VELOCITY OSCILLATIONS AND PLUME DISPERSION IN A RESIDENTIAL NEIGHBORHOOD DURING WINTERTIME NIGHTS

Measurements of velocity and tracer plume concentrations during stable atmospheric conditions were obtained in the Boise River Valley as part of the EPA Integrated Air Cancer Project during December, 1986. Wind speed, temperature, and wind direction were measured at two levels on a 30 m tower. Spectral and autocorrelation analyses of the velocity component data clearly indicate the occurrence of wave-like oscillations in the flow and the almost complete lack of turbulent energy. The predominate wave-like motion had an oscillation period of about 1000 s. Halogenated atmospheric tracers were released from as many as four houses during the night-time drainage conditions. Hourly averaged horizontal dispersion coefficients were very large compared to the Pasquill-Gifford curves and the urban McElroy-Pooler dispersion curves. The time-averaged dispersion coefficients formed an upper bound on very short-term dispersion coefficients obtained from mobile traverses of the tracer plume with a continuous SF6 analyzer. These results agree with the concept of a narrow instantaneous plume with a broad meander driven by the observed 1000 s oscillation. Vertical dispersion rates were slightly smaller than the Pasquill-Gifford class F curve. Results from a single tracer release from a side canyon near the neighborhood showed that drainage flow from the tributary impacted the main residential sampling site at Elm Grove Park and represented a significant fraction of the upstream air flow at Elm Grove Park. For sources with equal emission rates, a source in the tributary adds about 10% to the mean of the concentration caused by a neighborhood source. © 1990.