Road dust reduction strategies are designed to reduce the amount of fugitive dust (PM-10 and PM-2.5) that is suspended into the air by tires on roadways. Several different methods are available, including strategies geared toward paved roads and unpaved roads. These strategies generally have no impact, or minimal impacts, on other pollutants.
Road dust emission factors were drawn from EPA's Compilation of Air Pollutant Emission Factors: AP42, and do not account for new research which suggests that a larger portion of road dust is in the form of PM-2.5. Note that the EPA guidance recommends regions develop their own emission rates based on local silt loading data.
The following examples do not take into account any changes in exhaust emissions that may occur, such as in response to speed restrictions:
Surface treatments for road dust mitigation are control options requiring periodic reapplication and can be divided between two main categories. Wet suppression strategies add moisture to the road surface which conglomerates particles and reduces their likelihood to become suspended in the air when vehicles pass over the surface. The second major type of control is chemical stabilization treatment which attempts to change the physical characteristics of the surface.
This strategy focuses on PM reduction. Sometimes paving is not feasible for industrial roads subject to very heavy vehicles and/or spillage of material in transport. Watering and chemical suppressants, on the other hand, are potentially applicable to most industrial roads at moderate to low costs, though they require frequent reapplication to maintain an acceptable level of control. Chemical suppressants are generally more cost-effective than water but not in cases of temporary roads (which are common at mines, landfills, and construction sites).
Table 7-1. Unpaved Road Dust Mitigation Strategy - Overall Impact on Emissions
| PM-2.5 | PM-10 | CO | NOx | VOCs | SOx | NH-3 |
|---|---|---|---|---|---|---|
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N = No change; not quantified in EPA guidance
PM emissions from resuspended road surface material vary linearly with traffic volume. They also vary linearly with the fraction of silt (particles with diameters smaller than 75 micrometers [µm]). The silt fraction is the proportion of loose dry surface dust that passes a 200-mesh screen, based on the ASTM-C-136 method. Vehicle weight is also highly correlated with emissions on industrial sites, where heavy equipment is common. On public roads where passenger vehicles are more common, vehicle weight tends to be more uniform, thus not affecting emissions considerably. Moisture content is also highly correlated with PM emissions. Strategies to mitigate PM emissions from unpaved road dust are divided in three categories:
This calculates PM emissions from suspended road surface material on an unpaved road adjacent to a construction site in Southern California. The strategy proposed is watering control. Necessary inputs include:
Step 1: Calculate VMT Affected.
= (Average daily traffic) x (project length)
10 trucks x 2 miles x 2 (return trip) x 5 days/week x 52 weeks/year = 10,400 VMT
Step 2: Determine road silt content.
+8.5 percent
Step 3: Determine average vehicle weight.
+30 tons
Step 4: Determine number of days with significant precipitation (more than 0.01).
+40
Step 5: Calculate emission factor (lb/VMT).
E = [k.(S/12)a.(W/3)b]*[(365-P)/365], where:
E = Emission factor (lb/VMT)
S = Road silt content (%)
W = Average vehicle weight (tons)
P = Number of days in a year with at least 0.01 of precipitation
k, a, b = Constants (See table below)
| Variable | PM2.5 |
PM10 |
|---|---|---|
k |
0.23 |
1.5 |
a |
0.9 |
0.9 |
b |
0.45 |
0.45 |
Based on these calculations: EPM2.5 = 0.4232 lb/VMT ; EPM10 = 2.7599 lb/VMT
Step 6: Calculate total baseline emissions (TBE)
= (Total VMT) x (emission factor)
TBEPM2.5 = 4,401 lb = 2 tons
TBEPM10 = 28,703 lb = 13 tons
Step 7: Calculate updated emissions with watering controls (WE).
The application of 0.056 gallons of water per square yard of road is equivalent to 0.01 inch of precipitation, which is considered a "rainy day" for calculation purposes. To compensate for the dry months (June through September), water will be applied during these days. By repeating the calculations from the previous steps (with P increased to 160), the following results are obtained:
WEPM2.5 = 2,776 lb = 1.3 tons
WEPM10 = 18,105 lb = 8.2 tons
This represents a reduction of 37 percent in PM emissions.
Table 7-2. Total Emissions Reductions (tons/year) from Dust Mitigation Example
| Year | PM-2.5 | PM-10 | CO | NOx | VOCs | SOx | NH-3 |
|---|---|---|---|---|---|---|---|
| 2006 | 0.7 | 4.8 | NA | NA | NA | NA | NA |
| 2010 | 0.7 | 4.8 | NA | NA | NA | NA | NA |
| 2020 | 0.7 | 4.8 | NA | NA | NA | NA | NA |
Road paving reduces air pollution caused by dust particulates released into the air. According to EPA estimates, the difference between paved and unpaved emission rates is close to 572.32 g/VMT, which represents a significant reduction in PM-10 emissions due with implementation of this strategy. Typical projects include paving shoulders, curbs and gutters, roads, and access points.
This strategy focuses on PM reduction.
Table 7-3: Road Paving Strategy - Overall Impact on Emissions
| PM-2.5 | PM-10 | CO | NOx | VOCs | SOx | NH-3 |
|---|---|---|---|---|---|---|
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N = No change; not quantified in EPA guidance
PM emissions from suspended road surface material are affected by the following factors:
For EPA Guidance, see AP 42, Fifth Edition, Volume I, Chapter 13, Section 13.2.1, http://www.epa.gov/ttn/chief/ap42/ch13/draft/d13s0201.pdf and Section 13.2.2, http://www.epa.gov/ttn/chief/ap42/ch13/draft/d13s0202.pdf.
The city of Maricopa in central Arizona proposes to pave a 1.5 mile section of unpaved road. The project will pave highway lanes only (i.e., shoulders, curb, and gutter will remain unpaved). Necessary inputs include:
Step 1: Calculate VMT Affected.
= (Average daily traffic) x (project length) = 150 x 1.5 = 225 vehicle miles/day
Step 2: Calculate emissions reduced.
= (VMT Affected) x [(emissions factor for unpaved roads) - (emissions factor for paved roads)]
PM10: 225 x (573.91 - 1.59) = 128,772 grams/day = 47 tons/year
PM2.5: 225 x (143 - 0.4) = 32,193 grams/day = 12 tons/year
Table 7-4. Total Emissions Reductions (tons/year) from Road Paving Example
| Year | PM-2.5 | PM-10 | CO | NOx | VOCs | SOx | NH-3 |
|---|---|---|---|---|---|---|---|
| 2006 | 12 | 47 | NA | NA | NA | NA | NA |
| 2010 | 12 | 47 | NA | NA | NA | NA | NA |
| 2020 | 12 | 47 | NA | NA | NA | NA | NA |
Regular street sweeping on paved roads removes sand and/or other de-icing materials, and other deposition of dust or dirt on roads, reducing the amount of particulate matter released into the air. Projects may add street sweepers, replace non-certified sweepers with newer vehicles, use new vehicles to increase the frequency of sweeping in existing areas, or use new vehicles to expand the area that is regularly swept. Specific approaches to street sweeping include vacuum sweeping, water flushing, and broom sweeping and flushing.
Street sweeping projects affect only particulate matter associated with road dust, not direct vehicle emissions. While street sweeping removes material that can potentially be suspended in the form of particulate matter, the street sweeping equipment also produces exhaust emissions, which are generally minor, but may need to be considered in regard to other pollutants.
Table 7-5: Street Sweeping Strategy - Overall Impact on Emissions
PM-2.5 |
PM-10 |
CO |
NOx |
VOCs |
SOx |
NH-3 |
|---|---|---|---|---|---|---|
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N* = Generally considered to have no impact, but may increase emissions (associated with use of sweepers)
The reduction of PM emission due to street sweeping comes from the reduction of reentrained dust from vehicles traveling on roadways. It is important to note that sweeping of curb and gutter areas may increase emissions, given that it redistributes loose material onto the travel lanes. Factors affecting emission include:
For EPA Guidance, see AP 42, Fifth Edition, Volume I, Chapter 13, Section 13.2.1, http://www.epa.gov/ttn/chief/ap42/ch13/index.html
The city of Maricopa in central Arizona proposes the use of "PM10 Efficient Street Sweepers" for non-freeway streets. This strategy focuses on PM-10 only. Necessary inputs include:
Step 1: Calculate current emissions from reentrained dust from vehicles traveling on the road, assuming that there is no street sweeping performed.
VMT = (Average daily traffic) x (number of lane miles) = 5,000 x 200 = 1,000,000 veh. mile/day
PM10 = (Emission factor) x (VMT) x 52 weeks x 7 days = 400 tons/year
Step 2: Calculate proposed emissions from reentrained dust from vehicles traveling on the road.
Based on a 7-day cycle, the PM10 emission factor with an efficient sweeper is 0.6871 g/VMT.
PM10 = (Emission factor) x (VMT) x 52 weeks x 7 days = 250 tons/year
Step 3: Determine total emission reduction.
= (Baseline emissions) - (emissions with sweeper)
= (400 - 250) = 150 tons/year
The emissions from the sweeping process itself are negligible.
Table 7-6. Total Emissions Reductions (tons/year) from More Frequent Street Sweeping Example
| Year | PM-2.5 | PM-10 | CO | NOx | VOCs | SOx | NH-3 |
|---|---|---|---|---|---|---|---|
| 2006 | NA | 150 | NA | NA | NA | NA | NA |
| 2010 | NA | 150 | NA | NA | NA | NA | NA |
| 2020 | NA | 150 | NA | NA | NA | NA | NA |
