FIELD TESTING AND EVALUATION OF DUST DEPOSITION AND ...

particles through the top of the box musts always be in the upwards direction. These
conditions are closely met during a dust storm when the ground is a strong source, and
the vertical concentration profile does not vary over a large distance in the horizontal.
They are only met under neutral to unstable conditions for the case of emissions from
unpaved roads, and even then, only over very short downwind distances.
Fourth, the height of the wake generated behind a moving vehicle has an effect on
the fraction of particles removed as the plume travels downwind, though this effect is
only significant for stable conditions. Preliminary measurements suggested that the
vertical extent of the turbulence induced in the lee of a passing vehicle scales
approximately linearly with vehicle height. According to the ISC model, a larger fraction
of particles is removed as the dust plume travels downwind for small values of the
“injection height” than for larger values. This effect is more pronounced under stable
conditions when the dust plume is not rapidly expanded in the vertical direction. Overall,
in view of the uncertainties in quantifying deposition velocities, the “injection height” is
probably a secondary consideration.
Fifth, measurement of emission factors for seven different vehicles with gross
weights varying from 1,622 kg to 23,636 kg were at odds with the silt-based emission
factors suggested in AP-42 (USEPA). The Ft. Bliss measurements indicated that
emission factors were highly dependent on vehicle speed. Linear regression of emission
factor vs. speed for each of the seven vehicles resulted in R 2 values ranging from 0.37 to
0.95, with six of the seven vehicles giving values greater than 0.70. In contrast, the AP-
42 emission factor formulation does not consider vehicle speed. As a result, the AP-42
tends to over predict PM 10 dust emitted from unpaved roads when the vehicle speed is
less than about 20 to 25 mph. In addition, the use of an “average” vehicle weight in the
AP-42 equation tends to introduce a positive bias for small vehicles. According to the
measurements at Ft. Bliss, for passenger-sized vehicles (large, heavy trucks excluded),
the AP-42 appears to overestimate emissions for vehicle speeds under 35 mph. Emission
inventories often assume an average unpaved road travel speed of 20 or 25 mph. Under
those conditions, use of AP-42 would result in an overestimate of PM 10 emissions by
50% to 100% based on the field study results. This discrepancy, if it can be shown to
hold at locations other than Ft Bliss, would explain some of the incongruencies between
PM 10 dust emission inventories and the amount of crustal material found on filter samples
(Watson and Chow, 2000).
Sixth, prior to applying a correction to existing emissions inventories, it is
important to verify if a substantial amount of PM 10 is removed near the source and if it is
possible that there are other sources of error in the emissions estimates for unpaved road
dust. The conclusions of this report are based, in part, on two field tests that represent the
opposite extremes of atmospheric and land use conditions. In one case, the terrain
consisted of small roughness elements and the atmosphere was unstable. In the other
case, the roughness elements were building-sized and the atmosphere was stably
stratified. It is important to compare model data, ISC or other models, to measurements
performed under a range of conditions. Considering the uncertainties in deposition
velocities and dispersion parameters, model predictions must be taken with a grain of salt
until field data are available for comparison. The two field studies summarized in this
report utilized highly time resolved particle monitors operating at multiple heights. This
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