FIELD TESTING AND EVALUATION OF DUST DEPOSITION AND ...

Table 5-2 enumerates the speed-dependent emission factors estimated from the
measurements at Ft. Bliss alongside those determined according to Equation 5-6.
Samples of road dirt were obtained both prior to and at the end of the field campaign.
The silt contents for those samples were 7% and 4%, respectively. The higher silt
content at the beginning of the study reflects two phenomena. First, on the day prior to
sample collection, there was a windstorm that may have deposited some fine soil material
onto the unpaved road. Second, the field tests themselves may have resulted in the
depletion of fine soil material from the road. Thus, during the field tests, it is reasonable
to assume that the silt content on the unpaved road was somewhere in between 4% and
7%. The AP-42 factors calculated using these two values as lower and upper bounds
appear in Table 5-2.
04/23_22 Freightliner
EF (g/vkt)
2500
2000
1500
1000
y = 38.397x
R 2 = 0.9458
Average
Linear (Average)
500
0
0 10 20 30 40 50 60
Speed (mph)
Figure 5-12. Example of emission factor dependence on vehicle speed. The vertical bars are the
standard deviations among the three downwind towers.
In general, the AP-42 factors overestimate PM 10 emissions for vehicles traveling
at low speeds compared to the measurements. This is illustrated in Figure 5-13, where
the ratios of the AP-42 values to measured values are plotted as a function of vehicle
speed. The top panel in the figure summarizes the data for all the vehicles in Table 5-2.
The figure illustrates that, depending on the assumed silt content, for speeds
approximately less than 20 mph, AP-42 overestimates PM 10 emissions while for speeds
greater than 25 mph, AP-42 underestimates emissions. It is noteworthy that most
emissions inventories assume that vehicles traveling on unpaved roads average speeds of
either 20 or 25 mph. This suggests, that at least from the standpoint of emissions
inventories, use of the data from the Ft. Bliss experiments would give results similar to
the AP-42. However, the vehicles in this study are on average much heavier than one
would expect to find on rural unpaved roads (industrial haul roads may be an exception).
The bottom panel of Figure 5-13 shows that if only passenger-sized vehicles are
considered, then the AP-42 may substantially overestimate PM 10 emissions for speeds up
to about 35 mph. In fact, in the 20 or 25 mph range, the AP-42 values are 1.5 to 2 times
those measured at Ft. Bliss. This suggests that the AP-42 emission factors may be
positively biased for passenger-sized vehicles. This bias can be a source of discrepancy
5-18