Spatial distribution of traffic-related pollutant concentrations in street canyons

Spatial distribution of traffic-related pollutants within the street canyons in Guangzhou, China was monitored using a self-developed automatic sampling system of vertical section. The wind fields at the roof and street level were also field investigated. The results showed that average horizontal and vertical profiles of traffic-related pollutant concentrations within street canyon depended on wind direction at the roof level, leeward average concentrations were about I time higher than those observed at the windward side and there were differences in the daily variation profiles of pollutant concentrations between them; however, these concentration profiles at different heights of each side were similar, with CO, NO, NO2 and NOx concentrations decreasing with height above the ground. For ambient air at roof level, the daily variation profiles of the leeward pollutant concentrations with distinct diurnal fluctuation were different from those at different height level in the street canyon, but daily variation profiles of CO, NO, NO2 and NOx concentrations at windward roof level were consistent with those within the canyon, which corresponded with the traffic volume variation, except for O-3. It was deduced from these observed phenomena that pollutants from vehicular exhaust emissions in the urban street canyon were advected by wind vortices that covered most of the canyon from the windward side to the leeward side and ascended to the leeward roof edge with vortex. Then one part of these pollutants became part of circulating pollutants within the canyon by vortex being carried back to the windward side and sinking into the bottom of the street canyon and the rest of them diffused to the windward roof, but ambient air pollutant concentrations at the leeward roof were less affected by pollutants within the street canyon. Additionally, it was observed that O-3 daily variation with the concentration level increasing with height at the roof and on windward side of the street showed obvious diurnal fluctuation characteristic, and O-3 concentration levels at the roof were higher than those below the roof and there was no clear daily variation or vertical gradient at leeward side below the roof. (C) 2003 Elsevier Science Ltd. All rights reserved.