Environmental Engineering Reference
In-Depth Information
expected wind direction EWD in the receptor-oriented (HI, LO) sector is defined
according to Eqs.
3a
and
3b
. Consequently, the deviation from the meteorological
mean wind direction WD is zero for winds perpendicular to the road, but the devia-
tion increases as WD turns parallel to the road.
F = Ф{0.5[HI + LO)/2 - WD]/s
m
}
(3a)
EWD = WD + s
m
Ф
-1
(F)
(3b)
As illustrated in Fig.
1
, the distance between receptor R and the point source S on
the line source changes with meteorological mean wind direction WD. Mean wind
direction WD is now replaced by EDW. The effective distance DIS (Eq.
4
), where
y is the perpendicular distance of R from the line source, determines the distance
of the receptor from the road in the expected mean wind direction EWD. As EWD
becomes never 0° or 180°, the distance DIS has always a finite value allowing
always an analytical solution of the line source dispersion model.
DIS = |y / sin (EWD)|
(4)
The probability (Eq.
2
) is directly used to scale the emissions. Expected wind direction
(Eq.
3b
) and effective distance (Eq.
4
) are applied to the computation of vertical and
lateral dispersion parameters s
z
and s
y
. The modification of the line source disper-
sion equation is restricted to the dilution term Q/[U*sin(WD)* s
z
], which is now
presented as P*Q/[U*sin(EWD)*s
z
], where Q is emission strength and U is the
mean wind speed.
Results and Discussion
The distributions of the parameters defined above against meteorological mean
wind direction WD are graphically illustrated at the nearest monitoring site ST1
using the meteorological time series of Nordbysletta. In the second phase we compared
the observed measurements with predictions (backgrounds subtracted) by CAR-FMI
when meandering was accounted as defined in Eqs.
1
-
4
.
Dependence of Probability, Expected Wind Direction
and Effective Distance on Meteorological Mean Wind Direction
The mean flow is oriented perpendicular to the road when the meteorological mean
wind direction WD is 90°. Probability of the plume reaching the receptor decreases
to 0.5 when mean wind turns parallel to the road and is roughly 1.0 with nearly
perpendicular flows, as seen in Fig.
4a
. If WD is nearly parallel to the road, the
probability may be slightly below 0.5 in case of finite line sources, which indicates
that the allowed wind sector (HI, LO) is smaller than half of the temporal wind
direction range defined in Eq.
1
.
