treasure valley road dust study: final report - ResearchGate

6. MODIFICATIONS OF ROAD DUST EMISSIONS INVENTORIES FOR
DISPERSION MODELS
The discrepancy between fugitive dust emissions inventories and the relative contribution
of fugitive dust to ambient PM 10 concentrations is well-documented (Watson and Chow, 2000;
Countess, 2001). In particular, dispersion models that use fugitive dust emissions estimates as
input generally overstate the fraction of dust in ambient PM 10 samples. The principal reason for
this discrepancy is that a significant amount of dust is removed from the atmosphere within a few
hundred meters of the source. Since most regional scale air quality models employ grid
resolutions on the order of 1 km or greater, much of the short range dust removal is “missed” by
the models. Thus, there exists a physical disconnect between the near-field emissions
measurements used to assemble inventories and the PM 10 dust inputs used in the air quality
models.
In this section, we employ a simple dispersion model that accounts for near-field
deposition of particles with aerodynamic diameter less than 10 ?m. The purpose of this exercise
is to obtain an estimate of the fractional reduction in PM 10 that is due to deposition of particles
over the first several hundred meters after emission. This aspect of fugitive dustresearch has
received much attention in recent years and efforts are underway to refine modeling techniques to
account for this effect (e.g. the box model proposed by Gillette and described in Countess, 2001).
6.1 Methods
The one-dimensional, time-dependent atmospheric transport equation (e.g. Seinfeld, 1986)
was solved numerically for both neutral and stable atmospheric conditions. Invoking the K-theory
simplification, the full equation is:
? c
?
? t
? c
u
? x
?
? ?
K
? x?
xx
? c?
?
? x?
? ?
K
? y?
yy
? c?
?
? y?
?
? z
?
?
K
zz
? c?
?
? z?
S
i
?
S
o
(6-1)
where c is the concentration of the species of interest, u is the velocity in the x-direction, K ii are
the turbulent diffusivities in the x, y, and z directions, S i and S o are the sources and sinks. The
one dimensional form of Eq. 6-1 is independent of the velocity u:
? c
? t
?
? ?
K
? z?
zz
? c?
?
? z?
V
d
? c
? z
(6-2)
where V d , the deposition velocity is the source/sink term.
The numerical grid consisted of 41 nodes ranging from z = 0 m to z = 250 m. There were
8 nodes within the first meter AGL, 19 nodes within 5 m AGL, 26 nodes within 25 m AGL, 33
nodes within 100 m AGL, and 8 nodes between 100 and 250 m AGL. Figure 6-1 shows a
schematic of the numerical grid.
6-1