AMBIENT TURBULENCE FOR USE IN CALCULATING DISPERSION
Lateral Dispersion from Ambient Turbulence
For the direct and Indirect sources int ehCBL the ambient component of the lateral dispersion is determined as follows:
The direct source plume height hed is calculated using eq (116)
The form of eq (88), with a=78 and p=0.3, follows frim an anlysisofthe lateral spread measured in the Prairie Grass experiment (Barad, 1985)
To account for the variations in release height from that at Prairie Grass we set
The value of hs , when applied in eq. (89) is limited to a minimum of zPG .
For sources in the SBL, the ambient component of the lateral dispersion is determined from
The Lagrangian time scale, in eq. (90) has been inferred from analysis of ground level concentrations in the Prairie Grass (Barad, 1958) experiments (see eqs. (88) and (89)) and extrapolated to more elevated sources and/or plume heights. This analysis resulted in a TLys given by
and
z, zPG are the pollutant release heights. The appearance of zmax
in the above accounts for plume heights greater
than the Prairie Grass source height, zPG , (note that TLys increases
with release
height). Furthermore, 
vm
in the above is limited by eq. (44).
Substituting eq. (91) into eq. (90) yields a form for the lateral dispersion in the SBL that is similar to that for the CBL (eq. (88)).
The
ambient component of the lateral dispersion for the penetrated source
(yap
),
which has been released below zi, but penetrates above, is
calculated using eqs.(90) with hes set
equal
to hep (the height of the penetrated source). However, for the
injected source, i.e. our released
above zi, no substitution is needed since these sources are
modeled as a stable source. I
Vertical Dispersion from Ambient Turbulence
For sources in the SBL, and for injected sources, the ambient portion of the vertical dispersion is composed of an elevated and surface portion. To produce a smooth transition between the expressions the following interpolation formula is used:
The expression for calculating hes is found in eq.(127).
The elevated portion of the vertical dispersion for the stable source in AERMOD follows the form of the familiar equation:
The vertical Lagrangian time scale (TLz) used in eq. (93) is taken from Venkatram ,et al. (1982)as
This form for TLzs is also used in CTDMPLUS. The length scale, l, interpolates between the neutral length scale, ln , and the stable length scale, ls , as
Under very stable conditions or at large heights, the composite length scale, l, approaches the stable value, ls . When conditions are near neutral, N is very small, and l approaches ln .
Substituting eq. (95) into eq. (94) and eq. (93) results in the following expression that is used by AERMOD to compute the elevated portion of the vertical dispersion for the stable source:
Now, the surface portion of vertical dispersion for the stable source is given by (following from Venkatram, 1992)
In the CBL, the ambient portion of the vertical dispersion, for the Direct and Indirect sources, is also composed of an elevated and surface portion. The penetrated source is assumed unaffected by the underlying surface since this source is assumed to be decoupled from the ground surface by its location above zi . The total ambient components of the vertical dispersion for the Direct
and Indirect sources is:
The elevated portion of vertical dispersion for the Direct & Indirect Source is given by the following expression
The bj ’s in eq. (99) result from the assumed bi-Gaussian p.d.f. (see Weil et al., 1997) and are given by
with R = 2 and the aj ’s given by eq. (72).
The
first constant (0.6) on the right-hand side of the 
b
expression is included to
maintain consistency
in the neutral-limit forms of z
for a surface source in the CBL
and the SBL. In this limit, zs for
the CBL (eq. (103)) is zero, and from eq. (97) for the
SBL, we have TS
~ 0.8u*x/u
To
avoid this near-surface, near-neutral discontinuity, the elevated
form for z (eq. (99))
remains non-zero even for HP
= 0. That is, the b
(for HP = 0) in
combination with eqs. (98) and (99) and the neutral
limit for w (= 1.3 u* from eq. (38)) yields
surface z
= 0.8u*x/u in the CBL
(consistent with the neutral limit).
For the Direct & Indirect Sources (CBL), the surface portion of the vertical dispersion is
calculated from
where: bc= 0.5
The
parameterization of eq. (103) is based on Venkatram’s (1992)
results for z due to a surface z source in the unstable
surface layer; i.e., 
. The parameterization is designed
to: 1) agree with Venkatram’s result in the limit of a surface
release (i.e., HP = 0), 2)
provide good agreement
between the modeled and observed concentrations from the Prairie
Grass experiment , and 3) decrease with source height in the surface
layer (HP < 0.1 zi
) and ultimately vanish for HP
> 0.1 zi . The constant bc
was chosen to satisfy the
second requirement above.
As indicated above the vertical dispersion for the penetrated source should be unaffected by the ground surface. Therefore, the vertical dispersion for the penetrated source is computed as the elevated portion of a stable source (eq. (96) ) with N = 0 and with no contribution from the surface component. The Brunt-Vaisala frequency, H, assumes the neutral limit of zero because the penetrated plume passes through the well mixed layer prior to penetration and back through that layer in dispersing to receptors within the mixed layer.
As always, the injected source is modeled as any source in a stable layer.
6 The AMS/EPA Regulatory Model AERMOD
6.1
General Structure of AERMOD Including Terrain
6.2
AERMOD Concentration Predictions in the CBL
6.2.1
DIRECT SOURCE CONTRIBUTION TO CONCENTRATION
CALCULATIONS
IN THE CBL
6.2.2
INDIRECT SOURCE CONTRIBUTION TO CONCENTRATION
CALCULATIONS
IN THE CBL
6.2.3
PENETRATED SOURCE CONTRIBUTION TO CONCENTRATION
CALCULATIONS
IN THE CBL
6.3
Concentrations in the SBL Calculated by AERMOD
6.4
Estimation of Dispersion Coefficients
6.4.1
AMBIENT TURBULENCE FOR USE IN CALCULATING DISPERSION
6.4.2
BUOYANCY INDUCED DISPERSION (BID) COMPONENT OF _ AND _
y z
6.4.3
COMPONENT OF DISPERSION COEFFICIENTS DUE TO
DOWNWASH
6.5
Plume Rise Calculations in AERMOD
6.5.1
PLUME RISE IN THE CBL
6.5.2
PLUME RISE IN THE SBL
6.6
Source Characterization
6.7
Adjustments for the Urban Boundary Layer
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