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In micro-scale numerical modelling of street canyon pollution, an inverse proportionality of additional concentration (C) with wind speed (U_roof) is often assumed for cases without buoyancy, stability effects, solar radiation and traffic induced turbulence. Detailed data analyses of two comprehensive field datasets from Göttinger Straße in Hannover and Jagtvej, Copenhagen including concentration and wind field measurements in the street and above the roof are presented. A significant deviation from the 1/U_roof dependence up to wind speeds U_roof = 8 .. 10 m/s was observed. The dependence on wind speed, amount of traffic and wind direction could be demonstrated. The most likely interpretation of this effect is, that an additional turbulence is created by traffic. A correct modelling of the influence of traffic induced turbulence is crucial for the situations in which the highest concentrations are observed; as these are cases with low wind speeds and high traffic density.

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