Influence of the Near-Field Geometry on Field Measurements in Urban Street Canyons

The potential effect of nearby buildings on air quality observations in the field is inadequately understood. Rafailidis (1998) proposed a method for assessing this influence conservatively and on a long-term basis. It is based on subdividing the wind vector above the roofs into parallel- and normal- to street contributions (Figure 1). Under windy conditions, while U_n exceeds the critical threshold for canyon vortex formation, air circulation and street pollution dispersion are described satisfactorily by the wind tunnel modelling of Rafailidis (1997). Additional flushing by U. mainly advects pollution along the street, without overcoming asymmetries which may develop depending on street canyon configuration (Figure 2). To validate the potential of the above analysis and assess its impact on actual urban canyon situations, this paper examines field observations, focusing on the existence of the suggested persistent vortex circulation and canyon reaeration patterns.