ANALYSIS OF NUMERICALLY MODELLED LOCAL CONCENTRATION GRADIENTS IN STREET CANYONS: IMPLICATIONS FOR AIR QUALITY MONITORING

The main objective of this study is to analyse numerically-modelled, local concentration gradients in street canyons. In a preliminary analysis of the physical processes within an urban built environment, a single idealised street canyon has been used. It has been shown that very steep concentration gradients exist at the leeward lower corner of the street canyon. Consequently, small differences in monitoring station positioning may yield significant variations of measured mean concentration due to large local concentration gradients. The study has been extended to assess the behaviour of local concentration gradients using a more complex urban geometry such as: a staggered cross-road, an idealised complex configuration of street canyons, and finally a real configuration of street canyons. The numerical modelling results obtained in this study are compared with both the field measurements obtained during a continuous air monitoring campaign of Glasgow City Council, Scotland, and physical modelling in the wind tunnel at the University of Hamburg, Germany. These results have significant implications for the positioning of monitoring equipment, not just in street canyons, but in wind tunnel experiments as well. Nevertheless, the obtained results are tentative and require a detailed, specifically designed, physical modelling investigation