Consider the two receptors depicted in Figure 10. Both receptors are located at the same

distance xr from the source but at different heights above ground, i.e., zr1 and zr2. An example

profile of some parameter " is shown at the far left of the figure. The value of the effective

parameter used by AERMOD to represent transport and diffusion from source to receptor

depends on the location of the receptor. For receptor 1 the effective parameter value ~α

1

(shown in the figure as "eff1) is determined by averaging the values of " {z} between Hp {xr} and

zr1. Therefore, the layer over which this average is taken is smaller than the plume’s half-depth.

Whereas, (shown in the figure as "eff2) is determined by averaging " {z} over the full layer ~α

2

from Hp {xr} down through a depth of 2.15Fz {xr} since the receptor is located below the defined

lower extent of the plume.

Since F

z {xr }depends on the effective values of FwT and u, the plume size is estimated by

first using the plume height values of FwT {Hp } and u {Hp } to calculate F

z {xr}. As illustrated in

calculated. Once the averaging layer for a given plume and receptor is established the effective

values, , are computed as simple averages: ~α

where hb and ht are the bottom and top, respectively, of the layer of importance such that:

For all plumes, both limits are bounded by either the zr or Hp. For both the direct and indirect

sources ht, in eq. (43) is not allowed to exceed zi and if hb $ zi then α~ = α {z } . i

For plumes in stable conditions and for the penetrated source in the CBL, Hp is always set

equal to the plume centerline height (Δ h h ) , where hs is the stack height corrected for stack s s +

tip downwash and )hs is the stable source plume rise. The stable source plume rise )hs is

calculated from expressions found in Section 5.6.2.

In the CBL, the specification of Hp is somewhat more complicated. Because of limited

mixing in the CBL the center of mass of the plume will be the plume height close to the source

and the mid-point of the PBL at the distance where it becomes well mixed. Beyond final plume

rise, Hp is varied linearly between these limits.

Prior to plume stabilization, i.e., x < xf (distance to plume stabilization),

H h h , where )hd is the plume rise for the direct source (estimated from eq. (91)), p s dp = + Δ,

and )hp (= hep - hs) is the plume rise for the penetrated source, where hep (penetrated source

plume height) is calculated from eq. (94).

The distance to plume stabilization, xf , is determined following Briggs (Briggs 1975; Briggs

1971) as

 

 

 

 

 

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