treasure valley road dust study: final report - ResearchGate

4.3.2 Meteorological Effects on Unpaved Road Emissions
The TRAKER loop included a 1 km section of private unpaved road (Howry) that was
traversed each time the loop was executed. The emissions potential for each day the loop was
executed was averaged over the entire length of the unpaved road. The result appears as a time
series in Figure 4-8. The black circles correspond to the emissions potential in [g/vkt/mps]. The
black squares correspond to daily precipitation in mm. The gray line is a time trace of the
relative humidity. Emissions potentials fall into three distinct categories outlined by the gray
boxes: “dry conditions”, “day after rain”, and “wet conditions”. The emissions potentials and
daily precipitation share the same y-axis scale on the left, while the relative humidity uses the
right y-axis scale. During and immediately after a rain event while the relative humidity is still
high (3/3/01), emissions potentials are very low and correspond to “wet conditions”. 24 hours
after a rain event (“day after rain”), when the relative humidity has had a chance to sub side and
the top layers of soil are drying, emissions potentials are higher than under “wet conditions”. 48
hours or more after a rain event, emissions potentials are even higher (“dry conditions”). If we
consider the “dry conditions” as a baseline value, then under wet conditions, emissions are on
average 7.9% of the baseline value and under “day after rain”, emissions are 35% of their
baseline values. This trend seems to hold regardless of the amount of rainfall, though we note
that the range of daily rainfall amounts that were reported for the 2/23/01 to 3/20/01 period was
modest (0.25 mm - 0.77 mm or 0.01 in – 0.03 in).
AP-42 (USEPA, 1999) suggests the approximation that emissions from unpaved roads
should be set to zero for all days where the total daily rainfall exceeds 0.25 mm. The results of
the TRAKER loop indicate that in addition to reducing emissions for days when rainfall is
reported, unpaved road emissions should also be reduced for the 24-hour period that follows the
day that rainfall was reported. Physically, results from the TRAKER loop suggest that a thin
layer of soil at the surface dries quickly, but that the soil underneath may remain wet for at least
48 hours, considerably reducing the potential for dust emissions.
Emissions Potential [g/vkt/mps] Or rainfall in mm
10.00
Dry
Conditions
Day After
Rain
1.00
Wet
conditions
0.10
TRAKER
Rainfall
"RH"
2/23/01 2/28/01 3/5/01 3/10/01 3/15/01 3/20/01
Date
100
90
80
70
60
50
40
30
20
10
0
Figure 4-8. Plot of emissions potential of unpaved road on TRAKER loop, daily precipitation, and relative
humidity. Emissions potentials and rainfall use the left y-axis; relative humidity uses the right y-axis.
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