treasure valley road dust study: final report - ResearchGate

elemental carbon abundances that were less than 0.1%. The enhanced carbon content of paved
roads over unpaved roads was probably due to a combination of tire wear and exhaust particles
that have deposited on the road surface. While these two sources of carbon were probably also
present on unpaved roads, they were significantly dwarfed by the amount of geologic material
that is inherent to the road surface.
Source profiles from road sanding material used by ACHD had 10 to 100 times higher
abundances of soluble, sodium, chloride, and sulfate than winter source profiles from paved
roads. Material used for summertime chip-sealing had abundances of organic carbon that were 5
to 10 times less than for summer paved road samples. The source profile for chip-sealing
material was similar to that from unpaved roads, perhaps making it di fficult to distinguish the
two during CMB modeling.
8.5 Conclusions
The results of this study have several important implications for road dust emissions.
First, the potential of a road to emit dust appears to be highly dependent on the speed of the
vehicles traveling on that road. Practically, this means that both the “dirtiness” of the road and
the speed at which vehicles travel are important and interconnected factors that control the
emissions of particulate matter from roads.
Second, the potential for roads in the Treasure Valley to emit PM 10 dust is higher in the
winter than in the summer. This suggests that there are additional sources of road dust during
winter conditions. Wintertime road sanding may contribute to dust emissions. Though low in
silt content, sand applied to roads by the ACHD may be pulverized by passing vehicles and
potentially serve as a reservoir for PM 10 emissions. However, tests conducted as part of the
TVRDS indicated that road sanding is not likely to be the sole contributor to elevated wintertime
road dust emissions. Results from controlled experiments showed that road sanding increases
emissions from paved roads for only a few hours before emissions return to their baseline, presanding
values. This was found to be true regardless of whether or not the road was cleaned with
a street sweeper after sanding. A mass balance approach can be used to reach the same
conclusion. Average winter day emissions for Ada County were 64 metric tons per day while
annual average emissions were 46 tons per day. Assuming a 90-day winter season, this means
that approximately 1,600 additional tons of PM 10 are emitted during the winter compared to the
rest of the year. The sanding schedule shown in Table 4-9 indicates that approximately 1,700 m 3
or 5,100 tons (assuming a density of sand equal to three times that of water) were applied to Ada
County roads in the 2000-2001 winter season. In order for the road sand to account for all the
excess wintertime PM 10 road dust emissions, more than one-third (33%) of the sand applied to
Ada County roads must be emitted as particles with aerodynamic diameter less than 10 ?m.
Though there are little data on the evolution of the size distributions of road dust over time, it
seems highly unlikely that such a large fraction of road sand is emitted as PM 10 , especially since
the sand is usually removed by street sweepers within 48 hours after application. Therefore,
there must be a significant source of wintertime road dust other than just road sand applied for
traction control.
One possible explanation for elevated emissions is that wetter conditions during the
winter are more conducive to the trackout of mud from unpaved lots and driveways unto the
paved road network. Tracked mud can serve as a reservoir for PM 10 emissions. During the
summer, when conditions are drier, trackout is expected to have a lesser role. It is not possible to
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