above zim. Therefore, whether the plume is above or below zim, the region of low turbulence above

zim will have an appropriate effect on the concentration distribution within the mixing layer.

When the plume buoyancy carries the rising plume into the relatively non-turbulent layer

above zim, the reflecting surface is still placed at 2.15 Fzs above the effective plume height because

there will be plume spread due to plume buoyancy and downward mixing is still important.

Therefore, in the SBL, plume material is assumed to reflect off an elevated surface which is

defined as:

where Fzs in eq. (68) is determined from equations found in Section with FwT and u

evaluated at hes; not as an effective parameter. It is important to note that zieff depends on

downwind distance since Fzs is distance dependent. In fact, as eq. (68) suggests, this effective

reflecting surface is only folding back the extreme tail of the upward distribution. Also, if the

height of the receptor zr $ zieff then the effective reflecting surface is not considered. This

approach is also implemented for the penetrated source. For the penetrated and injected sources

zieff is calculated using eq. (68) with Fzs and hes replaced by Fzp and hep respectively.

5.4 Treatment of Lateral Plume Meander

In AERMOD we include the effect that lower-frequency, non-diffusing eddies (i.e., meander)

have on plume concentration. Meander (or the slow lateral back and forth shifting of the plume)

decreases the likelihood of seeing a coherent plume after long travel times. This effect on plume

concentration could best be modeled with a particle trajectory model, since these models estimate

the concentration at a receptor by counting the number of times a particle is seen in the receptor

volume. However, as a simple steady state model, AERMOD is not capable of producing such

information. AERMOD accounts for meander by interpolating between two concentration limits:

the coherent plume limit (which assumes that the wind direction is distributed about a welldefined

mean direction with variations due solely to lateral turbulence) and the random plume

limit, (which assumes an equal probability of any wind direction).

For the coherent plume, the horizontal distribution function (FyC) has the familiar Gaussian



where F

y is the lateral dispersion parameter (see Section 5.5). For the random plume limit, the

wind direction (and plume material) is uniformly distributed through an angle of 2B. Therefore,

the horizontal distribution function FyR takes the simple form:


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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