Dispersion modeling at urban intersections

Ambient concentrations at urban intersections are investigated using a fluid model (boundary-layer wind tunnel) and three mathematical simulation models. In the fluid model, quantitative gas-tracer methods are used to study dispersion at an intersection surrounded by a regular array of uniform low-rise rectangular blocks as well as an intersection with significant variations in the height of the adjacent blocks. For the uniform-block configuration, concentrations simulated by three mathematical models are compared against the fluid model pedestrian-level concentration measurements at 15 street-level intersection locations for each of eight wind directions ranging from avenue-parallel to street-parallel. Two of the simulation models are Gaussian dispersion models (HIWAY2 and CALINE4) frequently used in regulatory applications. The third model combines a mass consistent flow model with a Lagrangian dispersion model (Stör.lag) based on the Langevin equation. The performance of the two Gaussian models is predictably poor while performance of the Lagrangian model is significantly better. Four performance measures are used to evaluate the three models. Recommendations suggest areas for model improvement.