Evaluating the impact of different green infrastructure strategies on particulate pollutant concentrations in street canyons: a numerical simulation study in Qingdao

Green infrastructure (GI) in street canyons can reduce particulate matter (PM) through leaf deposition, but it may also promote pollutant accumulation by modifying ventilation and recirculation. To clarify this deposition-ventilation trade-off, this study combined field measurements and ENVI-met simulations to evaluate the effects of GI configurations on PM dispersion in a typical street canyon in Qingdao, China. Field measurements were conducted on the windward and leeward sides during winter and summer, and a validated ENVI-met model was used to simulate PM10 dispersion under three wind directions, namely parallel, perpendicular, and oblique winds, and eight GI configurations. The effects of GI were evaluated using the Average Relative Difference in Concentration (ARDC), total deposition, and deposition efficiency per unit vegetation volume. Field measurements indicated relatively higher windward-side PM concentrations in summer, whereas winter showed a tendency toward PM10 accumulation on the leeward side from midday to evening. Simulations showed that parallel winds produced the lowest average PM10 concentrations within the canyon. Shrubs and green walls had near-neutral effects on canyon-wide mean concentrations but helped suppress local near-ground peaks. In contrast, street trees and multi-layer GI configurations tended to increase PM10 concentrations in the leeward zone, with the maximum leeward ARDC indicating an approximately 9% increase relative to the non-vegetated reference scenario. Street trees contributed the highest total deposition, while shrubs showed the highest deposition efficiency per unit volume. These findings suggest that street-canyon GI design should prioritize axial ventilation corridors, avoid continuous tall or dense vegetation on leeward sides, and consider shrub-green wall combinations in pedestrian zones.