FIELD TESTING AND EVALUATION OF DUST DEPOSITION AND ...

four heights. The root mean sum of squares (RMS) was then computed from the relative
difference between the calculated velocities and the measured velocities. The sum of the
RMS over all 110 intervals was used as a weighting function. The final estimate for z 0
was obtained by minimizing the weighting function. Note that this method is superior to
performing a regression for each 15-minute interval, since it provides a single value for z 0
that is applicable to all intervals.
20
15
DustTrak Reading (mg/m3)
10
5
Vehicle passes by DT_1
0
Baseline
DT_1 DT_2
DT_3
-5
18:02:10 18:02:27 18:02:44 18:03:01 18:03:19 18:03:36 18:03:53 18:04:11
Time
Figure 5-2. Example of time series of DustTrak PM 10 concentrations after the passage of a vehicle on
the unpaved road at Ft. Bliss. The arrows in the figure illustrate the start and stop times estimated
for a baseline reading and the dust plume passing through the downwind towers DT_1, DT_2, and
DT_3.
The final value of 0.005 m obtained for z 0 lies between those expected for “level
desert” (0.001 m) and “lawn” (0.01 m) (Seinfeld and Pandis, 1999). Figure 5-3 shows the
distribution of u * values over the 111 intervals. Ninety-five percent of the values of u *
fell between 10 and 50 cm/s, with values between 30 and 40 cm/s occurring most
frequently (35%). Examples of how the wind speed fitted to a logarithmic vertical profile
compares with the measured wind speed are given in Figure 5-4 for conditions
corresponding to the lightest, most frequent, and heaviest winds observed over the 110
15-minute intervals. In general, the logarithmic fit is reasonable for all cases, though the
wind speed at 12.2 meters is overestimated in the case of light winds. This may be caused
by small differences in the resistance to motion among the four cup anemometers used.
The fact that in Figure 5-4a the measured wind speed at 12.2 meters is slightly less than
that at 5.7 meters supports this hypothesis.
5.1.1.1 Emission Tests
Road dust emissions were created by having a test vehicle travel back and forth
along the roadway for a number of passes. This ranged from a minimum of 20 to a
maximum of 102. The mean number of vehicle passes was 49. The test vehicles traveled
at set speeds of 16, 24, 32, 40, 48, 56, 64, 72, and 81 km/hr. Not all vehicles attained the
highest speeds for safety considerations. After two passes (one heading south and one
returning north) at the same speed, the vehicle speed was increased incrementally to the
5-4