treasure valley road dust study: final report - ResearchGate

lead to unsteady flow conditions between the front tire and the inlet. To avoid this possibility, a
minimum speed of 5 m/s is required to consider a data point valid. This criterion is relaxed for
unpaved roads (1.5 m/s) where traveling at high speeds can causethe DustTraks and PSAs on
board the TRAKER to be overwhelmed with high dust concentrations. Acceleration/deceleration
criteria (< 0.5 m/s 2 ) are also applied to the TRAKER measurement. During periods of high
acceleration, the flow regime around the inlets may be transient; during periods of deceleration,
dust from the brakes may influence the particle concentrations behind the front tire. In addition,
the wheel angle must be less than 3 degrees with respect to the vehicle body. This is to ensure
that the orientation of the inlets with respect to the front tires is not changing over the course of
the measurements. The criteria shown in Table 3-1 are based on empirical observations and
statistical analyses of the TRAKER measurement under a variety of driving regimes. They are
conservative and intended to ensure that the measurements used in this study are valid.
3.3.2 Inlet Loss
To assess the magnitudes of particle losses in the sample inlet lines, particle
concentrations at the inlet were compared to concentrations measured by the instruments
attached to the plena. The TRAKER was parked perpendicular to and 15 meters downwind of an
unpaved road. Two PSAs were placed within 5 cm of the inlet opening (Figure 3-9). Inside the
TRAKER, the manifold was outfitted with the usual suite of instruments, one PSA, one
DustTrak with a PM 10 inlet, and one DustTrak with a PM 2.5 inlet. Because they have directionsensitive
inlets, and due to their physical configuration, t iwas not possible to place a DustTrak
close enough to the inlet opening for comparison with the measurement inside the TRAKER.
Instead, test results for the DustTrak were inferred from the results for the PSA. A second
vehicle was driven back and forth on the unpaved road to simulate the particle concentration
range that would occur at the inlet during routine operation (Figure 3-10). This test was
performed for a minimum of 30 minutes for each of the three inlets. For the inl ets behind the left
and right front tires, the test was conducted for the normal configuration, as well as for the case
where the dilution system was in use. The concentrations measured by each instrument were
averaged over the period of the test for inter-comparison purposes.
Figure 3-11 shows the fractional line losses associated with each particle size measured
by the PSA for the three inlets. The figure also shows the collocated precision for the PSA. The
aerodynamic particle size shown on the x-axis is calculated by assuming that the optical particle
diameter is equivalent to the Stokes diameter and that the particles are spherical with a density of
2.6. Based on these assumptions, the aerodynamic diameter is equal to the optical diameter
times the square root of the particle density (Seinfeld and Pandis, 1998).
Line losses in the smallest size bin are not consistent with physical expectation and are
probably marred by artifacts introduced by instrument error. However, we note that the mass
associated with this bin represents less than 0.1% of the mass of PM 10 and that this error is
negligible in the context of the present work. Except for the smallest size bin, there are no
significant differences in line losses between the left, right, and background inlets, i.e.
differences in line losses between inlets are not greater than twice the precision which is the
criterion for deciding whether or not two measurements are significantly different. Thus, the
left, background, and right line losses are adequately represented by Figure 3-12 which shows
the average line losses for all three inlets. Note that though negative line losses do not have a
physical meaning, they are retained in order to correct for inter-instrument biases.
3-11