Environmental Engineering Reference
In-Depth Information
are smaller or larger respectively by factors such as 1.5-2.0 that can be estimated
from the dispersion parameter plots. If the area of application is larger by a factor
such as 100, i.e., 1 km
2
, then local concentrations downwind of sources would
probably be greater. Horizontal dispersion then merely mixes this air and most dilu-
tion is by vertical dispersion and the dilution factor discussed above would be of the
order of 10 rather than 60. There will also be contributions from evaporation from
other soils in the locality that have been subject to prior applications.
Due to uncertainty in calculating concentrations from the volatilization rate
Q
, it
is more convenient and probably more accurate to calculate downwind concentra-
tions from an assumed concentration at, for instance, 1 km from the source i.e.,
C
1 km
. By applying the equation for
C
at 1 km and at x km and taking the ratio, the
concentration at a distance x km can be shown to be
C
x
1
182
km
. The quantity
x
1.82
can be
.
regarded as a dilution factor. The wind speed cancels when the ratio of concentra-
tions is deduced. In practice, the exponent of x can be lesser, but this gives a reason-
able form of the dilution equation. When applied to monitoring data it was
determined that an exponent of 1.5 is more appropriate.
Combining the mass loss and the volume expansion gives the concentration
downwind as a function of
U, L,
and the CTT, which is represented by (
14
):
k
H
k
H
æ
ç
ö
÷
´
L
U
æ
ö
÷
´
CTT
Q
M
Q
M
-+
k
-
k
+
ç
R
R
C
=
)
´
e
or
C
=
)
´
e
(14)
(
(
p
´´ ´
U
r r
p rr
´´ ´
U
y
z
y
z
Or, more conveniently, by (
15
):
k
H
k
H
C
x
æ
ç
ö
÷
´
L
U
C
x
æ
ç
ö
÷
´
M
M
-+
k
-
k
+
CTT
R
R
1
182
km
1
182
km
´
e
or
´
e
(15)
.
.
It is these calculated concentrations (that do not include deposition) that can be
compared with monitoring results.
Limitations in predicting concentrations downwind of a source are caused by
the uncertainties inherent in the dispersion parameters,
Q, U
, and
L,
as well as the
possibility that a remote region has experienced CPY transport from multiple
sources. Equation (
15
) does, however, provide a basis for estimating concentrations
of CPY in air at more remote locations as far as 100 km from the source. If desired,
conservative assumptions can be applied. The effect of wind velocity can also be
evaluated. Lesser wind speeds cause an increase in the initial concentration, because
as the quotient
Q
U
increases and transit times
L
U
increase, the volatilized pesticide
is more concentrated in the region of application, there is more transformation
locally, and the impact of LRT would be reduced. The equation also enables the
relative roles of transformation and dilution by dispersion to be assessed. For exam-
ple, at relatively short distances downwind, dispersion dominates because the transit
time is short relative to the half-life for transformation. At greater distances and
Search WWH ::
Custom Search