Using Data from Urban Tracer Experiments for Model Evaluation

This paper compares results from the analysis of data from three tracer experiments designed to understand dispersion in urban areas. The first study was conducted in St. Louis over the period 1963-1965, the second in Barrio Logan, San Diego in 2001, and the third in Wilmington, Los Angeles in 2004. These cities differ in average building height and building density. The data from St. Louis have been summarized in terms of dispersion curves, referred to as McElroy-Pooler curves, and have been incorporated in regulatory dispersion models, such as the ISC model (Industrial Source Complex Model) or AERMOD (AMS/EPA Regulatory Model). The dispersion curves from St. Louis provided a poor description of the data from the field studies conducted in Barrio Logan in the summer of 2001 and Wilmington in the summer of 2004. In all three field experiments, a model that used on-site turbulence data in the urban boundary layer provided adequate descriptions of the observed horizontal plume spread as well as ground-level concentrations. A comparison of data from the tracer experiments indicated that while dispersion in urban areas is substantially enhanced over that in rural areas, its characteristics differ from city to city. Dispersion over urban areas depends on the flow properties of the urban boundary layer, which in turn is a function of urban surface characteristics and morphology. Thus, dispersion curves corresponding to data from one city cannot be readily transferred to another city.