treasure valley road dust study: final report - ResearchGate

Figure 3-6 shows the TRAKER coefficient of variation for the left and right PM 10
DustTrak signals as a function of the vehicle speed. The coefficient of variation is a measure of
the relative precision and is equal to the standard deviation of the measurement divided by the
average of the measurement. In the Figure, the measurement corresponds to multiple passes on
the same 1-mile stretch of road in the Treasure Valley. The Figure clearly shows that the
precision of the measurement improves with increasing vehicle speed. The precision is 84% at 5
mps, 30% at 9 mps, and approximately 10% above 14 mps. Note that most TRAKER
measurements occur at speeds greater than 9 mps (approximately 20 mph). The poor precision at
low speeds is probably due to the influence of fluctuating ambient winds on the flow regime
behind the front tires. As the vehicle speed increases, such fluctuations become less important
compared to the speed of the vehicle (see section 3.3.4 for discussion of the relationship between
the TRAKER signal and vehicle speed). The equivalent information for PM 2.5 DustTraks is
shown in Figure 3-7. Note that for PM 2.5 , DustTrak data at speeds less than 50 km/hr are below
the detection limits of the instruments. The trend of improved precision with higher speeds is
similar to the PM 10 case.
3.3.1.1 Inferring PM 10 from PM 2.5
Unpaved roads are by nature much dustier than paved roads. A dilution system was
designed for the TRAKER in order to reduce the particle concentration in the TRAKER inlets
and allow the DustTrak measurements to remain within the manufacturer’s specified operating
range. In some cases, the PM 10 DustTrak upper limit (150 mg/m 3 ) was exceeded even with the
dilution system in operation. For both the Treasure Valley Road Dust Study, and the calibration
tests at Ft. Bliss (described in detail in section 3.4) PM 2.5 DustTraks were collocated with PM 10
DustTraks in the sampling plena within the TRAKER. Figure 3-8 shows the relationship
between collocated PM 2.5 and PM 10 DustTraks. Based on the correlation shown in the figure,
when the PM 10 DustTrak was out of range, the PM 2.5 value was divided by 0.39 in order to
estimate the PM 10 value. Note that while useful for correcting out of range data, the DustTrak is
calibrated for PM 10 and measurements with a 2.5 ?m inlet only give a nominal value for PM 2.5 .
For a few cases, both the PM 10 and PM 2.5 DustTraks were out of range. Under such
circumstances, default values of 770 and 280 mg/m 3 were assumed for PM 10 and PM 2.5 ,
respectively. This occurrence is rare and applies to less than 0.5% of the unpaved road data.
3.3.1.2 TRAKER Validity Criteria
A TRAKER data point is only considered valid if it meets all of the criteria outlined in
Table 3-1. Criteria are applied to the speed, acceleration, deceleration, and the wheel angle of
the TRAKER vehicle. If a data point does not meet any one of the criteria, then that data point is
flagged as “Invalid” and is not used in any subsequent data processing activities. Note that the
TRAKER measurement uses the difference between the particle concentration measured behind
the front tire and the concentration measured through the front bumper (see equation 1 in section
3.3.4). Under certain conditions, the concentration at the front bumper may be higher than it is
behind the front tire resulting in a negative measurement. Negative values are NOT considered
invalid and are retained in the database. It is important to retain negative values so that a
systematic bias is not introduced into the dataset.
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