       where zPG = 0.46 m (Prairie Grass release height), and z MAX[z z ] . To insure that " does max PG = ;

not become unrealistically large for surface releases, z is not allowed below zPG (i.e., 0.46 m). In

the SBL, z = hes; in the CBL z = hs; for penetrated sources, z = hep. . As " becomes small for

large release heights, Fya would tend to grow linearly with downwind distance.

5.5.1.2 Vertical Dispersion from Ambient Turbulence

For sources in the SBL (and for sources in the CBL that are emitted directly into the stable

layer above the mixed layer), the ambient portion of the vertical dispersion (Fzas) is composed of

an elevated (Fzes) and near-surface (Fzgs) component. For hes < zi simple interpolation provides a

smooth transition between the two components.

For hes \$ zi Fzas is set equal to Fzes. The expressions for calculating hes are found in Section 5.6.2.

It should be noted, for sources in the SBL, that Fzas is the specific form of the ambient portion of

the vertical dispersion (i.e., Fza in eq. (75)).

In the SBL, the elevated portion of the vertical dispersion follows the form of eq. (76):

where FwT is the vertical turbulence due to the mechanical mixing (Cimorelli et al., 2004).

As with the lateral component, the Lagrangian time scale (TLzs) for the vertical turbulence can

be written in the form (Venkatram et al. 1984)

The length scale l is an interpolation between the limiting length scales for neutral conditions,

l h , and stable conditions :

where l h and . Under very stable conditions or at large heights, l n es = 0.36 l N s wT = 0 27 . ~σ

approaches ls. When conditions are near neutral, N is very small, and l approaches ln.

By combining eqs. (80), (81), and (82) we find the following expression that is used by

AERMOD to compute Fzes, the elevated portion of the vertical dispersion for the stable source:

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