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302 19. Transport and Dispersion of Air Pollutants
TABLE 19-2
Coefficients and Exponents for Brookhaven
C^iictinocc flaecoc
Type a b c d
B 2 0.40 0.91 0.41 0.91
B, 0.36 0.86 0.33 0.86
C 0.32 0.78 0.22 0.78
D 0.31 0.71 0.06 0.71
d
b
Note: cr y = ax ; cr z = oc (x is in meters).
Source: Adapted from Table 1 of Gifford (12).
parameters <j y and cr z are found by estimation from the graphs (Fig. 19-
6), as a function of the distance between source and receptor, from the
appropriate curve, one for each stability class (12). Alternatively, cr y and
cr z can be calculated using the equations given in Tables 19-4 and 19-5,
which are used in the point source computer techniques PTDIS and PTMTP
(16). These parameter values are most applicable for releases near the
ground (within about 50 m).
Other estimations of cr y and cr z by Briggs for two different situations,
urban and rural, for each Pasquill stability class, as a function of distance
between source and receptor, are given in Tables 19-6 and 19-7 (12).
TABLE 19-3
Pasquill Stability Categories
Isolation Night
Surface
wind Thinly overcast
speed or ^ 4/8 low =£3/8
(m/s) Strong Moderate Slight cloud cloud
<2 A A-B B _
2-3 A-B B C E F
3-5 B B-C C D E
5-6 C C-D D D D
>6 C D D D D
(for A-B, take the average of values for A and B, etc.)
Notes:
1. Strong insolation corresponds to sunny midday in midsummer in England; slight insol-
ation to similar conditions in midwinter.
2. Night refers to the period from 1 hr before sunset to 1 hr after sunrise.
3. The neutral category D should also be used, regardless of wind speed, for overcast
conditions during day or night and for any sky conditions during the hour preceding
or following night as defined above.
Source: From Pasquill (13).