where the subscripts y and z are deleted from F

b because Fyb is assumed equal to Fzb. With the

exception of the CBL’s penetrated source the form of eq. (75) applies to all source dispersion in

both the CBL and SBL such that Fy,z becomes Fys,zs and Fyjs,zj and Fya,za becomes Fyas,zas and Fyajs,zaj

for the SBL and CBL, respectively. For the penetrated source, the total dispersion is assumed to

include ambient and buoyancy induced turbulence only; building wakes are assumed to have little

influence. For the injected source, the total dispersion is calculated as if the source were in the



A comment on notation: eq. (75) applies for both lateral and vertical dispersion in the SBL

and CBL. In references to the SBL, F

z appears as Fzs in the dispersion equation; Fza appears as

Fzas. In reference to the CBL, F

z appears as Fzj for the dispersion expression applicable to the

direct and indirect sources and Fza appears as Faj; for the penetrated source F

z appears as Fzp in the

dispersion expression.

5.5.1 DISPERSION FROM AMBIENT TURBULENCE Lateral Dispersion from Ambient Turbulence

In general terms, the ambient component of the lateral dispersion is based upon Taylor (1921)

such that:

where p = 0.5, u is the wind speed, Fv is the root-mean-square lateral turbulence velocity, and TLy

is the Lagrangian integral time for the lateral turbulence. Application of eq. (76) in a preliminary

version of AERMOD yielded poor concentration estimates in comparison to those found in the

Prairie Grass field experiments (Barad 1958). Specifically, the lateral spread was not well

matched. Therefore, the lateral dispersion expression was reformulated to allow for an empirical

fit to the Prairie Grass data.

Using an approach similar to that of Venkatram et al. (1984) TLy is found to be l/ F

v where l is

an appropriate length scale for lateral turbulence. Equation (76) can be written in terms of the

non-dimensional downwind distance X and a non-dimensional height scale " as:



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