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Air pollution in cities, especially in existing urban canyons, poses a global significant long-term health risk to the urban population, and urban planners are faced with the challenge of increasing the transport of pollutants out of cities. In recent years, many research and review articles have explored passive methods of altering the urban canyon geometry using ‘obstacles’ that alter the dispersion of air flow in these canyons, thereby enhancing pollutant dispersion. These obstacles have been categorised (in previous review articles) into porous and non-porous barriers. Porous barriers include vegetated shrubs and trees, while non-porous barriers include parked cars, low boundary walls, roadside barriers, wind catchers and solar chimneys. This paper expands on previous reviews to highlight newer studies, their findings, and challenges in implementing these measures for existing urban canyons. In addition, it attempts to bring us one step closer to developing urban design guidelines that improve air quality in the urban environment.

This study analyses the interactions and impacts between multiple factors i.e., urban greening, building layout, and meteorological conditions that characterise the urban microclimate and thermal comfort in the urban environment. The focus was on two neighbourhoods of Lecce city (southern Italy) characterised through field campaigns and modelling simulations on a typical hot summer day. Field campaigns were performed to collect greening, building geometry, and microclimate data, which were employed in numerical simulations of several greening scenarios using the Computational Fluid Dynamics-based and microclimate model ENVI-met. Results show that, on a typical summer day, trees may lead to an average daily decrease of air temperature by up to 1.00 °C and an improvement of thermal comfort in terms of Mean Radiant Temperature (MRT) by up to 5.53 °C and Predicted Mean Vote (PMV) by up to 0.53. This decrease is more evident when the urban greening (in terms of green surfaces and trees) is increased by 1266 m2 in the first neighbourhood and 1988 m2 in the second one, with respect to the current scenario, proving that shading effect mainly contributes to improving the urban microclimate during daytime. On the contrary, the trapping effect of heat, stored by the surfaces during the day and released during the evening, induces an increase of the spatially averaged MRT by up to 2 °C during the evenings and a slight deterioration of thermal comfort, but only locally where the concentration of high LAD trees is higher. This study contributes to a better understanding of the ecosystem services provided by greening with regard to microclimate and thermal comfort within an urban environment for several hours of the day. It adds knowledge about the role of green areas in a Mediterranean city, an important hot spot of climate change, and thus it can be a guide for important urban regeneration plans.