Abstract
Busy street canyons can have a large flow of vehicles and reduced air exchange and wind speeds at street level, exposing pedestrians to high pollutant concentrations. The airflow tended to move with vehicles along the canyon and the 1-s concentrations of NO, NO2 and CO were highly skewed close to the road and more normally distributed at sensors some metres above the road. The pollutants were more autocorrelated at these elevated sensors, suggesting a less variable concentration away from traffic in the areas of low turbulence. The kerbside concentrations also showed cyclic changes approximating nearby traffic signal timing. The cross-correlation between the concentration measurements suggested that the variation moved at vehicle speed along the canyon, but slower vertically. The concentrations of NOx and CO were slightly higher at wind speeds of under a metre per second. The local ozone concentrations had little effect on the proportion of NOx present as NO2. Pedestrians on the roadside would be unlikely to exceed the USEPA hourly guideline value for NO2 of 100 ppb. Across the campaign period, 100 individual minutes exceeded the guidelines, though the effect of short-term, high-concentration exposures is not well understood. Tram stops at the carriageway divider are places where longer exposures to higher levels of traffic-associated pollutants are possible.
Highlights
IntroductionDeep urban canyons are commonly found in major cities
If the vortices are weak at the canyon bottom, the airflow can be dominated by other processes, such as vehicles or the prevailing wind along the street axis [21]
Turbulence and pulses of pollutants from passing vehicles caused a rapid variation in the concentrations, which could be extremely high for brief periods
Summary
Deep urban canyons are commonly found in major cities. These are places where limited air exchange leads to poor air quality [1], regardless of the wide range of turbulenceinduced fluctuating concentrations [2]. At the base of the canyon, the wind speeds can be low, even if the traffic-generated turbulence is strong. Architectural features, such as advertising signage, overhead walkways [3] and a range of street features [4] such as bus shelters [5] and roadside vegetation [6,7,8], inhibit the airflows. The ventilation within the canyon may be suppressed [9], and vortices can leave dead zones with stagnant air [2]
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