FIELD TESTING AND EVALUATION OF DUST DEPOSITION AND ...

the ground, where particles have a better chance to deposit. In addition, the areas of
stagnation on both the windward and leeward sides of the containers may have enhanced
the removal rate of PM 10 particles.
The conditions during the Ft. Bliss test may be more true to everyday reality than
the Dugway tests. This assertion is supported by two observations. 1.) Unpaved road
dust is more likely to be an air quality concern in arid, open-range regions characterized
by much of the American Southwest, and 2.) It is likely that the majority of travel on
unpaved roads occurs during the day when the atmosphere is either neutral or unstable.
With these observations in mind, it seems that in areas were unpaved road dust is of
greatest concern, the removal of particles by deposition is smallest in magnitude. This
goes against a recent movement to divide fugitive dust emissions by a factor of 2-4 in
order to account for deposition within several hundred meters of the road. We note
however that the conditions at Ft. Bliss are not representative of every area in the United
States that is concerned with road dust. There may be some instances where there is
thick vegetative cover very close to an unpaved road (e.g. farms). This possibility is
revisited later in the text.
Second, for the purposes of modeling the transport and removal of dust particles
near an unpaved road source, the Gaussian-style models, such as EPA’s ISC3, provide an
imperfect, but reasonable preliminary approach. Dispersion and deposition are the two
main processes that must be accounted for in any model used. The ISC model was
compared to the one-dimensional Atmospheric Diffusion Equation (ADE) and to data
obtained at Ft. Bliss. The ADE relies on similarity theory for the parameterization of
dispersion. Based on vertical concentration profiles measured as part of the Ft. Bliss
experiments, it was determined that ADE over predicts dispersion in the region very close
to the source (i.e. 500 m downwind or less) and that the ADE is not suited for modeling
over short downwind distances. The ISC model uses a distance-based dispersion
parameter that depends on atmospheric stability (σ). Comparison of predicted
concentration profiles with those observed at Ft. Bliss indicated that the ISC captures the
approximate shape of the dust plume. However, whereas the measured profiles varied in
steepness at different values of the friction velocity u * , the ISC profile was invariant.
This is because the friction velocity is not included in the parameterization of σ.
Several different models for dry deposition were compared. They produced
deposition velocities that were in agreement within a factor of two or so. The
formulation for deposition velocity in the ISC model has the advantage that it only
requires knowledge of three parameters: The friction velocity u * , the roughness height z 0 ,
and the Monin-Obukhov length L. U * and L may be estimated from commonly-measured
meteorological parameters. Z 0 can be inferred from existing land use databases (e.g.
EPA’s BELD database). In contrast, more complex formulations, including the widely
cited Slinn (1982) model, require many parameters as input, some of which are difficult
to obtain.
It is difficult to determine how well models for deposition velocity reflect realworld
values. This is largely because these models have not been widely-tested with
field data. In most cases, adjustable parameters are made to fit Chamberlain’s (1967)
wind tunnel tests for deposition on grass surfaces. As part of this study, the deposition of
particles to two different types of surrogate surfaces was measured. Comparison with the
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