Environmental Engineering Reference
In-Depth Information
10
4
10
3
A
A
B
10
3
C
B
D
E
10
2
C
F
D
10
2
E
10
A Extremely
B Moderately
A Extremely
B Moderately
F
Unstable
Unstable
C Slightly
E Slightly
C Slightly
E Slightly
D Neutral
D Neutral
10
Stable
Stable
F Moderately
F Moderately
1
1
10
2
10
2
10
4
Distance from source, m
10
3
10
5
10
4
Distance from source, m
10
3
10
5
FIGURE 6.32
Vertical and horizontal dispersion coefficients for use in the Gaussian air
dispersion model. (From Turner, D.B. 1969.
Workbook of Atmospheric Dispersion Estimates.
Washington, DC: HEW.)
The effective height of the stack
z
s
(m) is the sum of the actual stack height
z
1
(m)
and the plume rise
z
(m).The plume rise can be estimated using a number of different
expressions (Wark, Warner, and Davis, 1998).
Oneparameterthatisoftensoughtinairpollutionmodelingistheso-calledground-
level center-line concentration
Δ
Q
s
e
−
(z
s
/
2
σ
z
)
.
[
A
]
(x
,0,0
)
=
(6.146)
π
U
σ
y
σ
z
The maximum concentration at a receptor along the ground-level center-line will be
given by substituting
z
s
z
:
=
2
σ
0.1171
Q
s
U
[
A
]
max
=
.
(6.147)
σ
y
σ
z
E
XAMPLE
6.17 A
CCIDENTAL
S
PILL
S
CENARIO
Early morning, on a cloudy day, a tanker truck carrying benzene developed a leak
spilling all of its 5000 gallons on the highway, forming a pool 50 m
×
50 m. What is
the ground-level concentration 1 km downwind of the spill? The wind speed averaged
2 m/s and the ambient temperature was 25
◦
C.
The spill was 1500 m
2
in area of pure benzene. Evaporation of benzene from a
pure phase is gas-phase mass transfer controlled with
k
g
=
0.37 m/s.
P
i
=
0.125 atm.
Hence
Q
s
=
(
0.37 cm/s) (3600 s/h)(0.125 atm)(1500
×
10
4
cm
2
)
/[(8.205
×
10
−
5
cm
3
atm/K/mol)
(
298 K
)
]=
1.7
×
10
5
mol/h
=
3.7
×
10
3
g/s.At
U
=
2 m/s,foranovercast
continued
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