Quantifying Urban Air Pollution Mitigation by Tree Canopies Using Low-Cost Sensors

Urban environments are contaminated by a multitude of air pollutants. Tropospheric ozone (O3), nitrogen dioxide (NO2) as well as coarse particulate matter (PM10) and fine particulate matter (PM2.5) are the most dangerous for human health. However, urban greenery, in particular trees, offer a variety of ecosystem services, including the ability to improve air quality. We planted 170 young trees in the city of Florence using five species with proven capabilities to remove air pollutants, and open-field research was conducted to assess their pollution removal potential. Multi-sensor monitoring devices were used to monitor air pollutant concentrations and meteorological parameters from the first three years after planting. The devices were installed inside/outside the plantation and above/below the canopies. The experiment showed that the selection of suitable species effectively led to an improvement in air quality, with a reduction in monitored air pollutants below the canopy. In detail, a reduction in O3 and NO2 was detected for the second (2023) and third (2024) growing seasons, while a reduction in PM10 was only observed in 2024. The highest average reduction percentage was found for O3 (−9.1%) and PM10 (−24.5%) during 2023 and 2024, respectively. These findings highlight that nature-based solutions are really effective in air pollution mitigation, suggesting their implementation to further expand urban reforestation programmes and preserve human health.