treasure valley road dust study: final report - ResearchGate

4.1.1 Converting TRAKER Data Into Emissions Potentials
All invalid TRAKER data points (according to the criteria outlined in Section 3) were
removed from the dataset. The parameter used in considering valid TRAKER data is the
emissions potential of the road. Recall from Section 3 that:
T ? T T
? T B
(4-1)
T ? T
* s 3
(4-2)
EF ? bs
(4-3)
1/ 3
EF ? kT
(4-4)
where T T is the PM 10 concentration at the vehicle tire, T B is the background aerosol
concentration measured at the front bumper, T is the differential TRAKER signal, EF is the
emissions factor, s is the vehicle speed, and T * , b, and k are all constants for a given road
segment. It follows that
3
?(
k?
T / s s?
1/
EF ? )?
(4-5)
1/ 3
b ? ( k?
T / s)
(4-6)
where the constant, b, is the road dust emissions potential and is related only to the “dirtiness” of
the road. That is, for a given road, b is constant regardless of the speed that the TRAKER
traverses the road and has units of [g/vkt/mps] where vkt is the vehicle kilometers traveled and
mps is the speed of travel in meters per second. Actual emissions factors for the road (units of
g/vkt) are obtained by multiplying b by the speed of the vehicle that is traversing the road.
The emissions potential was calculated for each valid TRAKER data point. Note that in
some cases, the differential signal T can be negative. This occurs if the background PM
concentration measured on the bumper of the vehicle is higher than the influence concentration
behind the tire due to instrument noise or a time delay between when a “dirty” air pocket passes
by the bumper and then the tires. It is important to retain these negative values in the database so
as not to introduce a bias when calculating averages.
4.1.2 Associating TRAKER Data With Roads and Settings
All data were imported into a GIS-format database. Using the GIS coverages shown in
Table 4-1 and a software utility for joining spatial data, each TRAKER measurement was
associated with a road segment or type, county, and setting (urban/rural). For non-residential
roads, the TRAKER measurement was associated with a link in the Traffic Demand Model
(TDM) that was provided by COMPASS. Each link in the TDM is attributed with a road class
(e.g. principal arterial, collector, etc). Residential roads are not reflected in the TDM as physical
entities; instead, they are grouped and represented as “Centroid Connectors”. For residential
roads, the TRAKER measurement was flagged as “residential” based on spatial joins with
separate Ada County and Canyon County street coverages, shown in Table 4-1 as “adastr” and
“canstr”, respectively.
When attributing a road dust emissions potential to an individual road segment, 1-second
TRAKER data for that road segment (link) were averaged. If there were fewer than 10 valid data
points for that road segment, then the road segment was not considered to have a valid
measurement.
4-2