FIELD TESTING AND EVALUATION OF DUST DEPOSITION AND ...

average wind speed measured at 12 meters over all emissions tests was 6.2 m/s. Thus, a
transport time of 30 seconds is probably, on average, a conservative overestimate.
The wind speed and direction were measured only at the far downwind tower
(DT_3). Therefore, for using Equation 5-1, it was necessary to use wind speeds and
directions measured at DT_3 for DT_1 and DT_2. The possibility of introducing errors
by this method was examined. The horizontal PM 10 fluxes were calculated for DT_3 only
and averaged over the test vehicle speed. This procedure was performed twice, once with
wind data that corresponded to the time of the passing of the dust plume, and once with
wind data that was retarded by 30 seconds. For example, the flux at 13:30:00 was
calculated using wind data measured at 13:30:00 and also with wind data measured at
13:29:30. The purpose of this exercise was to subject the flux calculations for DT_3 to
the same uncertainties that the data from DT_1 and DT_2 would be experiencing. In
comparing the two resultant sets of horizontal fluxes, no significant difference were
found (slope = 1.01, R 2 =0.98, n=53, intercept was forced to 0), indicating that the use of
DT_3 wind direction and wind speed to calculate horizontal fluxes of PM 10 at DT_1 and
DT_2 would not introduce errors.
5.1.2 Sonic Anemometer Tests
A three-dimensional sonic anemometer (“A” style probe, applied Technologies
Inc) was used to assess the effect of vehicle size on the initial distribution of the dust
plume generated. The anemometer was first collocated with the cup anemometer and the
wind vane at the 12.2 m height on DT_3 for three sample days (4/18/02 – 4/20/02). The
sonic anemometer was set to measure the U, V, and W components (corresponding to the
x, y, and z directions) of the velocity at a frequency of 10 Hz. Figure 5-5 shows the
comparison between wind speeds measured with the cup anemometer and a sonic
anemometer; part a. shows a time series; parts b. through d. show comparisons of 1-
second averages and 1-minute averages. In general, the cup and the sonic anemometers
track each other well, though the sonic anemometer gives smaller values of wind speed as
indicated by the slopes of the regressions in Figure 5-5 b-d (0.91 – 0.94). The significant
improvement in R 2 values between the 1-second (0.84-0.89) and 1-minute averaged data
(≥ 0.98) is to be expected since the cup anemometer has a nominal response time of 1
second (approximately the time required to register 1/3 of the difference between changes
in wind speed) while the sonic anemometer response is practically instantaneous.
On 4/21/02 between 19:44 and 22:15 a series of tests were conducted to assess the
magnitude of the turbulent wake behind a passing vehicle. The tests occurred at night
because under stable nighttime atmospheric conditions, the turbulence generated by a
vehicle can be more readily identified with a sonic anemometer than when the
atmosphere is unstable and the background turbulence level is much higher. The
downwind tower closest to the unpaved road was moved to the edge of the road. Two
markers were placed on the unpaved road, one at a distance of 2 meters from the edge of
the road (and the tower) and the other at a distance of 6 meters. The sonic anemometer
was mounted on the tower at a height of 0.9 meters above ground level (AGL) and
protruded 1.1 meters towards the road (i.e. approximately 1 meter from the near marker).
Three vehicles with considerably different profiles were tested: a Dodge Neon, a 1979
Chevy cargo van, and a 24-foot moving truck. Each vehicle was driven through the two
markers on the road once heading north and once heading south at 16, 32, 48, and 64
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