Environmental Engineering Reference
In-Depth Information
longer transit times, transformation is more influential. The equation can also be
used to estimate the fraction of the volatilized mass of CPY that will travel a given
distance, or be deposited, or the fraction of the applied mass that can reach a
specified distance. The quantity of transformation products can also be estimated.
Results of the model can also be used to design more targeted monitoring. The
model equation for
C
as a function of
C
1 km
, distance and time is applied later to test
agreement with monitoring data.
2.3
Formation and Fate of Chlorpyrifos-Oxon
Despite uncertainties in partitioning and reactivity of CPYO, it is possible to esti-
mate CPYO's rate of formation and concentrations in distant atmospheres relative
to CPY. These estimates can also be compared with monitoring data. It is assumed
forillustrativepurposeshere,thatCPYreactswith•OHtoformCPYOinairoron
surfaces with a molar yield of 30%; CPYO also reacts by the same mechanism.
Half-lives are assumed to be 3 and 12 h for the reactions of CPY and CPYO, respec-
tively. In the later evaluation, we assume a more conservative yield of 70%.
A parcel of air containing
M
0
mol of CPY will change in composition with time
and distance, forming CPYO, which in turn is degraded. This decay series is analo-
gous to a radioactive decay series. The quantity of CPY (
M
1
) will follow first order
kinetics, which can be described as (
16
):
dM
dt
1
=- ´
Mk
(16)
1
1
This can be integrated to give (
17
):
(
)
MM
kt
1
-´
=
(17)
1
0
Where:
k
1
is the first order rate constant. The corresponding differential equation for
CPYO
(M
2
) is given by (
18
):
dM
dt
2
=´´-´
YkMkM
(18)
1
1
2
2
Where:
Y
is the upper reported molar yield of 0.3, i.e., 30% and
k
2
is the transformation
rate constant of CPYO. It is likely that
Y
is larger than is stated above because other
transformation products are at lesser yields. Integration of this function gives (
19
):
Yk
kk
Me e
´
-
´´ -
(
)
-´ -´
kt kt
M
=
1
(19)
1
2
2
0
2
1
When
k
1
< k
2
, a “secular” or “transient” equilibrium is established with an
approximately constant ratio of the two species. In this case,
k
1
> k
2
and a “no
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