AN EXAMINATION OF THE PASQUILL-GIFFORD-TURNER DISPERSION SCHEME

The Pasquill-Gifford-Turner (PGT) dispersion curves are used in mast a ir quality models recommended for regulatory analysis of small industr ial sources. In this paper, we first examine the origin of these dispe rsion curves. We then examine the theoretical foundation of these curv es in the light of current understanding of dispersion, which we refer to as surface-layer similarity theory (SST). Numerical estimates of v ertical spread from PGT are then compared with those from SST for cond itions corresponding to the Prairie Grass experiment (Barad, 1958). We find that while the PGT dispersion estimates are broadly consistent w ith those from SST, they only represent a limited description of dispe rsion in the surface boundary layer. The use of the PGT dispersion sch eme requires careful consideration of the reference height used for th e wind speed that appears in the Gaussian dispersion equation. In fact , choosing a reference height (as PGT does) is not meaningful, because vertical spread is not independent of the transporting wind speed. Th e shortcomings of the PGT scheme can be readily corrected by adapting a version of a scheme based on surface-layer similarity theory. We sho w that the recommended method is just as easy to use as the PGT disper sion scheme, even within the Gaussian framework.