Circulation analysis and characteristic time scales in an urban canopy

The concept of ”city breathability”, namely the concept of ventilating the urban environment, is a topic of growing interest within the scientific community, as it is strongly linked to the dangers that atmospheric pollutants can pose to human health. Studying air circulation down to the smallest scales of the urban boundary layer allows for a general understanding of how pollutant concentrations are transported within the city. Atmospheric pollutants in urban environments can be harmful to health, particularly in relation to their residence time within the urban canopy. For these reasons, the objectives of this thesis are to characterize air circulation in two urban canyons in the city of Bologna and to provide turbulence time scales that can be used as indicators of pollutant residence time in the air. To carry out these analyses, data were selected from the dataset from the experimental field campaign con- ducted in Bologna during the summer of 2023 within the framework of the H2020 I-CHANGE (Individual Change of HAbits Needed for Green European transition) project. Measurements were taken in Irnerio and Berti Pichat, two urban areas with different morphological characteristics, streets orientation and vegetation cov- erage. The study led to the characterization of four different snapshots of the air circulations in the two canopies according to the different direction of the wind approaching each canyon, highlighting differences in the turbulent aspects of wind flow between the two areas, mainly due to the diverse morphological and geometrical characteristics of the two areas and the uneven pres- ence of vegetation in one site compared to the other. The use of time scales related to mechanical turbulence, combined with the characterization of airflow regimes and turbulent mechanisms favouring air refreshing through the canyon top inter- face, proved effective in describing the temporal evolution of nitrogen monoxide concentrations at street level.