5.2.2 INDIRECT SOURCE CONTRIBUTION TO CONCENTRATION CALCULATIONS

IN THE CBL

The concentration due to the indirect source is calculated from:

where Q

rj= Qdj -)hi, andzis eitherzr.(for the horizontal plume state) orzp(for the terrainfollowingstate). As shown in Figure 14, the indirect plume is modeled as a reflected version of

the direct plume with an adjustment ()

hi- calculated from eq. (92)) to the reflected plume heightto account for the delay in vertical mixing due to plume lofting at the top of the boundary layer.

5.2.3 PENETRATED SOURCE CONTRIBUTION TO CONCENTRATION

CALCULATIONS IN THE CBL

For the penetrated source the concentration expression has a Gaussian form in both the

vertical and lateral directions. The concentration due to this source is given by:

where

zieffis the height of the upper reflecting surface in a stable layer (see Section 5.3) andziseither

zrfor the horizontal plume state orzpfor the terrain-following state. The vertical dispersionparameters (F

zp) are calculated as described in Section 5.5.1.2.The penetrated plume height,

hep, is taken as the height of the plume centroid above themixed layer and is calculated from eq. (94).

5.3

Concentrations in the SBLFor stable conditions, the AERMOD concentration expression (

Csin eq. (48)) has theGaussian form, and is similar to that used in many other steady-state plume models (e.g., HPDM

(Hanna and Paine 1989)). The

Csis given by

where

zieffis the effective mechanical mixed layer height, Fzsis the total vertical dispersion in theSBL (see discussion in Section 5.5), and

hesis the plume height (i.e., stack height plus the plumerise - see Section 5.6.2).

Above the mechanical mixed layer height,

zim(eq. (26)), the turbulence level is generallyexpected to be small and thus supports little vertical mixing of the plume. AERMOD is designed

(in the SBL) with an effective mixing lid,

zieff, that retards but does not prevent plume materialfrom spreading into the region above the estimated mechanical mixed layer. When the final

plume height is well below

zim, the plume does not interact withzim. When the plume is belowzimyet the “upper edge” (plume height plus 2.15 F

zs) of the stabilized plume reacheszim, the effectivemixing lid is allowed to increase and remain at a level near the upper edge of the plume. In this

way, AERMOD allows the plume to disperse downwards, but where the turbulence aloft is low,

vertical plume growth is limited by an effective reflecting surface that is folding back only the

extreme tail of the vertical plume distribution. There is no strong concentration doubling effect as

occurs with reflections from an assumed hard lid. Downward dispersion is primarily a factor of

F

waveraged from the receptor to the plume height. If the plume height is above the mixed layerheight, the calculation of the effective F

wwill include regions in which Fwis likely to be small.This, in effect, retards plume growth by an amount dependent upon how much of the plume is

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castellano: DISPER CUSTIC DESCAR RADIA italiano:

castellano: DIS CUS DES RAD english: DIS CUS DES RAD

português: DIS CUS DES RAD italiano: DIS CUS DES RAD