Ambient concentrations at urban intersections are investigated using a
fluid model (boundary-layer wind tunnel) and three mathematical simul
ation models. In the fluid model, quantitative gas-tracer methods are
used to study dispersion at an intersection surrounded by a regular ar
ray 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 mat
hematical 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 stre
et-parallel, Two of the simulation models are Gaussian dispersion mode
ls (HIWAY2 and CALINE4) frequently used in regulatory applications. Th
e third model combines a mass consistent flow model with a Lagrangian
dispersion model (Stor.lag) based on the Langevin equation. The perfor
mance of the two Gaussian models is predictably poor while performance
of the Lagrangian model is significantly better. Four performance mea
sures are used to evaluate the three models. Recommendations suggest a
reas for model improvement.