Impact of Street-Level Trees on Pollutant Dispersion in Real Urban Canyons: A Comparative Study of Bologna and Nicosia

Urban air pollution in densely built city poses a significant threat to public health, with pollutant concentrations frequently exceeding the thresholds indicated in WHO guidelines. Nature-Based Solutions (NBS), particularly street trees, are increasingly considered as mitigation measures, yet their effectiveness in real urban environments remains poorly understood. Most studies assess tree effects in idealised canyon geometries, while evidence of their effectiveness in reducing pollutant concentration from real, morphologically complex urban settings is scarce. This study addresses this gap by investigating the influence of real urban trees and green areas on pollutant dispersion in two historical European city centres: Bologna (Italy) and Nicosia (Cyprus). The comparison is deliberately designed to test whether urban geometry which differs substantially between the two cities plays a determining role in governing the effectiveness of tree-based NBS, an aspect that has not been systematically investigated to date.

For each city, the case study consists of a real urban neighbourhood of the historical city center. In Bologna, the study focuses on the street canyon of Via Irnerio, characterised by a dense, irregular building arrangement with a high aspect ratio. In Nicosia, a previously validated urban geometry is adopted from Antoniou et al. (2019), representing a broader Mediterranean urban layout with different canyon proportions and building configurations. Urban trees and green areas are implemented as porous media using a validated drag-force formulation, with crown characteristics derived from municipal tree data website. Traffic-related pollutant emissions are represented by a ground-level line source centred within the street canyon, consistent with established modelling practice.

Reynolds-Averaged Navier–Stokes (RANS) simulations with the standard k-ε turbulence model are conducted. The Bologna baseline is validated against water-channel experiments. The Nicosia baseline validation follows Antoniou et al. (2019), providing an independently verified starting point for both cities.

By comparing pollutant concentration distributions and canyon ventilation efficiency with and without trees across both sites, the study aims to assess whether morphological differences between cities lead to substantially different NBS outcomes. The findings are expected to provide novel, evidence-based guidance on the role of urban geometry in determining the effectiveness of street-level vegetation as an air quality mitigation strategy, with relevance for urban planners.