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.
Highlights
Outdoor air pollution causes 35 000-50 000 premature deaths per year in the UK 1, and more than 1 million worldwide[2], in addition to increased morbidity[3]
This study focuses on NO2 and PM10, which are the dominant pollutants in most urban areas, where they are largely derived from vehicle emissions
As leaves typically present favorable surfaces for particle capture, but wind speeds in canyons and immediately above roofs are likely to be low relative to the above-roof mean wind speed, the relatively conservative value of 0.64 cm s-1 used by Nowak[11,12] is adopted. Use of this commonly-used deposition velocity aids comparison of the results with those of previous studies. This Vd of 0.64 cm s-1 is considered suitable for this study because it is comparable with that predicted by the process-based model of Petroff and Zhang[41] for deposition to grass (LAI 1-2; the same as the assumed leaf area index (LAI) for a green wall) for PM10 mass distributions measured in polluted street canyons[42,43], and broadly comparable than the ~1 cm s-1 measured for particles of comparable size over moorland by Nemitz et al.[44]
Summary
Outdoor air pollution causes 35 000-50 000 premature deaths per year in the UK 1, and more than 1 million worldwide[2], in addition to increased morbidity[3]. Use of this commonly-used deposition velocity aids comparison of the results with those of previous studies This Vd of 0.64 cm s-1 is considered suitable for this study because it is comparable with that predicted by the process-based model of Petroff and Zhang[41] for deposition to grass (LAI 1-2; the same as the assumed LAI for a green wall) for PM10 mass distributions measured in polluted street canyons (mass distribution peaking at 310 μm)[42,43], and broadly comparable ( smaller) than the ~1 cm s-1 measured for particles of comparable size over moorland by Nemitz et al.[44].
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