treasure valley road dust study: final report - ResearchGate

6.3 Discussion
The depletion of PM 10 in the proximity of the emissions source has important implications
for the use of road dust emissions in air quality models. PM 10 dust emissions used to estimate
emissions factors are measured within 5 to 50 m of a road. Emissions calculated based on those
factors do not account for the depletion of PM 10 as the plume travels further downwind. Many of
the frequently used urban and regional scale air quality models assume that ground level emissions
are initially uniformly mixed over a certain height, typically the distance from the ground to the
first node in the numerical grid (20 – 40 m). This assumption does not adequately allow for the
removal by deposition of the larger particles in the PM 10 size range. Consequently, air quality
models that use road dust emissions as input without accommodation for near-source removal of
particles will likely overestimate the ambient concentrations of road dust PM 10 .
Table 6-4 shows how changes in the parameters used in the model can affect the fraction
of PM 10 that remains suspended after 60 seconds. The model is sensitiveto some of the
parameter values and fairly insensitive to others. For example, varying the roughness length under
neutral conditions over several orders of magnitude has only a modest effect on the fraction of
particles remaining in suspension. On the other hand, the value used for deposition rate due to
impaction can have substantial consequences. If impaction is not accounted for at all (V i = 0) and
particles were only allowed to deposit by gravitational settling, then the fraction that remains in
suspension is 95%, considerably higher than the 65% achieved with the model assumptions here.
Another important parameter is the assumption for the initial distribution of particles in the
vertical direction. A factor of two increase or decrease in the initial pulse height can vary the
fraction remaining between 45% and 80%.
Table 6-4. The effects of perturbation of model inputs on fraction of particles remaining in
suspension at t = 60 seconds.
Parameter Stability Baseline value
Baseline
fraction
remaining
Perturbed value
Fraction
remaining
z 0, u * Neutral 0.01 m, 0.058 m/s 65% 1 m, 0.17 m/s 60%
z 0, u * Neutral 0.01 m, 0.058 m/s 65% 0.1 m, 0.087 m/s 62%
z 0, u * Neutral 0.01 m, 0.058 m/s 65% 0.001 m, 0.043 m/s 67%
V i Neutral 0.08 m/s 65% 0.16 m/s 63%
V i Neutral 0.08 m/s 65% 0.04 m/s 68%
V i Neutral 0.08 m/s 65% 0 m/s 95%
L Stable 10 m 64% 1 m 70%
L Stable 10 m 64% 1000 m 64%
Initial distribution Neutral 0-2 m 65% 0-4 m 80%
Initial distribution Neutral 0-2 m 65% 0-1 m 46%
Z i Neutral 200 m 65% 400 m 65%
Z i Neutral 200 m 65% 100 m 65%
The model presented here is useful for demonstration of the behavior of road dust
emissions near the point of origin. However, several simplifying assumptions have been made in
the formulation of this model and many of the parameters only represent approximations. In
future work, and in keeping with the suggestions of both Watson and Chow (2000) and Countess
(2001), it is important to better estimate the conditions under which emissions from roads are
transported. Most notably, better field measurements of the impaction term V i and the initial
plume distribution should be obtained under neutral and stable atmospheric conditions.
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