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1.1.5 The Dispersion Parameters

1.1.5.1 Point Source Dispersion Parameters.

     Equations that approximately fit the Pasquill‑Gifford curves (Turner, 1970) are used to calculate σy and σz (in meters) for the rural mode.  The equations used to calculate σy are of the form:

where:

In Equations (1‑32) and (1‑33) the downwind distance x is in kilometers, and the coefficients c and d are listed in Table 1‑1.  The equation used to calculate σz is of the form:

where the downwind distance x is in kilometers and σz is in meters.  The coefficients a and b are given in Table 1‑2.

     Tables 1‑3 and 1‑4 show the equations used to determine σy and σz for the urban option.  These expressions were determined by Briggs as reported by Gifford (1976) and represent a best fit to urban vertical diffusion data reported by McElroy and Pooler (1968).  While the Briggs functions are assumed to be valid for downwind distances less than 100m, the user is cautioned that concentrations at receptors less than 100m from a source may be suspect

                          TABLE 1‑1

      PARAMETERS USED TO CALCULATE PASQUILL‑GIFFORD σy

σy = 465.11628 (x)tan(TH)

 

TH = 0.017453293 [c - d ln(x)]

Pasquill

Stability

Category

 

 

             c

 

 

                 d

A

24.1670

2.5334

B

18.3330

1.8096

C

12.5000

1.0857

D

8.3330

0.72382

E

6.2500

0.54287

F

4.1667

0.36191

where σy is in meters and x is in kilometers

 

 

 

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