Nantes to characterize the influence of trees on the turbulent exchanges of momentum, heat, humidity and particles, under different atmospheric conditions and along the continuum street – urban canopy – city – atmospheric boundary layer. The project will lead to recommendations for introducing trees into cities in order to improve both the urban microclimate and air quality.