treasure valley road dust study: final report - ResearchGate

3.2 TRAKER Measurements
For the TVRDS, several improvements have been made over the original test vehicle. In
place of a Jeep Cherokee, a 1979 Chevy van is equipped with three exterior steel pipes that act as
inlets for the onboard instruments (Figure 3-2a). Two of the pipes are located behind the left and
right front tires and are used to measure the emissions from the tires. The third pipe runs along
the centerline of the van underneath the body and extends through the front bumper. This pipe is
the inlet for background air. Dust and exhaust emissions from other vehicles on the road can
cause fluctuations in the particle concentration above the road surface. The background
measurement is used to correct the measurements behind the tires for those fluctuations.
The three exterior pipes enter the cargo compartment of the van through the underbody.
Each pipe then goes into a plenum/manifold; the plenum distributes the sample air to up to five
instruments (Figure 3-2c). Three types of instruments have been used over the course of this
study: TSI DustTrak monitors, GRIMM optical particle size analyzers, and filter-based MiniVol
samplers.
A central computer collects all the data generated by the onboard instruments (Figure
3-2d). Data from TRAKER measurements are imported into a Microsoft Access database for
subsequent data processing and analysis.
3.2.1 Inlets
Unlike gases, particles have inertia; as a result, sampling of particles through an inlet
necessarily results in some particle losses to inlet surfaces. These losses could be due to
diffusion of particles towards inlet walls or impaction/settling of particles upon inlet walls.
Diffusion is a phenomenon that governs the motion of very small particles (less than 0.1 ?m).
Since road dust is comprised primarily of larger particles (greater than 0.3 ?m), diffusion is not
an important consideration for TRAKER. Impaction and gravitational settling can be important
processes for particles with aerodynamic diameters greater than 1 ?m. In general, gravitational
settling can be minimized by reducing the amount of time a particle spends in the inlet lines (e.g.
by increasing the speed of the flow). On the other hand, particle impaction can be minimized by
reducing the speed of the flow turns within the inlet lines.
The inlet lines, visible in Figure 3-2a are 19 mm (3/4”) in diameter and 2.3 m (7.5’) in
length for the tire lines and 3.7 m (12’) in length for the background line. The influence inlets on
the right and left are in slightly different positions with respect to the tires. On the right, the inlet
is 165 mm (6.5”) above the ground, 50 mm (2”) behind the tire, and 63 mm (2.5”) in (towards
the center of the vehicle) from the outside edge of the tire. On the left, the inlet is 165 mm (6.5”)
above the ground, 63 mm (2.5”) behind the tire, and 63 mm (2.5”) in from the outside edge of
the tire. Because of the vehicle configuration, it is not possible to avoid bends in the inlet lines.
However, wherever necessary, bends have been kept as shallow as possible in order to minimize
losses of particles to the inlet walls. Each of the inlet lines feeds into a 600 mm (20”) long
torpedo-shaped plenum (Figure 3-2c). All particle sampling instruments are connected through
the plenum via short Tygon tubes that are in turn attached to 200 mm (8”) long steel tubes that
extend into the body of the plenum. Flowrates through the inlets are 75 liters per minute (lpm),
corresponding to an inlet face velocity of 4 meters per second (mps) and 0.3 mps in the plena.
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