From canopy to boundary-layer scales: in-situ quantification of urban-trees effects on turbulent exchanges, ventilation, and pollutant dispersion

Trees located within the urban canopy are known to mitigate the urban heat island effect and heat waves by lowering the canopy air temperature via evapotranspiration and surface shading. They are also invoked as a nature-based solution to improve air quality through the absorption and deposition of anthropogenic pollutants. However, urban trees  can have the adverse effect of reducing ventilation within the canopy as well as the dispersion of pollutants. Heat and mass exchanges between the urban canopy and the overlying atmosphere play a key role in air temperature and air quality at the pedestrian level. These exchanges are driven by turbulent motions developing at different scales, within the urban canopy, in the overlying atmospheric boundary layer, and at their interface. Quantifying the role of urban trees in these exchanges and their links to ventilation or dispersion processes within the canopy is challenging due to the complexity of urban surfaces and the various processes involved in surface-atmosphere interactions.

The CITRY field campaign was carried out to characterize turbulent exchanges of momentum, heat, and mass as well as fine particles concentrations in an urban environment from canopy to boundary-layer scales, under various stability conditions and seasonal tree characteristics, specifically the presence of leaves. The experimental set-up was installed in a residential area of the city of Nantes (France) over a period of 10 months (March-December 2025).  A wide range of instruments was deployed at different heights from the roof of a 14m-tall building and within the urban canopy. Within the urban canopy, four ultrasonic anemometers were installed on two masts, at 6 m and 10 m above ground level (a.g.l.). Two ultrasonic anemometers and one Doppler LiDAR wind profiler were positioned on the rooftop to measure wind and turbulence at 20 m and 24 m a.g.l., and between 55 m and 400 m, respectively. Within and above the canopy, gas analyzers (LI‐COR) were also installed at 10 and 24 m a.g.l., close to the ultrasonic anemometers, to deduce water vapor and carbon dioxide turbulent fluxes. Particulate matter sensors, covering particle diameters from 0.2 to 40 micrometers, were deployed across the site, on streetlight poles and on masts at several heights within and above the urban canopy, to capture the spatial variability of particle concentration.

We will present a preliminary analysis of this observational dataset, including a statistical assessment of the effects of wind sector, wind speed, atmospheric stability, and leaf state on water vapor and carbon dioxide fluxes and particulate matter concentrations. Additionally, for selected periods, the main characteristics of the turbulent structures responsible for canopy–atmosphere exchanges will be shown, as well as their size dependency to the thermal stability.