Street canyon design is an important parameter which determines the microclimate conditions within street canyons. It is essential to improve the ventilation within urban street canyons so as to minimise the exposure of the population to traffic-related pollutants at street level. The residence time of pollutants within street canyons depends on meteorological conditions such as wind speed and direction, geometric layout, heterogeneity of the street and local parameters. Meteorological conditions influence the dispersion of pollutants within the street because, depending on the direction of the wind above the canopy, different flow patterns develop in the street. The influence of local parameters and local obstructions within the street has been taken into account in modelling studies in (eg Vardoulakis et al, 2003) by introducing a roughness coefficient and in laboratory studies (Gayev and Savory (1999)) by considering penetrated internal roughness. Gallagher et al (2011) studied the influence of parked cars on both sides of the street using LES under different wind directions for a symmetrical canyon. Their study showed the potential benefit that the existence of parked cars might have in street canyons, as the cars act as a baffle wall to control pollutant flow and improve air quality on pedestrian footpaths.

An experimental study was carried out to investigate the influence of local obstruction parameters on the dispersion of traffic-related pollutants, under meteorological constant wind conditions for different street canyon geometries: symmetrical (uniform building heights on both sides of the street), non-symmetrical (“step-down” case: uniform building heights but lower on one side of the street) and heterogeneous (non-uniform building heights on both sides of the street). The position of local obstruction parameters such as position of traffic lane and the influence of parked cars within the street were examined. Laboratory experiments were carried out in a water channel and simultaneous measurements of velocity field and concentration scalar levels within and above the street canyons were obtained using PIV and PLIF techniques. Traffic-related emissions were simulated using a line emission source. Two traffic lane positions were examined for all street geometries: a line emission source was placed in the centre of the street canyon; the line emission source was placed off the centre of the street. Gaps between buildings were examined for the heterogeneous street canyon. Moreover, the parked cars were examined for all the street geometries.

Results show a strong dependence of the dispersion of traffic related pollutants on local parameters for the different street geometries. For the symmetrical canyon pollutants with traffic lane placed at the centre of the canyon were twice higher on the upwind building than on the downstream building while when the traffic lane place off centre of the canyon pollutants on the upwind building were reduced by 20%. When the traffic lane was placed off centre of the street, under perpendicular wind conditions, the mixing rate within the heterogeneous street canyon was increased and the pollution levels were reduced by 30% as shown in Figure 1a and 1b. In the step-down case, pollution levels were reduced by 15-20% on the downwind building surface while on the upwind building surface pollution levels were reduced by 10%. In this case more pollution accumulated at the bottom of the canyon at the centre of the street compared with the case when the traffic lane was placed at the centre of the street.

In case where parked cars existed within the street in the heterogeneous street canyon case turbulence levels were increased at the lower zone plane of the street and pollutants were reduced (see Figure 2). Our results show that as more complex parameters exist within the street level the ventilation of pollutants out of the street improved.

Figure 1: a) Pollution dispersion within a heterogeneous street canyon, line source emission at the centre of the street, b) Pollution dispersion within a heterogeneous street canyon, line source emission off the centre of the street at 11mm to the right

Figure 2: Pollution dispersion within a heterogeneous street canyon, with line source emission off centre of the street  at 11mm to the right and parked cars close to the upwind building wall