Environmental Engineering Reference
In-Depth Information
Those authors also present tracer data for SF
6
that suggest a dilution factor of 100
from a source 9 km SW of Lindcove to Ash Mountain, i.e., a distance of approxi-
mately 31 km. It is monitoring data of this type that can provide quantitative infor-
mation on LRT and assist in calibrating models.
A semi-quantitative interpretation of measured concentrations of CPY and
CPYO, assisted by use of the fate and transport model developed in this study is
provided here including the effects of transformation, transport, and dispersion/
dilution processes on downwind concentrations. A half-life of CPY of 3 h in air
(Table
6
) is assumed, but to test the sensitivity of the results to this half-life, the
effect of a value of 12 h is also used. A wind speed of 15 km h
−1
(4.16 m s
−1
) is
assumed for estimating the CTD. In the model, the concentration at a distance
downwind C
L
and distance x km can be estimated from the concentration C
1 km
at
1 km by (
20
) to give C
L
as:
æ
ç
k
H
ö
÷
´
L
U
æ
ç
k
H
ö
÷
´
C
x
C
x
M
M
-+
k
-
k
+
CTT
1
km
N
R
1
km
N
R
´
e
or
´
e
(20)
The exponent
N
is assigned an illustrative value of 1.5,
k
H
M
is assigned a value
of 0.002 h
−1
, which is small in comparison to the reaction rate constant of 0.231 h
−1
.
L
U
is the transit time in h. The rate constant for CPYO reaction was increased to
0.139 h
−1
, i.e., a half-life of 5 h and the yield of CPYO from CPY was increased to
æ
ç
k
H
L
U
ö
÷
æ
ç
ö
÷
´
70%. The CTD of CPY occurs when -+
k
M
is 1.0, or equivalently
R
U
when
L
is
.
k
H
æ
ç
ö
÷
k
+
M
R
These parameter assignments were selected by comparing available monitoring
data to predicted values from the simulation model and adjusting parameters by
hand until the selected input parameters resulted in simulated results that were com-
parable to measured concentrations. The objective was not to rigorously calibrate
the model, but rather to test the feasibility of developing and applying the LRT
model to estimate concentrations of CPY and CPYO at locations remote from site
of application. The results of applying the model developed in this study are sum-
marized for CPY (Table
10
) and illustrated for CPY and CPYO (Fig.
4
). Near the
area of application, such as at a distance of 1 km and assuming a 0.1 h air transit
time, air concentrations (C
1 km
) were assigned a value of 100 ng CPY m
−3
(~700 nPa).
At these short transit times, relatively little of the CPY would have been trans-
formed, although there might be transformation to CPYO on the surface and adjacent
atmosphere if conditions are sunny and favor greater concentrations of •OH.
Concentrations of CPY are primarily controlled by rates of evaporation and disper-
sionratherthanreactionswith•OH.
At a distance of 120 km and 8.4 h transit time, which is equivalent to two CTDs,
84% of the volatilized CPY would have been transformed and 16% would remain.
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