FIELD TESTING AND EVALUATION OF DUST DEPOSITION AND ...

situations. Some of these limitations are listed here with a detailed discussion given in a
later section.
1. If the flux through the ceiling above the road dm ceil /dt is not negligible. This
implies that the thickness of the first layer of a regional air quality model is
important to the transport of dust.
2. If the flux upwind of the road dm ambin /dt is not negligible compared to the flux
from the road. If the flux upwind from a source is not negligible, a regional
model must accommodate carrying the dust from upwind to downwind.
3. If the approximation of a well mixed dust concentration from the ground to the
top of the control volume is not a good description of the natural situation.
4. If the assignment of the flux through the ceiling of the control volume as KC(x) is
different from the diffusion taking place in the natural situation.
5. If the assignment of the deposition as V d C(x) is different from the deposition
taking place in the natural situation.
6. If steady state conditions not observed.
7. If the downwind flux dm ambout /dt is not of the order of dm ambin /dt within a few
hundred meters to a kilometer, the result of the model would not be applicable to
most regional scale regional models. Since the grid-scale of regional models is of
the order of a kilometer, dm ambout /dt should be of the order of dm ambin /dt within a
few hundred meters. The downwind flux equilibration distance may vary
depending on local surface variables. This may have some implications for the
minimum spatial resolution of a regional dust transport model. Perhaps in
modeling, the distance could be varied as a function of the soil and vegetative
cover—which in turn implies the need of good geographic data bases of such
information.
8. The analysis above was done for the wind perpendicular to the road. For wind at
an angle to but not parallel the road, the analysis would be almost the same, but
with a wider “effective road width.” For an infinitely long road parallel to the
wind, however, the assumptions of the analysis would be violated and the solution
would not be useful
4.1.3.2a
Weaknesses of the Box Model
1. The model continues to pass dust mass through the CV ceiling even though realworld
situations exist where no vertical flux takes place or there is vertical flux
from above the ceiling into the CV.
2. The model assumes dust to be well mixed in the control volume even though there
are situations where dust continues to lie close to the ground (possibly caused by
atmospheric stratification.)
4.2 Inter-comparison of the ADE, ISC3, and the Gillette Box Model
Since the model Gillette has proposed is mechanistically different from the ADE
and the ISC3, we begin with a discussion of the assumptions in the Box Model.
Modeling results from the ADE and the ISC3 are then compared with those obtained
from the Box Model. Note that except were noted, all simulations for deposition apply to
particles with aerodynamic diameters of 8 µm. PM 10 is the mass of all particles with
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