6.4 Stability
Stability
typing is employed in air quality dispersion modeling to facilitate
estimates oflateral and vertical
dispersion parameters [e.g., the standard deviation of plume
concentration in the lateral (
y
) and vertical (z
)] used in Gaussian plume models. The preferred stability typing scheme,
recommended for use in regulatory air quality modeling applications is the
scheme proposed in an article by Pasquill in 1961 [33]; the
dispersion parameters associated with this scheme [often referred to as the
Pasquill-Gifford (P-G) sigma curves] are used by default in most of the EPA
recommended Gaussian dispersion models.
Table
6-3 provides a key to the Pasquill stability categories as originally
defined; thoughimpractical for
routine application, the original scheme provided a basis for much of the
developmental work in dispersion modeling. For routine applications
using the P-G sigmas, the Pasquill stability category (hereafter referred to
as the P-G stability category) should be calculated using the method
developed by Turner [34]; Turner's method is described in Section
6.4.1. Subsequent sections describe alternative methods for estimating the
P-G stability categorywhen representative cloud cover and ceiling data are
not available. These include a radiation-based method which uses
measurements of solar radiation during the day and delta-T at night (Section
6.4.2) and turbulence-based methods which use wind fluctuation statistics
(Sections 6.4.3 and 6.4.4). Procedures for the latter are based on the
technical note published by Irwin in 1980 [35]; user’s are referred
to the technical note for background on the estimation of P-G stability
categories.
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cloud
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