Instantaneous Spread of Plumes in the Surface Layer

Data are presented from two recent tracer campaigns regarding relative diffusion of surface-level plumes. One study consists of tests performed amid flat, rural terrain near Galen, Montana, while other experiments were conducted above a poplar forest with uniform canopy density and height near Boardman, Oregon. In both cases, sulfur hexafluoride was released near the surface at a Constant rate, and fast-response analyzers were used to measure plume concentrations along crosswind traverses and at fixed locations within 1 km of the source. This paper characterizes horizontal plume spread on near-instantaneous time frames during 29 tests,and the field data are used to test seven empirical and theoretical approaches for estimating relative diffusion coefficients using on-site wind data. Five of the equations utilize simple turbulence statistics to predict plume spread as a function of downwind distance; one method utilizes stability-based power-law formulas; and the last technique invokes: second-order closure. Overall, the seven approaches predict values similar to observed diffusion coefficients within a factor of 2 or better for most of the tests.