The spatial variability in concentrations of a traffic-related pollutant in two street canyons in York, UK–Part II: The influence of traffic characteristics

The influence of traffic characteristics on the dispersion of carbon monoxide (CO) was investigated over a period of one month in two street canyons of differing geometry in the city of York, UK. Electrochemical sensors were used to measure CO concentrations at several locations along the two street canyons. Bi-directional traffic characteristics were measured in both streets using inductive detector loops of the Split, Cycle and Offset Optimisation Technique (SCOOT). The SCOOT data allowed comparison of free, unstable and congested traffic conditions with the spatial and temporal variability of CO concentrations in both streets. During free flow traffic conditions a significant linear relationship was found to exist between traffic flow and CO concentration in each street. The relationship fails during congested conditions, as the mean CO concentration was found to be more dependent on detector occupancy. In general, the highest mean CO concentrations occurred during high-occupancy conditions, indicating the likely influence of stop–start conditions and the accelerating driving mode that is associated with higher emission rates. By considering the measured above roof level wind direction, it was shown that during perpendicular winds, the leeward CO concentrations were a factor of at least two greater than on the windward side of the canyon. Overall, the greatest mean CO concentrations occurred during congested conditions with perpendicular above-roof level winds. The significance of the work for both emissions and dispersion modelling of traffic-related pollutants within urban areas is discussed.