However, research has shown the aerodynamic influence of trees which, in certain configurations, can reduce ventilation and the dispersion of pollutants emitted into the streets. By jointly studying the influence of trees on the urban microclimate and on the dispersion of particulate pollutants, the aim of the CITRY project is to propose strategies for the introduction of trees in cities, which will enable an overall improvement in the quality of the urban atmosphere.

A better understanding of the influence of urban trees on the microclimate and dispersion of pollutants is needed to propose appropriate urban greening strategies. The complexity of the processes involved lies in the multi-scale and multi-physical nature of the turbulent exchanges of mass, momentum and heat at the top of the urban canopy and in the street.
The originality of the CITRY project is to propose an evaluation of the direct and indirect impacts of trees on the urban microclimate and on the dispersion of particulate pollutants, taking into account :

• The multi-scale processes at the origin of street ventilation, from local turbulent flow to exchanges with the atmospheric boundary layer; 
• Tree-induced changes in dynamic and thermal turbulence, and in atmospheric thermal stability.

The CITRY project is based on two complementary approaches: numerical modelling of the urban atmosphere and urban site observation, with the aim of improving our understanding of the various interactions along the street - urban canopy - city - atmospheric boundary layer continuum.

Two experimental campaigns are planned for winter and summer 2025 in a district of the city of Nantes, where long-term observations are also being carried out as part of ONEVU. The additional resources deployed as part of the CITRY project are aimed at characterizing the heterogeneity of micrometeorological variables and particle concentrations in the neighbourhood, and obtaining information on the characteristics of the atmospheric boundary layer. Analysis of experimental data for different atmospheric conditions and leaf densities will improve our understanding of the influence of trees on turbulent exchanges of momentum, heat, moisture and particles.

In this project, we will develop an innovative multiscale, unsteady approach to simulate the urban atmosphere and the dispersion of particles ranging in size from submicron to several tens of microns. Our modelling strategy will cover the continuum of scales from city to street, progressively refining the effects of urban canopy heterogeneity and multi-physics interactions between trees, the built environment and the atmospheric boundary layer. Application of the model to the various situations encountered during the experimental campaigns will complete the analysis of the experimental data. Once qualified, the model will be used to study different tree planting scenarios and assess their influence on microclimate and particle dispersion.

The objectives of the CITRY project concern most of the cities in the world facing heat waves and air quality problems. The ambition of densifying trees in cities is on the program of many cities, but without really knowing its overall impact on micro-meteorology and air quality. In addition to the major new scientific advances expected on urban atmospheric quality, the CITRY project will provide recommendations:

• To urban planners and local authorities on the best strategies to introduce trees in cities in order to regulate the microclimate while avoiding the deterioration of the air quality, specifically with regard to particulate pollution;
• To developers of operational Air Quality models to better take into account trees in their models.