Canyons and Canopies: Effects of building configuration and green infrastructure on urban air quality

Urban air quality is influenced by both street geometry and vegetation, yet their combined effects remain insufficiently understood, particularly where dry deposition to vegetation is explicitly represented. We applied the PALM model coupled with the VIDA deposition scheme to an urban area in Gothenburg, Sweden, to quantify how building configuration and street-level vegetation jointly affect concentrations of PM 10 , PM 2.5 and NO 2 . Four building layouts and two vegetation densities were evaluated under westerly and easterly winds. Building configuration was the primary determinant of pollutant. Narrow, poorly ventilated street canyons resulted in the highest concentrations, while more permeable layouts, featuring breaks in building lines and varied building heights, substantially reduced PM and NO 2 concentrations. Vegetation consistently decreased PM 10 and PM 2.5 concentrations across all scenarios, with focus area mean PM 10 reductions exceeding 30%. In contrast, NO 2 responses depended strongly on ventilation: sparse vegetation lowered NO 2 in well-ventilated streets, whereas dense vegetation increased NO 2 in confined street canyons by impeding dispersion. A barrier building located upwind of the study area under easterly winds reduced NO 2 by ∼11%, demonstrating the potential of structural shielding from major emission sources. The results highlighted that building geometry determined the underlying ventilation regime, while vegetation modulated pollutant concentrations within that regime. Accurate representation of dry deposition is essential for capturing benefits of vegetation on particulate matter. Effective air-quality improvements therefore require a combination of permeable urban form, strategically placed vegetation adapted to local airflow, and species selection that accounts for unwanted effects of vegetation like BVOC and pollen emissions.