The development and evaluation of dispersion models for urban areas using tracer experiments

This paper summarizes results from a five year project, funded by the California Air Resources Board and the California Energy Commission, to develop dispersion models that can be used to examine the air quality impact of urban sources of pollution at source-receptor distances ranging from a few meters to several kilometers. Because there is little data on dispersion within urban areas, this project conducted several field studies to collect data that could be used to evaluate existing models and develop new models. These tracer field studies were conducted in a variety of urban locations, which included the residential neighborhoods of Barrio Logan, San Diego, and Wilmington, Los Angeles, which are located close to industrial sources. In addition, tracer experiments were conducted inside a model urban canopy constructed at the Dugway Proving Ground, Utah, and around a small building located in a parking lot at the University of California, Riverside.

In most of these experiments, sulfur hexafluoride, the tracer, was sampled at ground-level along arcs ranging from meters to kilometers from the release point. Sonic anemometers, sodars, and temperature profilers provided detailed meteorological information. The data from these experiments provided understanding of dispersion in urban areas relative to that in rural areas, and were used to develop and evaluate a set of dispersion models in which horizontal and vertical plume spreads were related to meteorology of the urban boundary layer. These models also account for special features of urban areas, such as low wind speeds and the enhancement of turbulence by buildings and the heat island effect. At the coastal Wilmington site, the stability of the onshore flow limited the vertical growth of the plume. These results indicate that estimating dispersion at scales of hundreds of meters requires information on the structure of the urban boundary layer.

At source-receptor distances of meters, when the plume is still embedded within the urban canopy, horizontal dispersion is enhanced through building-induced wind meandering. A model that uses on-site meteorology to account for this meandering provides adequate estimates of observed concentrations even when downwash effects are not modeled explicitly.