Estimation of Dispersion Coefficients
The
standard deviations for both the lateral and vertical concentration
distributions (
y and z
respectively)
result from the combined effects of: ambient turbulence a
); turbulence induced by plume buoyancy (b); and, enhancements from
building wake effects (d).
Dispersion
(ya,za
), induced by
ambient turbulence, is known to vary significantly with height, having its strongest
variation near the earth’s surface. Unlike present regulatory
models, AERMOD has been designed to account for this height
variation.
In
our earlier AERMOD formulations for ys
& za we attempted to account
for vertical variations in turbulence through
treatment of vertical in homogeneity. However, comparisons made using
the Prairie Grass data showed this approach to be inadequate.
Therefore, the current expression for za
is a combination of a direct
treatment of surface dispersion and the more traditional approach based on Taylor
(1921) for elevated dispersion. With this, we obtained good results
for all SBL comparisons. However, the results in the CBL indicated
that our treatment of lateral dispersion near the surface was
problematic. We corrected this with an empirical relationship for ya near the surface using the
full (CBL and SBL) Prairie Grass data set. In the remainder of this section we will
describe those formulations for ya & za
that resulted from this
empirical analysis.
In
the CBL, although the ambient induced dispersion for the Direct (D)
and Indirect (I) sources is treated differently than for the
Penetrated (P) source, the general approach of combining the effects
from ambient turbulence, buoyancy and buildings, is the same. For the
Direct and Indirect sources, the total dispersion coefficients (y
or z ) are calculated from the
following general y z expression (Pasquill and Smith, 1983):
For the penetrated source, the total dispersion is calculated as follows:
and building wakes are assumed to have little influence.
For the injected source, the total dispersion is calculated by eq. (87) as a source in table conditions.
In the SBL, the total dispersion is calculated as follows.
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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