Flow characteristics and particle mass and number concentration variability within a busy urban street canyon

Mean and turbulent flow characteristics together with particle concentrations were measured in a busy urban street canyon in Essen, Germany, at five (flow characteristics) and three heights (particles) above ground, respectively. Particle mass and number concentrations were sampled in the size range 0.3<Dp<10 μm. The flow characteristics within the canyon were significantly influenced by canyon geometry and were shown to have significant impact on particle concentrations. During flow being directed perpendicular to the canyon a vortex circulation leads to a doubling of ambient particles when the measurement site is situated upwind to ambient flow. The vertical profiles of fine particles have maximum vertical differences of 12% between measurement levels. In the upper part of the canyon, concentrations decrease due to enhanced turbulence and mixing. Significant differences in the dynamics of particle number concentration for different size ranges are analysed. While submicron particles are inversely related to turbulence parameters, i.e. lower concentrations during enhanced turbulence, coarser particles (1<Dp<10 μm) are positively correlated to mixing within the canyon.