FIELD TESTING AND EVALUATION OF DUST DEPOSITION AND ...

k
C int
=
peakend
k
C
peeakstart
dt
Equation 5-5
where C k int is the integral of the concentration C k at tower location k. For the purpose of
comparison, it was necessary to use a common scale for concentration. This was
accomplished by normalizing the concentrations to a common value for each tower.
While the location of the particle monitoring instruments (DustTraks) varied slightly
from one tower to the next, all towers were equipped with one monitor at a height of 1.26
m. Thus, the integrated concentrations for each individual pass and at each height on a
tower were normalized by the integrated concentration at 1.26 m for that tower. The
concentration profile from each of 109 passes was placed in a category according to the
value of the friction velocity, u * , associated with that pass. The normalized concentration
profiles were then averaged within each friction velocity category. There were four
categories in all for u * , corresponding to bins centered at 0.2 m/s, 0.3 m/s, 0.4 m/s, and
0.5 m/s. For example, all concentration profiles with a u * between 0.15 m/s and 0.25 m/s
were lumped together under the u * =0.2 category.
The validity of using Equation 5-5 to estimate the concentration profile from a
line source based on multiple passes of a moving point source (one vehicle traversing the
road) was assessed. On two occasions during the field campaign, large military convoys
traveled on the road near the towers. The convoys, consisting of tens of vehicles spaced
approximately 10 meters apart, are an excellent approximation to a true line source.
Figure 5-7 shows a comparison between the concentration profiles from a line source and
those from multiple moving point sources. Allowing for some scatter, especially in the
line source curves where only one data point was used for each convoy, the two sets of
profiles are very similar for both the 50 m (T2) and 100 m (T3) downwind towers. This
indicates that the treatment of an unpaved road as a line source is reasonable at the scale
of 100 meters or so. At larger distances, the observed differences between a true line
source and multiple moving point sources is expected to be small.
The integrated concentration profile for three towers at multiple downwind
distances from the road (7, 50, and 100 m) are shown in Figure 5-8 for two values of u * .
All tests were performed on sunny days after 10:00 AM. Therefore atmospheric stability
ranges from neutral to moderately unstable. As expected, the concentration profile is
steep near the road at the first tower. The profile becomes less steep as particles mix
vertically while being transported further downwind past the second and third towers.
The profiles for the u * =0.3 m/s case indicate a greater degree of dispersion than those for
the u * =0.5 m/s. This seems counter-intuitive since it is expected that higher values of
friction velocity would enhance dispersion. The data shown in Figure 5-8 do not
contradict that expectation; higher values of friction velocity also imply higher ambient
wind speeds and shorter transport times. Thus, while on a time basis, dispersion
increases with friction velocity, on a distance basis, the opposite appears to be true, at
least for the conditions of the experiment at Ft. Bliss.
5-11