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.

Mobility is at the heart of the stakes for urban sustainable development and transportation policies that are set up in cities increasingly incorporate environmental components. Urban mobility plans (PDU) are, in France, an essential tool of urban mobility policies. Their environmental features have received increasing attention during the last decade, so that environmental impacts assessment of the PDU actions is now compulsory. The Eval-PDU project aims at developing PDU environmental impact assessment methodologies. It leans towards the case of Nantes Metropole and its present PDU (2000-2010), following a research-action program conducted by the IRSTV in 2007-2008 with the urban community of Nantes. This situation is a good illustration of the need of local public authorities for rigorously based tools to assess a series of impacts (air quality, noise, ...) effectively associated with various actions (or groups of actions) they lead. Beyond the monitoring of objective indicators, it is a matter of understanding and quantifying a cascade of physical and social causalities and, further, its consequences for the quality of life and its perception by the inhabitants. The need concerns as much ex post evaluations of what has already been done, as ex ante evaluations of what is being planned. The selected approach is based on multi-factor numerical simulations (air quality, noise, energy consumption, greenhouse gas emission) representing a set of alternative scenarios (before/after, with/without) figuring changes in transportation generated by series of PDU actions. These environmental impact simulations are based on traffic simulations by means of a multimodal mobility model (especially road traffic, a key parameter for these environmental stakes). The traffic data allow to evaluate the consumption of energy and the emissions of atmospheric pollutants and greenhouse gases in the shape of an emission inventory. Modelling the interactions with the urban morphology further allows to evaluate and map the concentrations and exposures to pollutants. An analogous construction is used for the exposure to noise. Following, the assessment of the socio-economic consequences of these alternative simulations will help to produce synthetic indicators of the social benefits of the PDU actions for the environment. Several approaches will be tested. We will try to analyze the influences of air quality and noise – characterized by the results of the numerical simulations – on two types of socio-economical indicators : (1) well-being and declared satisfaction indicators, collected by an ad-hoc survey; (2) property values of housings. The econometric methods should allow to isolate the environmental factors effects among those of other preponderantly influent factors. In parallel, it is planned to explore alternative approaches, aiming at a first order assessment of some environmental impacts skipping all or parts of the numerical model chaining while keeping the capability of analysing the processes from the socio-economic point of view. The first tested approach consists in evaluating the environmental consequences of the changes in behaviour linked to one or several key actions of the PDU. It is expected to drive a survey among specific inhabitants especially concerned with these actions, to identify the individual strategies to adapt to transportation offer variations, especially as concerns individual car journeys. We will try to deduce a direct evaluation of the differential environmental impact on energy consumption and air quality. Finally, an alternative assessment method using composite indicators will also be tested. It will be applied to construct a health impact indicator of PDU actions, associated to atmospheric pollution, further compared to the results of the numerical models. This construction, based on a cascade of numerical models of the physical processes, coupled with surveys and geo-statistic analyses for the socio-economic aspects, raises a series of research questions: - the relevance of scenarios built on PDU actions; -the capability of a simulation model cascade, starting from the traffic model, to evaluate the environmental impacts of these actions; - the ability of relating the physical environmental indicators, established from the simulation models, with socio-economic indicators; - the assessment of some environmental impacts by alternative approaches without the complex models of the physical processes. Only strong interdisciplinarity allows to touch these questions on.

Urban populations are nowadays overexposed to extreme heat and pollution episodes, degrading their life quality and affecting their health. CoolPath project aims to provide communities and residents with knowledge and recommendations for identifying, quantifying, and reducing their exposure to overheating and pollution risks, in particular by proposing alternative pedestrian itineraries in urban areas. The aim of the proposed work is, on the one hand, to enhance knowledge of the spatial practices and lifestyles of residents in "real-life" situations. On the other hand, it will deepen our understanding of people's response and adaptation to overheating and pollution, depending on the frequency and the severity of their exposures. New models, adapted to rapid changes in physical and physiological variables over time, at different spatial scales and for different environments, will emerge from this work. To achieve the project's objective, a reproducible, multi-scale and multi-physics methodology will be implemented. This interdisciplinary methodology, combining human and social sciences, physiology and urban physics, will be based on interviews, modeling and field measurements. The results will highlight new interdisciplinary indicators of exposure and adapted discomfort thresholds, making it possible to better assess the vulnerability of different population profiles on different urban itineraries and scales ranging from the pedestrian to the city, via the building and the neighborhood. Promoting an interdisciplinary approach, this project involves academic and applied research centers, specialized in the urban environment, an air quality monitoring association, the local authority of Nantes and the French national agency of health.

Each year, pollution claims thousands of lives across Europe, yet city administrations struggle with limited resources and the need to balance the multiple and uncertain effects of potential solutions. PUREPOLIS is set to transform urban health by pioneering advanced tools that empower policymakers to take high-impact action. At its core, PUREPOLIS drives data-driven, citizen-powered, and science-backed decision-making. A breakthrough Citizen Science Application (PR1) will enhance engagement and real-time monitoring. The Near Real-Time Pollution Source Apportionment tool (PR2) and Risk-based Health Impact Assessment framework coupling epidemiology and toxicology approaches (PR3) will precisely link exposure to 35 pollutants and their combination across air, noise, water, and soil to 10 critical health risks, including wellbeing. To maximize the efficiency of Zero Pollution Measures, PUREPOLIS will deploy cutting-edge analysis (PR6) to assess their effectiveness in terms of health, economic, and climate impacts, embracing a complexity perspective. These insights will feed into a Dynamic Abatement Strategies Toolbox (PR7), ensuring seamless policy integration into co-created, tailored comprehensive strategy for each of the two case studies in France and Spain, optimising co-benefits with multiple policy sectors. PUREPOLIS also pioneers a Vulnerability framework (PR4) and Behaviour framework (PR5) to drive behavioural changes for more successful interventions and foster equitable and sustainable solutions. Beyond core cities in France and Spain, a next-gen Digital Twin (PR8) and policy reports (PR9) will ensure replicability, scalability, and real-world impact across Europe. With nineteen partners from eight countries over thirty-six months, PUREPOLIS is an unprecedented step forward for healthier, more resilient cities. By fusing science, digital innovation, and citizen-driven action, PUREPOLIS sets a new gold standard for pollution mitigation in Europe.