Influence of urban morphology on street level concentration: Water channel and field study in three Southern Californian Cities

Water channel experiments were conducted to model three Southern Californian urban areas with different building configurations and scales: high-rise settlement of Los Angeles, mid-rise settlement of Long Beach, both at 1:800 scale, and low-rise settlement of Huntington Beach at 1:200 scale. Tracer plumes were released from ground level line sources to simulate pollutants emitted from vehicles in arterials. Simultaneous Particle Image Velocimetry/Planar Laser Induced Fluorescence (PIV/PLIF) technique was applied to acquire the flow velocity and plume concentration data in selected horizontal and vertical planes.

From the field experiments that we carried out in these three urban areas, it was found that traffic load in arterials is less important compared with the influence of surrounding building arrangements on street level concentrations of pollutants, so in this study, we investigated the plume dispersion from three levels of source strength for each urban area. Finally, we compared the nondimensional concentration C_*=CuH²/Q (where C is the concentration of pollutant [mg/m³], u is the free stream velocity [m/s], H is the characteristic length [m] taken as the average building height, and Q is the pollutant emission rate [mg/s]) of plume 1) for each building arrangement with varying source strength and 2) for the same source strength through different building arrangements. Mean velocity, turbulence intensity, mean concentration and concentration fluctuation intensity were compared for all cases.

In order to understand the variation of turbulent and plume structures for different building arrangements, the integral turbulent length scale L_χ was estimated from the velocity spectrum and the integral time scale of concentration fluctuations T_χ was calculated. The values of L_χ and T_χ for three urban areas in water channel were reported and compared. The influence of building configurations and the physical mechanisms governing ground level concentrations will be presented.