DETERMINATION OF ALONG-WIND DIFFUSION
FROM A QUASI-INSTANTANEOUS LINE SOURCE
Gennaro H. Crescenti and Thomas B. Watson
Field Research Division Air Resources Laboratory
National Oceanic and Atmospheric Administration
Idaho Falls, Idaho
Robert E. Keislar
Desert Research Institute
Reno, Nevada
Christopher A. Biltoft
Meteorology & Obscurants Division
Dugway Proving Ground
Dugway, Utah
Little data exist to characterize along-wind diffusion, especially for distances of more than a few kilometers. While there is a wealth of information on cross-wind and vertical diffusion, relatively few studies have been specifically designed to measure the along-wind diffusion parameter sx and how it varies with distance or atmospheric stability. In contrast to continuous sources of industrial air pollution in which along-wind diffusion can be neglected, sx is an important parameter when considering instantaneous or quasi-instantaneous sources. The lack of understanding of along-wind diffusion is a major concern in military operations in which ground forces may be subject to chemical and/or biological agents discharged from sudden explosive releases. In civilian applications, accidental releases of toxic pollutants from stationary or mobile containment vessels (e.g., chlorine tank, train car) can pose an immediate threat to life and property. Inadequate or erroneous parameterization of sx in transport and dispersion models could have disastrous consequences if used for military or emergency response decisions.
The investigation of sx can be aided by use of a two-dimensional geometry (i.e., line release) which eliminates cross-wind variability. This was attempted during the Over-Land Atmospheric Dispersion (OLAD) field experiment conducted at the Dugway Proving Ground (DPG) in September 1997. The objective OLAD was to acquire a database on along-wind diffusion over 2 to 20 km distances for verification and improvement of the Vapor, Liquid and Solid Tracking (VLSTRACK) model and the Second-Order Closure Integrated Puff (SCIPUFF) dispersion model. A series of early and late morning trials was conducted in which sulfur hexafloride (SF6) was released by truck or aircraft along a line approximately perpendicular to the mean wind. Three lines of whole-air samplers and continuous analyzers were used to measure SF6 concentrations downwind of a quasi-instantaneous line source. Surface and upper-air meteorological measurements were also acquired. The assumption of quasi-two-dimensional geometry is tested by comparison of intra-line variability with duplicate sample variability. While edge-effects and possible cross-wind mesoscale influences were observed, the assumption is generally acceptable. Values for along-wind diffusion as a function of distance and atmospheric stability are presented