Urban air pollution from road traffic remains a major public health concern, with its spatial variability at neighbourhood scales strongly influenced by urban morphology. This study investigates how urban form affects the dispersion of traffic-related PM_2.5 in four Italian cities (Lecce, Bari, Milan and Rome) representing diverse climatic and morphological contexts. Seasonal simulations were conducted using the ADMS-Roads dispersion model, integrating detailed road geometries, standardized traffic emissions, and city-level meteorological data for 2019–2021. Urban morphology was characterized at 100 m resolution using building plan area fraction (λ_p), street-canyon aspect ratio and mean building height derived from GIS analyses. Statistical analysis combined random forest regression with partial dependence plots and quantile regression to explore both average and distributional effects. Results reveal a generally negative association between λ_p and PM_2.5 in Lecce, Milan, and Rome, particularly at higher concentration quantiles, suggesting that denser urban fabrics may mitigate extreme pollution episodes. Bari exhibits a weaker and more heterogeneous response, highlighting the influence of local wind regimes and traffic distribution. Wind speed and temperature consistently reduce PM_2.5 across all cities, while street geometry effects are non-linear and season-dependent. These findings demonstrate the importance of considering urban morphology alongside traffic and meteorology when designing strategies to reduce exposure. Importantly, the methodological framework presented here, combining high-resolution dispersion modelling with interpretable machine-learning analyses, is transferable to other urban contexts, providing a robust approach to assess morphology–pollution interactions beyond the studied cities.

This research, carried out within the framework of the PNRR Italian National Centre for Sustainable Mobility CNMS (European Union – Next Generation EU – PNRR – MISSIONE 4 – COMPONENTE 2 – INVESTIMENTO 1.4 – Spoke 7 – Code CN00000023, CUP: F83C22000720001), investigates the impact of vehicular emissions in urban environments, with a particular emphasis on the vicinity of sensitive receptors such as schools and densely populated residential areas. Recent studies suggest that current legislation limits may not adequately safeguard vulnerable populations, particularly children and adolescents.To address this issue, a network of high-resolution monitoring instruments will be deployed in Lecce and its surrounding areas (Southern Italy). Specifically, measurement campaigns will take place in urban sites and on a bridge outside the urban area. The setup includes AIRQINO air quality and meteorological stations, as well as vehicle-counting cameras. These instruments will continuously collect data on key atmospheric parameters (wind speed and direction, temperature, precipitation, humidity, solar radiation) and traffic patterns (vehicle classification and speed). The collected data will be integrated into the ADMS-Roads dispersion model, simulating pollutant dispersion dynamics, while air quality measurements will validate the model outputs for NOx, PM10, PM2.5 and O3 concentrations.The complementary use of urban and extra-urban sites will provide a more comprehensive characterization of pollution transport mechanisms and their dependence on meteorological and traffic conditions. This approach enhances the accuracy of dispersion modeling and enables a detailed evaluation of emission hotspots.By combining real-time monitoring with advanced dispersion modeling, this study will develop targeted mitigation strategies to reduce air pollution exposure in high-risk urban areas, providing scientific evidence to support air quality policies and public health protections, particularly for vulnerable populations in sensitive locations.