Estimating Plume Dispersion-A Comparison of Several Sigma Schemes

Abstract

The lateral and vertical Gaussian plume dispersion parameters are estimated and compared with fieldtracer data collected at II sites. The dispersion parameter schemes used in this analysis include Cramer'sscheme, suggested for tall stack dispersion estimates, Draxier's scheme, suggested for elevated and surfacereleases, Pasquill's scheme, suggested for interim use in dispersion estimates, and the Pasquill-Gifford schemeusing Turner's technique for assigning stability categories. The schemes suggested by Cramer, Draxler andPasquil estimate the dispersion parameters using onsite measurements of the vertical and lateral windvelocity variances at the effective release height. The performances of these schemes in estimating the dispersion parameters are compared with that of the Pasquill-GitFord scheme, using the Prairie Grass andKarlsruhe data. For these two experiments, the estimates of the dispersion parameters using Draxier's schemecorrelate better with the measurements than did estimates using the Pasquill-Giffoth scheme. Comparisonof the dispersion parameter estimates with the measurements suggests that Draxier's scheme for characterizingthe dispersion results in the smallest mean fractional error in the estimated dispersion parameters and thesmallest variance of the fractional errors. Centerline values of surface concentration are estimated using the Gaussian plume model and comparedwith The concentration values determined during five field experiments three for near-surface releases andtwo for elevated releases. Comparison of the concentration estimates with the measurements from the fivefield experiments suggests that Draxier's scheme for characterizing the dispersion results in the smallest meanfractional error in the concentration estimates and the smallest variance of the fractional errors. Twoalternative characterizations of the dispersion, formed by altering Draxler's scheme to be independent ofrelease height, are shown to perform almost as well as Draxier's scheme.