Coupling of urban street canyon and backyard particle concentrations

Differences in particle mass and number concentrations between a busy urban street canyon (north-south orientation, about 50,000 vehicles 24 h-1) and an adjacent backyard were measured with optical particle counters. The influence of meteorological quantities, especially turbulent flow within the urban canopy layer, was also studied. Particle mass concentrations PM10 and PM1 were consistently larger within the street canyon due to enhanced emission and resuspension. For the study period this resulted in higher concentrations in the canyon of on average 30 % (PM10) and 22 % (PM1). Although elevated transport of submicrometer particles was related to easterly wind directions, the largest relative concentration differences between both sites were associated to cross-canyon flow from westerly wind directions. This is due to the canyon vortex being able to direct polluted air masses to the measurement site during flow being directed perpendicular to the canyon axis. For less polluted air within the backyard the backyard vortex is of minor influence. We found different influence of thermal and mechanical turbulence on the temporal evolution of concentration differences at both sites. Thermal turbulence was positively correlated with particle concentrations, while the latter was characterised by negative correlation coefficients.