5.5.2 BUOYANCY INDUCED DISPERSION (BID) COMPONENT OF Fy AND Fz

For all plumes, the buoyancy induced dispersion (BID) is calculated following Pasquill

(Pasquill 1976) and Weil (1988b) as

where )h is the plume rise appropriate for each of the plume types (direct, indirect, penetrated,

and stable plumes). The direct source plume rise is calculated from eq. (91), stable plume rise

()hs ) is calculated from eq. (95) and the plume rise for the penetrated source Δh h h p ep s = −

(where hep is calculated from eq. (94)).

5.5.3 TREATMENT OF BUILDING DOWNWASH

AERMOD incorporates the Plume Rise Model Enhancements (PRIME) (Schulman et al.

2000) algorithms for estimating enhanced plume growth and restricted plume rise for plumes

affected by building wakes(U.S.Environmental Protection Agency 1995). PRIME partitions

plume mass between a cavity recirculation region and a dispersion enhanced wake region based

upon the fraction of plume mass that is calculated to intercept the cavity boundaries. These

boundaries are established from estimates of the locations of the lateral and vertical separation

streamlines. Dispersion of the recirculated cavity mass is based on building geometry and is

assumed to be uniformly mixed in the vertical. At the boundary of the cavity region, cavity mass

is emitted into the wake region. Here, it is combined with plume mass that was not captured by

the cavity and dispersed at an enhanced rate based on source location, release height and building

geometry. The enhancement of turbulence within the wake decays gradually with distance,

allowing for a smooth transition to ambient levels of turbulence in the far-field. A probability

density function model and an eddy diffusivity model (Weil 1996) are used for dispersion

estimates in the near-wake and far-wake regions, respectively. Plume rise, for sources influenced

by a building, is estimated using a numerical model that includes effects from streamline

deflection near the building, vertical wind speed shear, enhanced dilution from the turbulent wake

 

 

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