Environmental Engineering Reference
In-Depth Information
and vertical dimensions. This is difficult to quantify because it depends on terrain
and local and recent meteorology. It is especially difficult if part of the parcel of air
is subject to fast upward convective transport (thermals) or during a storm. This
mass of air could be conveyed to higher altitudes and into a region of lesser concen-
trationsof•OH,fastertransport,andlowertemperatures,whichcouldenablethe
associated CPY to travel thousands of km. It is thus not surprising that small but
detectable concentrations can be found in remote locations such as Svalbard
(Hermanson et al.
2005
; Muir et al.
2004
). The largest concentration of 16 ng CPY L
−1
was found in ice from Svalbard in the 1980s, but concentrations measured more
recently are generally <1 ng CPY L
−1
. Although CPY is unlikely to be used for
agricultural purposes in such near-polar locations, there is a possibility that it was
used locally for other purposes, such as for control of biting insects.
A simple but approximate approach to estimate concentrations of CPY at dis-
tances from sources is to use a dispersion model to estimate concentrations at
ground level from a ground level source using standard air dispersion parameters
(Turner
1994
). To estimate concentrations at ground level downwind of applica-
tions, a simplified and approximate version of the Gaussian air dispersion model for
a ground level source can be used, which can be described mathematically (
12
) as:
Q
C
=
´´ ´
(12)
(
)
p
U
r r
y
z
In (
12
),
C
is concentration (g m
−3
),
Q
is emission rate (g h
−1
),
π
is the mathemati-
cal constant that is the ratio of the circumference of a circle to its diameter,
U
is
wind velocity (m h
−1
) and
ρ
y
and
ρ
z
are respectively the horizontal (crosswind) and
vertical Pasquill-Gifford dispersion parameters (m) that depend on downwind dis-
tance (km) and atmospheric stability class. This equation must be applied with cau-
tion because of variation of
U
as a function of height and topography, but it is used
here to suggest the form of an appropriate correlation.
Q
can be estimated from the
total quantity applied and an assumed fraction volatilized during a specified time
period. Plots of
ρ
y
and
ρ
z
(m) versus downwind distance x (km) have been given
(Turner
1994
), and can be expressed as correlations for stability class C (
13
):
091
.
0 91
.
r
=´ =´
100
x
and
r
61
x
(13)
y
z
For example, at 1.0, 10 and 100 km (the maximum distance)
ρ
y
is 100, 776 and
6,026 m, respectively and corresponding values of
ρ
z
are 61, 496 and 4,030 m. For
an evaporation rate of 1.0 g h
−1
into a wind of 1 m s
−1
, the concentrations are
14 ng CPY m
−3
at 1 km, 0.23 ng CPY m
−3
at 10 km and 0.0037 ng CPY m
−3
at
100 km. There is approximately an 8-fold increase in plume width and height from
1 to 10 km, and thus, there is about a 64-fold decrease in concentration. At 100 km,
there is a further 61-fold decrease in concentration. For larger areas of application,
concentrations of CPY would be correspondingly greater. Under other conditions of
moderate atmospheric stability, e.g., categories D or B, the dispersion parameters
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