The spatial variability in concentrations of a traffic-related pollutant in two street canyons in York, UK—Part I: The influence of background winds

The influence of background wind flow on the dispersion of carbon monoxide (CO) was investigated over a one-month period in two street canyons of differing geometry in the city of York, UK. Electrochemical sensors were used to measure CO concentrations at various heights and locations along each canyon. Six ultrasonic anemometers were used to measure the airflow and turbulence within one of the canyons. A seventh anemometer measured the above-roof (or background) winds at approximately twice the street building height. Bi-directional traffic flow was measured in each street using the Split, Cycle and Offset Optimisation Technique (SCOOT) system. The investigation indicates that differences in the street geometry and the background winds result in contrasting in-canyon wind flows and dispersion characteristics. Whilst there is evidence of a single cross canyon vortex in one street canyon for certain background wind directions, there are also a number of three-dimensional (3D) flow structures formed in the canyon due to the influence of flow channelling, both within the canyon and from adjoining side streets. For background winds with a strong perpendicular component relative to the canyon axes, the mean CO concentrations on the leeward side of the street canyons were on average, a factor of two greater than on the windward side. This feature can be attributed to the formation of vertical and horizontal vortices. However, when the concentrations were normalised by the background wind speed and total traffic flow, a three-fold difference exists between the windward and leeward sides of the street canyon. Evidence of flow channelling and helical flow regimes was also detected for background flows with a component parallel to the canyon axes. The difference in mean CO concentrations between the two street canyons was approximately two-fold, with the highest mean concentrations in the narrower canyon, which has a smaller cross-sectional area.