treasure valley road dust study: final report - ResearchGate

8.3 Differences Between Emissions Measured Roadside and PM 10 Fluxes on the Grid
Scales of Air Quality Models
A simple one-dimensional, time-dependent dispersion model was used to estimate the
removal of particles by deposition as a plume of dust is transported downwind of a source. The
model was solved numerically for dispersion parameters that correspond to both neutral and
stable atmospheric conditions. Particle concentrations were assumed to be uniformly distributed
from the ground to a height of 2 m at the time of emission. Particles were subsequently allowed
to disperse vertically or deposit at the grid cell nearest the surface (20 cm in height) by a
combination of gravitational settling and inertial impaction. The simulation was per formed for
multiple particle sizes ranging from 1.44 to 9.87 ?m in diameter.
Preliminary results showed that the horizontal flux of 1.44 ?m particles remains
relatively constant over transport times on the order of 500 seconds. In contrast, under neutral
conditions, fluxes of 9.87 ?m particles were diminished by 28% after 30 seconds (~30 – 90 m
downwind of the source) and 64% after 500 seconds (~500 – 1500 m downwind of the source).
Stable atmospheric conditions enhanced these reductions to 28% and 74% at30 seconds and 500
seconds, respectively. Based on the size distribution of road dust emissions obtained during the
TVRDS and the assumptions of the model, PM 10 road dust emissions fluxes measured 500 –
1500 m downwind of a road may be 55 – 65% lower than fluxes measured roadside.
The model input parameters were systematically perturbed to assess the effects of model
assumptions. The rate of removal of particles by inertial impaction was found to have a
profound effect. In particular, the model indicated that in the absence of inertial impaction, the
reduction of horizontal dust flux 500 m downwind of a road compared to roadside is only on the
order of 5%. The initial height of the uniformly distributed dust plume was also found to be a
significant parameter (2 m in the base case). A decrease in the value of this height enhances
particle removal by deposition while an increase has the opposite effect.
8.4 PM 10 and PM 2.5 Source Profiles for Road Dust
Silt samples collected during the winter and summer field campaigns of the TVRDS from
paved and unpaved roads, samples of road sanding material obtained from ACHD, and samples
of material used in Treasure Valley summertime chip-sealing operations were resuspended in a
specially-designed chamber. Size-selective inlets were used to collect both the PM 10 and PM 2.5
fractions of the resuspended material on Teflon and quartz filters. The filters were analyzed for
mass, major ions, ammonium, chemical elements, organic carbon, and elemental carbon. Results
of these chemical analyses were used to compile chemical source profiles in the PM 2.5 and PM 10
size ranges for paved road dust, unpaved road dust, and road sanding material used by ACHD.
The percent content of species on a mass basis, also referred to as the abundance, was used to
compare the characteristics of the source profiles.
There were no marked differences between source profiles collected in the winter and
those collected in the summer. In both PM 10 and PM 2.5 size ranges, all of the samples showed
appreciable abundances of elements associated with geologic material: aluminum (Al), silicon
(Si), potassium (K), calcium (Ca), and iron (Fe). Abundances of organic and elemental carbon
varied amongst samples. For paved road dust, abundances of organic carbon varied between
12% and 16% while those for elemental carbon varied between 1.2% and 2.3%. In contrast,
samples from unpaved roads had organic carbon abundances between 2.5% and 4.5% and
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