Effectiveness of Green Infrastructure for Improvement of Air Quality in Urban Street Canyons

TLDR

Urban street canyons often exceed air‑quality standards for NO₂ and PM, and although deposition onto vegetation can reduce concentrations, prior studies have only modestly quantified this effect and rarely considered the enhanced residence time within canyon geometry. Planting vegetation in street canyons can cut NO₂ levels by up to 40 % and PM by up to 60 %, offering rapid, sustained air‑quality gains that persist even after traffic is removed.

Abstract

Street-level concentrations of nitrogen dioxide (NO(2)) and particulate matter (PM) exceed public health standards in many cities, causing increased mortality and morbidity. Concentrations can be reduced by controlling emissions, increasing dispersion, or increasing deposition rates, but little attention has been paid to the latter as a pollution control method. Both NO(2) and PM are deposited onto surfaces at rates that vary according to the nature of the surface; deposition rates to vegetation are much higher than those to hard, built surfaces. Previously, city-scale studies have suggested that deposition to vegetation can make a very modest improvement (<5%) to urban air quality. However, few studies take full account of the interplay between urban form and vegetation, specifically the enhanced residence time of air in street canyons. This study shows that increasing deposition by the planting of vegetation in street canyons can reduce street-level concentrations in those canyons by as much as 40% for NO(2) and 60% for PM. Substantial street-level air quality improvements can be gained through action at the scale of a single street canyon or across city-sized areas of canyons. Moreover, vegetation will continue to offer benefits in the reduction of pollution even if the traffic source is removed from city centers. Thus, judicious use of vegetation can create an efficient urban pollutant filter, yielding rapid and sustained improvements in street-level air quality in dense urban areas.