Clustering and mapping the patterns of street canyons and vegetation related to NO2 concentration in Milan, Italy

Air pollution is the biggest environmental risk to human health. In cities, the hot spots of air pollution are generally on some street canyons, which are narrow inner urban roads flanked by a continuous row of high buildings on both sides. These street canyons promote the accumulation of traffic-induced pollution due to their lack of natural ventilation. Trees are quoted as effective measures to allow pollutant deposits on their leaves. However, street trees with big canopies can lead to higher pollutant concentrations by reducing airflow in urban canyons. Previous studies have investigated the impact of street canyon geometry and street vegetation on air pollution. However, most of these studies focus on individual street canyons, while city-wide evaluations on this topic remain scarce. In 2023, NO2 concentrations were measured in 473 in the Municipality of Milan (Italy) as part of the citizen-science project “NO2 No Grazie!”. This research applies this data to study the impact of indicators of street canyon geometry and street vegetation (e.g., aspect ratio, sky view factor, green view factor, tree height, tree crown diameter, and tree stand density) and traffic volumes on measured NO2 concentration. The potential result is that the traffic volume and aspect ratio are positively correlated to the measured NO2 values. On the contrary, measured NO2 values are negatively related to green view factor and tree stand density. These indicators are used to classify the air quality in street canyons. Finally, several typical street canyon types were defined, which can be of fundamental interest for urban planners and policymakers.