The Impact of the Prairie Grass and the St. Louis Dispersion Experiments on Dispersion Modeling Practice

The Prairie Grass experiment was one of the most comprehensive field studies ever conducted to collect the data required to understand dispersion of releases in the surface boundary layer. It was carried out in Nebraska in 1956 (Barad, 1958) over a flat site covered with grass stubble 5-6 cm high. The experiment, conducted during July, August 1956, consisted of 70 runs, each lasting 10 minutes. The runs consisted of releasing SO2 at a height of 46 cm, and making concentration measurements at sampling arcs at distances of 50 m, 100 m, 200 m, 400 m, and 800 m from the release point. The concentration was measured at 1.5 m, except for the arc at 100 m where measurements were made on 6 towers at heights ranging from 0.5 m to 17.5 m. Meteorological measurements consisted of wind speed, temperature, and humidity at heights between 25 cm to 16 m on a tower. The concentration and meteorological data collected during the Prairie Grass experiment were used by Pasquill (1961) to construct the rural dispersion curves incorporated in the Industrial Source Complex (ISC) model; which until 2005 was USEPA's regulatory model. The data were also used to develop and evaluate the dispersion theory (van Ulden, 1978) that is used in the current generation of dispersion models such as AERMOD (Cimorelli et al., 2005).

The St. Louis study, conducted over the period 1963-1965, consisted of a series of 26 daytime and 16 evening experiments in which fluorescent zinc cadmium sulfide particles were released near ground level at two different locations in downtown St. Louis. Each release, which was typically 1 hour long, was sampled on arcs at 800 m to 16 km from the release point. A meteorological network of three stations on the outer area of the sampling area and an instrumented television tower tracked wind, temperature, and relative humidity. McElroy and Pooler (1968, Volume II) grouped the measured plume spreads using meteorological indices of dispersion, such as Richardson number and Pasquill stability class. Briggs (1974) fitted analytical curves to the data presented by McElroy-Pooler to formulate the urban dispersion curves used in EPA models such as ISC. The dispersion curves have also been used to interpret data collected in a recent field study conducted in Salt Lake City (Hanna et al., 2003).

This paper provides details of the design and implementation of these two classic dispersion experiments, the Prairie Grass and St. Louis experiments. It then describes the analysis of the observed data (Venkatram, 2005), and discusses the major impact of these field studies on the development of dispersion models that are currently used.

References:

Briggs, G.A., 1974: Diffusion estimation for small emissions. In ERL, ARL USAEC Report ATDL-106, U.S. Atomic Energy Commission, Oak Ridge, Tennessee.

Cimorelli, A., Perry, S.G., Venkatram, A., Weil, J.C., Paine, R.J., Wilson, R.B., Lee, R.F., Peters, W.D., and Brode, R. W., 2005: AERMOD: A dispersion model for industrial source applications. Part I: General model formulation and boundary layer characterization. J. Appl. Meteorol., 44, 682-693.

Hanna, S., Britter, R., and Franzese, P., 2003: A baseline dispersion model evaluated with Salt Lake City and Los Angeles tracer data. Atmos. Environ., 37, 5069-5062.

McElroy, J. L. and Pooler, F., 1968: The St. Louis dispersion study-volume II-analysis. National Air Pollution Control Administration, Pub. No. AP-53, US DHEW Arlington, 50 pages.

Pasquill, F. 1961: The estimation of the dispersion of wind-borne material. Meteor. Mag. 90, 33-49.

Van Ulden, A. P., 1978: Simple estimates for vertical dispersion from sources near the ground. Atmos. Environ. 12, 2125-2129.

Venkatram, A., 2005: An examination of the urban dispersion curves derived from the St. Louis dispersion study. Atmospheric Environment, 39, 3813-3822.