treasure valley road dust study: final report - ResearchGate

3.4.2 Flux Calculation from Meteorological and Particulate Concentrations
Horizontal flux emissions were calculated from the 1-second DustTrak data and the
vertical wind profiles measured on the towers. Figure 3-17 shows typical time series of the PM 10
concentrations measured by the DustTraks at each tower. The concentration spikes in the figure
represent individual vehicle passes. The time series are labeled based on the position of the
sampler. DW1_2 is the second from the bottom DustTrak on downwind tower 1 (See Figure
3-18 for DustTrak heights). The results of the close downwind tower 1 are shown in the upper
panel of the figure while the results from the farther downwind tower 2 are shown in the lower
panel.
The figure shows that the concentrations at the close downwind tower typically decrease
with height. At the far downwind tower, the second highest sampler typically measures the
highest concentrations. More over, the duration of the plume impact on the close tower is
approximately 3-7 seconds while the duration at the far tower is 10-15 seconds. These results
indicate that the road dust emissions plume is dispersing both vertically and horizontally as the
plume moves downwind of the road.
Figure 3-18 shows the vertical distribution of the plume at 9 m and 50 m downwindof
the road. Concentrations have been normalized so that they sum to unity. The profile at the far
downwind tower clearly shows that the plume is dispersing vertically.
Analysis of the DustTrak data from the three towers indicated that baseline drift over the
course of a day did not affect all instruments equally. In order to calculate emissions fluxes from
the PM 10 concentrations, a baseline for each instrument was determined from the peaks shown in
Figure 3-17. This baseline concentration was subtracted from the PM 10 time series so that the
resulting concentration time series was due only to the emissions of the vehicle passing the
tower.
PM 10 emissions fluxes were calculated using the assumption that each DustTrak monitor
represented a uniform concentration over an interval half way between the monitor and the
neighboring monitor on the tower. For example, the lowest monitor on downwind tower 1
(DW1_1) was representative of the PM 10 concentration from the ground to 1.83 m above ground
level (AGL). The second monitor represented the PM 10 concentrations from 1.83 m AGL to 3.56
m AGL, and so on. The flux of particles at each tower was calculated using the following
equation:
?
4
EF ? cos( ? ) u C ? z ? t
( 3 )
i?
1
i
where EF is the emissions factor of PM 10 in grams per vehicle kilometer traveled, ? is the angle
between the wind direction and a line perpendicular to the road, i is one of the four positions of
the monitors on the tower, u i is the average wind speed in m/s over the interval represented by
the i th monitor, C i is the average PM 10 concentration in mg/m 3 as measured by the i th monitor
over the interval ? t, ? z in m is the vertical distance represented by the th i monitor, and ? t is the
duration that the plume impacts the tower in seconds. A database of the emissions factors was
assembled for the parameters: vehicle speed, vehicle type (i.e. Ford Ranger, Chevy Van,
HumVee), date, and road orientation (i.e. East-West or North-South). The database is presented
in Table 3-3.
i
i
3-18