treasure valley road dust study: final report - ResearchGate

Table 6-1. Aerodynamic particle sizes simulated and corresponding removal rates due to
impaction
Aerodynamic Diameter (? m)
Removal rate due to
impaction, V i (m/s)
9.78 0.1
7.21 0.1
5.59 0.08
3.95 0.07
2.88 0.02
2.04 0.02
1.44 and less 0
For the initial condition (t=0), the concentration of particles was assumed to be uniform
over the first 2 m AGL and zero for the remainder of the domain. The top boundary condition
was that dc/dz was zero at z=240 m. The initial conditions were marched forward in time to t=
500 seconds in increments of 50 ms for the first 2.5 seconds, followed by 100 ms for the next 17
seconds, followed by 200 ms thereafter. This slow ramping of the time steps allows for
minimization of numerical errors when spatial gradients are large due to the initial conditions.
6.2 Results
The results of the numerical simulation are shown for all the particle sizes examined in
Table 6-2 and Table 6-3 under neutral and stable conditions, respectively. The tables report the
fraction of a particle size that remains suspended after the initial emissions at t=15 , 30, 60, 120,
240, and 500 seconds. The tables show that under both neutral and stable conditions, very little
removal by deposition occurs for 1.44 ?m particles within the time frame examined. In contrast,
for the largest particles examined under neutral conditions (9.87?m) only 72% of the original
emission remains in suspension after 30 seconds and only 36% remains after 500 s. Figure 6-2
shows a time series of 7.21 ?m particles remaining in suspension. The figure shows that the
particles are removed quickly within the first 200 seconds and then continue to fall out at a lesser
rate. The particle removal rates are approximately the same for both neutral and stable conditions
during the first 30 seconds. Thereafter, the curves for the two atmospheric conditions deviate
from one another and removal rates are noticeably higher under stable conditions. Figure 6-3
shows the time series progression of the vertical profiles of concentration for 7.21 ?m particles.
Under neutral conditions, particles disperse vertically more quickly than under stable conditions.
Effectively, this makes particles less available for removal by deposition near the ground under
neutral conditions than under stable conditions.
The fraction of particles remaining in suspension shown in Table 6-2 and Table 6-3 can be
applied to the entire size distribution of particles emitted by a particular source. The size
distribution of road dust for the TVRDS at the point of emissions was discussed earlier in section
4.1. That size distribution has been used to estimate the fraction of PM 10 mass (mass
concentration of all particles with aerodynamic diameter less than 10 ?m) that remains suspended
and the results are shown in Figure 6-4. According to the model results, under neutral conditions,
PM 10 is depleted by approximately 25% just 30 seconds downwind of the point of emissions and
6-4