An Investigation of Instantaneous Diffusion and Concentration Fluctuations

Abstract

Instantaneous plume behavior is investigated via experimental and modeling results from a recent field campaign. The data consist of wind velocity and concentration measurements collected 700 m from a point source of sulfur hexafluoride during stable and neutral conditions. Fixed-point and traverse concentration data are analyzed in terms of concentration fluctuation statistics and diffusion coefficients. For the fixed-point data during this study, concentration fluctuation intensities vary between 0.7 and 4.5, intermittency factors are between 0.18 and 0.95, and peak-to-mean ratios range from 4.5 to 41.1. For traverse data, two methods are used to estimate the instantaneous diffusion coefficient σ?1, which is defined as the standard deviation of the crosswind concentration distribution of the instantaneous plume. Using the moment method, coefficients for this dataset range from 10.3 to 132.1 m. Using the peak concentrations and an assumption of a Gaussian concentration distribution in the vertical and horizontal directions, coefficients are between 6.7 and 22.4 m. The instantaneous diffusion coefficients derived from measured peak concentrations are shown to be less sensitive to plume meander than values calculated from the moment method. The values are related to simple meteorological parameters with three empirical equations, and all three equations predict diffusion coefficients within a few meters of the measured values. When used in a meandering plume model, the diffusion coefficients provide a way to estimate plume concentration fluctuation statistics from simple measurements of the wind. Overall, the model predictions are within a factor of 2 or better for the concentration mean, intensity, intermittency factor, and peak-to-mean ratio under stable through neutral conditions in the absence of large horizontal wind meander.