FIELD TESTING AND EVALUATION OF DUST DEPOSITION AND ...

In the laboratory, the deposited material was weighed then separated into four size
bins (d 30 µm, 10 d < 30 µm, 3 d < 10 µm and d < 3 µm) by gravity settling in
water, based on the Stokes velocity difference between various size particles (see
Equation 2-4). The dry and wet fractions collected in the field were analyzed separately
and the results were added to get the total mass deposited. The size-separated fractions
were dried and weighed and the amount of deposition on the AV in each size range was
calculated by mass balance. The gravity separation method was validated in preliminary
experiments using surrogate mixtures of particles with known size and the separations of
the field samples were run in duplicate after splitting the sample using a riffler. The
quality of the size separations obtained from the field samples was also verified using
SEM.
For the artificial vegetation (AV) experiments, the deposition velocity was
determined using Equation 3-2 expressed on a mass basis. The GRIMM particle number
data were converted to particle mass. The amount deposited was determined from
weighing the 3 d < 10 µm and d < 3 µm wet separated fractions. The deposition
velocity was calculated based on the horizontal projected area of the collector outline.
3.1.3 Flat Substrate Method Testing
This study of field measurement of vehicle-generated dust deposition required
testing to evaluate alternative procedures, materials, sampling locations, sampling times,
and data analysis methods. Twenty experiments measuring particle deposition on flat
substrates were collected using various locations and numbers of vehicle trips. Samples
were collected 1 or 2 m above grade at 5 and 10 m from the unpaved road. In addition, a
blank sample was collected 100 m upwind from the road.
Four substrates were tested to determine which surface gave the best data:
1. Dry Polycarbonate filters (Isopore Membrane filter, 47 mm diameter,
0.2 µm GTTP, Millipore)
2. Oiled Polycarbonate filters, created by spraying DOW 316 (Slipicone®
316 release agent, Dow Corning Corp., Midland, MI) on the filters and drying them
3. Carbon tape (Catalog # 16073®, 8 mm, Ted Pella Inc, Redding, CA)
4. Double sided clear tape (Scotch ®, 3M, St. Paul, MN)
3.1.4 Vegetative Method Testing
Six experiments were conducted to evaluate deposition velocities by the artificial
vegetation method. A combination of different distances from the road (5 and 50 meters)
and different artificial vegetation substrates (Fir Garland and artificial Ivy) were used.
Deposition velocity was calculated using different areas in Equation 3-2:
horizontal, total and projected. The horizontal projection area was used because it
compares to what Slinn used in his calculations. It measures the enhancement of removal
provided by vegetation. It is also easy to measure and is applicable to flux studies. The
frontal projection area of the vegetation was used since it relates to the SEM flat substrate
experiments. It represents the area of direct contact with the bulk of the dust cloud. The
total area represents all surfaces available for deposition. Since not all exposed surfaces
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