Dynamical and chemical impacts of urban green areas on air pollution in a city environment

Green Nature-Based Solutions (NBS) can be utilized to mitigate air pollution in cities; however, other factors associated with urban morphology and local meteorological conditions contribute to pollution concentrations. Urban dispersion models are frequently used for analysing these aspects, but they may not account for vegetation effects on pollutant dynamical and chemical processes. The present study implements a methodology to evaluate the impact of vegetation on city pollution levels. The aerodynamic effect of vegetation is introduced by modifying the surface roughness using the morphometric method. Tree emissions are modelled as point sources, considering tree characteristics and environmental conditions. The ADMS-Urban model is utilized to reproduce the variability of pollutant concentrations during summer 2023 in a selected neighbourhood of Bologna, Italy, characterized by densely vegetated areas. Results are verified by comparing the results of numerical simulations with pollutant observations measured within an ad-hoc measuring campaign, part of the I-CHANGE H2020 project. Pollutant emissions from green areas locally increase O3 concentrations up to 7.4 % during the day, generating a persistent accumulation in surrounding areas. Thus, tree emission has a significant impact on pollutant concentrations, comparable to building elements in urban morphology. This highlights the need to include vegetation emissions to accurately assess urban air quality and to minimise possible side effects of green NBS.