Figure 13: Instantaneous and corresponding ensemble-averaged plume in the CBL

In addition to the non-Gaussian Fz, AERMOD has the following features. For buoyant

releases, there is no “final” plume rise assumed. Instead, the plume or particle trajectories are

determined by the addition of a distance-dependent plume rise and the random vertical

displacement caused by the vertical distribution of w. Ground level concentrations first appear

when the negative or downdraft velocities are sufficiently large to overcome the plume rise

velocity and carry plume sections to the surface. The direct transport of plume material to the


ground is treated by the “direct” source located at the stack. That is, the direct source treats that

portion of the plume’s mass to first reach the ground, and all subsequent reflections of the mass at

z = zi and 0 (where zi is the mixed layer height in the CBL (Cimorelli et al., 2004). For plume

segments or particles initially rising in updrafts, an “indirect” or modified-image source is

included (above the mixed layer) to address the initial quasi-reflection of plume material at z = zi,

i.e., for material that does not penetrate the elevated inversion. This source is labeled “indirect”

because it is not a true image source (i.e., as is found in models such as ISC) - the plume is not

perfectly reflected about zi. Thus, the indirect source treats that portion of the plume’s mass that

first reaches zi and all subsequent reflections of that particular mass at z = 0 and zi, For the

indirect source, a plume rise ()hi) is added to delay the downward dispersion of material from the

CBL top (see Figure 14); this mimics the plume’s lofting behavior, i.e., the tendency of buoyant

plumes to remain temporarily near zi and resist downward mixing. For non-buoyant sources the

indirect source reduces to the first image source (as found in ISCST3) resulting from the first

reflection at z = zi. Additionally, a “penetrated” source or plume (above the CBL top) is included

to account for material that initially penetrates the elevated inversion but is subsequently

reentrained by and disperses in the growing CBL.


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