Fluctuating concentrations in atmospheric dispersion

Abstract

This thesis presents theoretical models and experimental results of average and instantaneous concentration measurements from a series of atmospheric dispersion experiments conducted under both unstable and stable meteorological conditions. The experiments were undertaken at two different sites, over both flat and gently rolling terrain. Two types of surface-level point aerosol sources were used. One is a fog-oil smoke and the other is a hexachloroethane chemical smoke. Measurements of concentration at points along crosswind transects were taken over time periods of an hour at distances to several kilometers from the source. These measurements included both aerosol photometer records of the instantaneous concentration taken at a 1 Hz sampling rate and aspirated filters for mean concentration measurements.The flat terrain site was located at Camp Atterbury, Indiana, while the gently rolling terrain site was near Red Bluff, California. Meteorological measurements at these sites included both tower measurements and upper-air balloon soundings. These measurements were used in determining the atmospheric boundary layer scaling parameters in the unstable tests and in characterizing the complex wind field for the stable tests.The data compare favorably with developed models for both the mean and variance in concentration. Concentration fluctuation intensity ranges from 2 near the plume centerline to greater than 20 at the plume edge. Intermittency is important at all locations, with positive concentrations recorded on the mean plume centerline only 20% to 50% of the time. Point concentration histograms are shown to agree with the exponential distribution for concentrations greater than zero.Spectra of the concentration data show an inertial-convective subrange with a $-$5/3 power law versus frequency behavior. Integral time scales of the concentration records at all individual sampling points are approximately constant within a test and are equal to the mean duration of episodes or bursts in which the concentration is greater than zero. The probability distribution of individual burst durations at each sampler shows an exponential distribution.U of I OnlyETDs are only available to UIUC Users without author permissio