CBL. This is handled by combining the penetrated and non-penetrated weighting factors ( F

h2|CBL). That is,

where fp (see eq. (55)) is the fraction of the source material that remains trapped in the CBL.

Using eq.(74), concentrations in the CBL (Cc) are calculated from eq. (71) with ( F

5.5 Estimation of Dispersion Coefficients

The overall standard deviations ( F

y,z) of the lateral and vertical concentration distributions

are a combination of the dispersion (represented by Fya, Fza) resulting from ambient turbulence,

and dispersion ( F

b) from turbulence induced by plume buoyancy. Building induced dispersion is

not included here since a separate approach (see Section 5.5.3) is taken for situations in which

building wake effects contribute to the total dispersion. Dispersion 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

the effect of variations of turbulence with height on dispersion through its use of “effective

parameters” (see Section 4.2), which are denoted by an overscript tilde, e.g., .

~σ

wT

AERMOD treats vertical dispersion from ambient turbulence (Fza) as a combination of a

specific treatment for surface dispersion and the more traditional approach based on Taylor

(1921) for elevated dispersion. Using this approach good agreement with observations was

achieved in the SBL. However, the results in the CBL indicated that the treatment of lateral

dispersion near the surface was problematic. This problem was corrected through the

development of an empirical relationship for Fya near the surface using the full (CBL and SBL)

Prairie Grass data set. A description of the resulting formulations for Fya & Fza is presented in the

next section.

The approach used to combine the above contributions to dispersion assumes that the effects

are independent of one another. Thus, the total dispersion coefficients, for situations that do not

include building downwash effects, are calculated from the following general expression (Pasquill

and Smith 1983):

 

 

n951 - n952 - n953 - n954 - n955 - n956 - n957 - n958 - n959 - n960 - n961 - n962 - n963 - n964 - n965 - n966 - n967 - n968 - n969 - n970 - n971 - n972 - n973 - n974 - n975 - n976 - n977 - n978 - n979 - n980 - n881 - n982 - n983 - n984 - n985 - n986 - n987 - n988 - n989 - n990 - n991 - n992 - n993 - n994 - n995 - n996 - n997 - n998 - n999 - n1000

 

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