Evaluation of the NO2 burden within a joint street canyon and tunnel portal micro environment

Substantial breaches in NO2 limits in the vicinity of a tunnel portal and street canyon micro environment provided the motivation for this paper. The impact of portal and surface road emissions was assessed and mitigation scenarios computed, in order to aid the authorities responsible. Emphasis was placed on detailed emission processing. A unique data set from nine NO2 monitors located near the portal and within the street canyon was used to evaluate the model approach using a Lagrangian particle model. Good agreement with measurements was obtained using simple NOx/NO2 conversion approaches. The NOx/NO2 concentration pattern is dominated by surface road emissions. A kerbside area of ~30 m in the driving direction is clearly dominated by the portal emissions indicating effective diffusion at the portal. Major uncertainties are related to the interaction of flow and turbulent mixing from ambient atmospheric conditions and traffic, particularly at low wind speed conditions.