FIELD TESTING AND EVALUATION OF DUST DEPOSITION AND ...

0.8
Vertical velocity W (m/s)
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
-0.1
-0.2
Background
interval #1:
-37 to -7
seconds
Influence
interval: -7
seconds to
+15 seconds
Recorded
pass time
Background
interval #2:
+15 to +45
seconds
00.5 10.0 19.5 29.0 38.5 48.0 57.5 07.0 16.5 26.0 35.5
Time (seconds)
Figure 5-6. Example of W-velocity as measured by the sonic anemometer in the vicinity of a vehicle
pass. The vertical lines indicate the start and end times for the two background intervals and the
influence interval.
5.2 Results
5.2.1 Dispersion and deposition downwind of the unpaved road at Ft. Bliss
For a ground-level release, it is as important to accurately represent dispersion as
deposition. The two processes act simultaneously and the removal of particles downwind
of a dust source is strongly dependent on both. The parameterization used for dispersion
in the box model was discussed in Chapter 4. This section examines the ability of the
numerical solution of the Atmospheric Diffusion Equation (ADE) and the Gaussian
distribution used in the ISC3 model (USEPA, 1995) to represent the concentration profile
downwind of a ground-level source. The results of the Ft. Bliss experiments are
compared with predicted ADE and ISC concentration profiles. This comparison is only
applicable to the distance between the unpaved road and the furthest downwind tower
(100 m). Data for comparison of concentration profiles over longer distances may be
obtained from a long-range scanning LIDAR that was operating concurrently with the
tower measurements. However, those data were not available at the time of the writing
of this report.
Both the ADE and the ISC3 are designed to handle continuous release sources
and consequently, downwind concentrations that do not vary with time (steady-state).
The experiments at Ft. Bliss examined the downwind plume from a vehicle that traversed
an unpaved road, first in one direction, then in the other. The result is that the time series
of the concentration measured at the downwind towers more resembles an instantaneous
puff release than a continuous source (see Figure 5-2). To compare the Ft. Bliss data
with the models, the concentration time series was integrated for each vehicle pass and
each location on the tower. That is
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