Impact of vegetation on ventilation mechanisms in real urban street canyons: the Bologna iSCAPE case study

Traffic induced emissions have been commonly addressed as one of the major sources of air pollution in cities. The typical structures of European historical city centres, such as street canyons where the presence of densely built up areas limits the efficacy of atmospheric winds to disperse pollutants, enhance near ground concentrations. It is known that tree planting in urban street canyons influences pollutants dispersion and exchange with the free atmosphere by affecting wind ventilation at street and neighbourhood levels. Nevertheless, the actual quantification of tree influence depends on different and mutual interacting factors: synoptic meteorological conditions, wind stress at the top of the canyon, geometry of the street canyon, morphology of the neighbourhood and vegetation type. Within the recently EU-funded H2020 project iSCAPE (www.iscapeproject.eu), two intensive experimental campaigns (summer 2017 and winter 2018) have been conducted in two different urban street canyons in the city of Bologna (IT) with the aim of quantifying the impact of vegetation (two lines of trees at the edge of both sides of the road) on the street canyon in mitigating pollutant concentrations within the urban environment. Specifically, multi-levels sonic anemometers coupled with thermo-hygrometer data, as well as high resolution pollutant concentrations of several compounds (e.g. NOx, CO, PMx), have been used to quantify near surface pollutant dispersion using ventilation mechanisms in both vegetated and non-vegetated street canyons. New ventilation parameters, based on turbulent fluxes and evaluated during non-synoptic conditions, have been developed to explain the pollutant distribution under different wind directions and tree crown characteristics (i.e. porosity). Results are sought for extension to other European cities that are characterized by low synoptic conditions and similar morphological structures.