and turbulence data at plume height when applying CTDMPLUS in a regulatory application. As

previously stated, the model development goals for AERMOD include having methods that

capture the essential physics, provide plausible concentration estimates, and demand reasonable

model inputs while remaining as simple as possible. Therefore, AERMIC arrived at a terrain

formulation in AERMOD that considers vertical flow distortion effects in the plume, while

avoiding much of the complexity of the CTDMPLUS modeling approach. Lateral flow

channeling effects on the plume are not considered by AERMOD.

AERMOD captures the effect of flow above and below the dividing streamline by weighting

the plume concentration associated with two possible extreme states of the boundary layer

(horizontal plume and terrain-following). As is discussed below, the relative weighting of the

two states depends on: 1) the degree of atmospheric stability; 2) the wind speed; and 3) the

plume height relative to terrain. In stable conditions, the horizontal plume "dominates" and is

given greater weight while in neutral and unstable conditions, the plume traveling over the

terrain is more heavily weighted.

5.1 General Structure of AERMOD Including Terrain

In general, AERMOD models a plume as a combination of two limiting cases: a horizontal

plume (terrain impacting) and a terrain-following plume. Therefore, for all situations, the total

concentration, at a receptor, is bounded by the concentration predictions from these states. In

flat terrain the two states are equivalent. By incorporating the concept of the dividing

streamline height, in elevated terrain, AERMOD’s total concentration is calculated as a weighted

sum of the concentrations associated with these two limiting cases or plume states(Venkatram et

al. 2001).

The AERMOD terrain pre-processor (AERMAP) uses gridded terrain data to calculate a

representative terrain-influence height (hc) for each receptor with which AERMOD computes

receptor specific Hc values. Through this approach, AERMOD handles the computation of

pollutant impacts in both flat and elevated terrain within the same modeling framework thereby



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