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Vehicle-based road dust emission measurement—Part II: Effect of precipitation, wintertime road sanding, and street sweepers on inferred PM10 emission potentials from paved and unpaved roads
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Atmospheric Environment 37 (2003) 4573–4582
Vehicle-based road dust emission measurement—Part II: Effect of precipitation, wintertime road sanding, and street sweepers on inferred PM10 emission potentials from paved and unpaved roads H. Kuhnsa,*, V. Etyemezianb, M. Greenb, Karin Hendricksonc, Michael McGownc, Kevin Bartond, Marc Pitchforde a
Desert Research Institute, Division of Atmospheric Sciences, 2215 Raggio Parkway, Reno, NV 89512-1095, USA b Desert Research Institute, Las Vegas, NV, USA c Idaho Department of Environmental Quality, Boise, ID, USA d Ada County Highway District, Boise, ID, USA e National Oceanic and Atmospheric Administration, Las Vegas, NV, USA Received 30 August 2002; received in revised form 4 February 2003; accepted 13 June 2003
Abstract Testing Re-Entrained Kinetic Emissions from Roads (TRAKER) is a new technique to infer paved and unpaved road dust PM10 emission potentials based on particulate matter (PM) measurements made onboard a moving vehicle. Light scattering instruments mounted in front and behind the vehicle’s tires measure the differential particle concentration of dust suspended by the vehicle’s tire in contact with the road surface. Through empirical regressions relating the differential concentration (i.e. TRAKER signal) with the vehicle speed and the downwind flux of PM10 particles from the road, an equation is derived to infer the speed independent road dust emission potential from the measured TRAKER signal. Measurements from TRAKER offer a new perspective on the processes that affect road dust emissions. The system was used to investigate temporal changes in emission potentials from paved roads in both the winter and summer in the Treasure Valley in Southwest Idaho. During the 3-week wintertime sampling period, the residential road dust PM10 emission potential decreased by B50%. Summertime PM10 emission potentials were similar to those observed at the end of the winter sampling and showed no upward or downward trends. Wintertime unpaved road emissions increased consistently with the number of days since the last rainfall. Measurement of road dust emission potentials after road sanding on dry roads indicated a 75% increase in PM10 emissions after 2.5 h. This effect was short lived and emission potentials returned to their pre-sanding levels within 8 h of the sand application. Street sweeping with mechanical and vacuum sweepers was found to offer no measurable reduction in PM10 emission potentials. On several roads, the PM10 emission potentials actually increased immediately after vacuum sweeping. Long term effects of street sweeping on road dust emissions were not evaluated as part of this study and may offer some overall reduction in PM emissions from paved roads. r 2003 Elsevier Ltd. All rights reserved. Keywords: TRAKER; Road dust; Emissions; Road sanding; Street sweeping; Paved road; Unpaved road; PM10
1. Introduction *Corresponding author. Tel.: +1-775-674-7111; fax: +1775-674-7060. E-mail address: [email protected] (H. Kuhns).
This is the second paper in a three part series describing the results of paved and unpaved road dust
1352-2310/$ - see front matter r 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S1352-2310(03)00529-6
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emission experiments using the Testing Re-Entrained Kinetic Emissions from Roads (TRAKER) system. TRAKER is a vehicle equipped with real time particle monitors mounted in front of the vehicle and behind the front tires to measure the concentration of road dust suspended from the road surface while the vehicle is in motion. The vehicle configuration and the calibration of the TRAKER with respect to horizontal particle flux measurements are described in the first paper of the series (Etyemezian et al., 2003a). This paper discusses how the TRAKER system was used to evaluate the effects of precipitation, road sanding for traction control, and street sweeping on road dust emission potentials in the Treasure Valley, Idaho. The third paper in the series (Etyemezian et al., 2003b) describes how dust emission potentials vary with road characteristics such as the posted speed limit, volume of traffic, and geographic setting. Particulate matter (PM) emission inventories for most urban areas indicate that paved and unpaved road dust are the largest source categories. For example, for the 2001 base year in the state of California, paved and unpaved road dust emissions account for 17% and 27% of the total estimated PM10 emissions, respectively. Despite the importance of geologic material to the total PM10 and PM2.5 emission budgets, limited quantitative information is known about how road dust emissions change throughout the seasons and what controls, if any, can be effectively implemented to reduce their overall levels. More detailed emission factors are required as input into increasingly sophisticated computer models to simulate the transport and dispersion of emissions within urban and regional air sheds. Knowledge of the effectiveness of emission control strategies is needed to develop practical strategies for reducing ambient pollutant concentrations. This type of information is necessary for states to develop credible State Implementation Plans (SIP’s) to demonstrate how new or existing regulations will bring ambient air quality into attainment of US national standards. With respect to street sweeping, one published study found no discernable differences in airborne geologic PM10 concentrations measured either during or after one 10- and one 7-day long street sweeping campaigns in a Reno, Nevada neighborhood (Chow et al., 1990). Other studies attempting to quantify road dust emission reductions as a result of sweeping have had only limited success since ambient PM10 concentrations are influenced by meteorology and other emission sources that add large uncertainties to what appears to be a small effect (Cowherd et al., 1982; Cuscino et al., 1983; PEDCo-Environmental Specialists, 1981; Seton, 1983). Street sweepers have been studied in the context of road dust impacts on storm water quality. Rainfall has been found to increase material picked up by routine street sweeping in Portland, Oregon (Condor, 1989;
Sutherland and Jelen, 1995). Wintertime (rain season) sweeper pickups were up to 500% greater than summertime sweeper pickups, perhaps because less material is washed onto roads during the summer. Although sweepers effectively pick up visible material from the road surface, some studies have shown that both mechanical and vacuum sweepers are not very effective at collecting small particles. Sweeper pick up efficiencies for particles less than 44 mm range from 3% to 15% (Clark and Cobbins, 1963; Sartor et al., 1972). This paper uses data from a field study conducted in the Treasure Valley, ID to investigate the seasonality of inferred PM10 road dust emission potentials, the relative effectiveness of mechanical and vacuum sweepers in reducing emissions, and the temporal response of emission potential from unpaved road dust after rainfall.
2. Experimental methods The methods of TRAKER operation and calibration are described in detail in a companion paper (Etyemezian et al., 2003b). The inference of PM10 emission potential from the TRAKER measurement is reviewed here. TRAKER directly measures PM10 concentrations in front of and behind a vehicle’s tires while the vehicle is in motion. The difference in concentration between the two particle sensors was empirically related to the speed of the vehicle by the equation T ¼ TT TB ¼ as3 ;
ð1Þ
where TT is the aerosol concentration at the vehicle tire, TB is the background aerosol concentration measured through the vehicle’s front bumper, T is the difference in concentrations (i.e. TRAKER signal in mg/m3), and s is the speed of the vehicle. The variable a is a characteristic of the road being sampled. Experiments using towers to measure the horizontal flux of PM10 particles downwind of an unpaved road found an empirical power relationship between the TRAKER signal T and the PM10 emission factor (in grams per vehicle kilometer traveled) measured at 9 and 50 m downwind of the road: EFðg=vktÞ ¼ 8:8ðsg ¼ 1:5ÞT 0:3370:06 ;
ð2Þ
where EF is the PM10 emission factor. The geometric standard error (sg ) of the regression coefficient (8.8) was 1.5. Note that range of emission factors used for the calibration, 10–150 g/vkt, is generally one or two orders of magnitude higher than the typical emission factors for paved roads. Also, the vehicle speeds used to establish the relationship in Eq. (2) ranged from 2 to 8 m/s (5– 15 mph or 8–23 kph) on unpaved roads. Measurements downwind of unpaved roads indicate that PM10 emission factors increase linearly with vehicle speed. By dividing the PM10 emission factor by the vehicle speed,
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the resultant term (i.e. the emission potential in (g/vkt)/ (m/s)) is solely a property of the suspendable material on road and is independent of the travel speed of the vehicle operating on that road. This paper explores how the PM10 emission potential changes with season, precipitation, road sanding, and road sweeping. The precision of the TRAKER signal pertains to its ability to repeatedly measure consistent values over identical conditions. Repeated measurements of the TRAKER signal (T in Eq. (1)) on a 500 m stretch of paved road in Treasure Valley, Idaho and a 1200 m stretch of paved road in El Paso, Texas resulted in relative precision values between 10% and 20% for speeds greater than 10 m/s (20 mph or 36 km/h) and precisions between 20% and 85% for slower speeds. Since the emission potential is proportional to the cube root of the TRAKER signal, the relative precision of the emission potential is approximately one-third of the relative TRAKER signal precision (i.e. o7% for speeds greater than 10 m/s and o30% for speeds less than 10 m/s). The accuracy of the TRAKER measurement can be defined as its ability to infer the true PM10 emission potential from a road. The downwind horizontal flux of PM10 from the road is generally assumed to represent the ‘‘true’’ PM10 emissions from the road. However the precision of the flux measurement ranges from 10% to
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nearly 50% (Etyemezian et al., 2003a, Table 1). Although the accuracy of the emission potential based on the TRAKER signal cannot be better than the precision of the horizontal PM10 flux measurements, a precise TRAKER signal may be able to resolve relative differences in emission potentials that are less than the relative precision of the flux measurement itself. Thus, it is reasonable to state that the relative precision of the TRAKER emission potential (7–30%) is less than the relative accuracy of the emission potential (10–50%).
3. Temporal measurements of road dust variation To investigate temporal changes in road dust emission potentials, a 50 km loop was selected near Boise, Idaho spanning a variety of road types (interstate, arterial, collector, local, and unpaved) in both urban and rural areas. The loop was surveyed with the TRAKER 7 times during the winter season (26 February 2001 to 17 March 2001) and 5 times during the summer season (10 July 2001 to 28 July 2001). A map of the loop is shown in Fig. 1. Paved road sampling took place only when the road was visibly dry. The TRAKER loop included a 1 km section of private unpaved road that was sampled during the winter season only. The effect of precipitation
1 mile
Fig. 1. Loop of streets surveyed 7 times during the winter and 5 times during summer 2000 using the TRAKER vehicle in the Boise area in southwest Idaho.
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on unpaved road emission potentials was examined by comparing precipitation and relative humidity measurements collected at the Boise Airport to TRAKER measurements from this unpaved section.
4. Road sanding and sweeping experiments 4.1. Wintertime road sanding experiment The winter road sanding experiment took place on the morning of 15 March 2001 (Thursday) at two different locations in the Boise area. The first test location was on the rightmost eastbound lane on Chinden Road between 50th and 42nd St (Fig. 2). In this area, Chinden is a principle arterial with commercial zoning on both sides of the street. The road has four traffic lanes and a turn lane in the middle. The shoulder of the road is paved and at least 1 m wide in areas where there are no ingress– egress points such as driveways and intersections. Traffic counts obtained by the Ada County Highway District on 24 September 1997 (Wednesday) were 14,192 vehicles
Chinden Road (6000 ADT per lane; 45 mph)
per day in one direction or approximately 7000 per lane. The local traffic demand and forecasting model calculated year 2001 average daily traffic (ADT) of 23,392 in both directions or approximately 6000 ADT per lane. The standard error of the regression of modeled ADT versus actual traffic counts in Ada county is B2000 ADT or 30% of the average modeled ADT. Sections 1–3 were 500, 600, and 600 m long, respectively. The posted speed limit on this road is 45 mph (20 m/s or 72 kph). The second location was the westbound lane on Rose Hill/Franklin Road between Owyhee and Orchard. Rose Hill Road turns into Franklin Road west of Roosevelt. This road section has 2 traffic lanes and a turn lane. It is located in a residential neighborhood with curbing on both sides. Between 16 March 2001 4:00 (Friday) and 18 March 2001 5:00 (Sunday), traffic counters were deployed on the westbound section of Rose Hill 60 m west of Owyhee. Total one-way counts were measured at 5700 on Friday and 4200 on Saturday with a median speed of 33 mph (15 m/s or 53 kph). Modeled 2001 ADT for the road was 17,910 in both directions or approximately 9000 ADT per lane. The posted speed limit on
Rose Hill/Franklin Road (9000 ADT per lane; 35 mph)
Section 1
Section 2 Section 3 Section 2
1 0.9
0.7 0.6 0.5 0.4 0.3 0.2
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Fig. 2. Maps of street sanding and sweeping locations. The upper panels show the maps of the sanding and sweeping locations. The lower panels show the time series of PM10 emission potentials for each test section before and after treatment.
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Rose Hill/Franklin is 35 mph (16 m/s or 56 kph). Since ADT was not measured throughout the week, the Friday and Saturday measurements may not accurately reflect the weekly average ADT on Rose Hill. Lengths of Sections 1–3 were 400, 400, and 800 m, respectively. Both roads were surveyed twice with the TRAKER vehicle early on the morning of 15 March 2001. After initial surveying, a sand truck was operated at 5 mph (2.2 m/s or 8 kph) over all sections of both roads. The rate of sand flow was measured at 2.0 kg/s. The swath of sand thrown from the truck was estimated to span 6 m in diameter. Visual observation of the sand on the test sections indicated that the truck did not uniformly disperse the sand across the lane. The sand in the truck was wet and had a tendency to clump as it was applied. While the sand deposits were not uniform in the across-lane direction, there is no basis for presuming that any test section received more or less sand than the others. Immediately after sanding, a vacuum sweeper (Elgin Whirlwind) began operating on Section 1 and a mechanical broom sweeper (Johnson model HSD) began operating on Section 2. Sweeper operators were instructed to follow routine sweeping procedures to collect all visible sand within their respective sections. The mechanical broom sweeper uses a broom to lift material from the street surface onto a conveyer belt. The material is then delivered to a collection hopper. The vacuum sweeper uses a gutter broom to loosen dirt and debris from the road surface and direct it to a vacuum nozzle that sucks it into a hopper. Section 3 was used as a control and was not swept after sanding. Once the sand had been swept from Sections 1 and 2, the TRAKER vehicle resurveyed the test sections. TRAKER surveys were repeated at several intervals after sweeping to evaluate how emissions from the three test sections evolve over time.
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5. Results 5.1. Seasonal changes in paved and unpaved road dust emissions Data from the TRAKER loop are shown in Fig. 4 as a time series of emission potentials segregated by road class. The figure is split into a winter and a summer portion with the double vertical line delineating the two sampling seasons. Several trends are readily apparent. First, the figure shows that high-speed roads such as interstates are likely to have lower emission potentials than lower speed roads such as those in residential neighborhoods. This result is consistent with what Etyemezian et al. (2003b) found for Southwestern Idaho on the whole. The second observation is that emission potentials decreased steadily over time during the winter sampling period. The largest relative reductions were observed on the residential streets. We hypothesize that the steady decrease over the 18-day sampling period in winter is due to the slow removal of geologic material associated with an earlier event. The geologic material may have been due to road sanding operations or trackout of mud from dirt lots or driveways. Since precipitation and low temperatures are more prevalent in winter, dirt lots and roads are wet for longer periods of time than in summer. Vehicles leaving muddy areas may carry out substantial amounts of mud on the tires and body unto the paved road network. Third, summer emission potentials were lower than those for winter and were also relatively constant over the 10 days of measurements. This indicates that by the time the summer study had begun, excess wintertime geologic material associated with winter conditions had been removed from Ada County roads.
4.2. Summertime street sweeping experiment
5.2. Effects of precipitation on unpaved road dust emissions
During the summer, tests were conducted between 22 July 2001 and 24 July 2001. These tests were intended to evaluate the effectiveness of street sweeping during periods when roads are not impacted by sanding (i.e. spring through fall). The TRAKER vehicle surveyed several roads before and after sweeping to evaluate the relative change in emission potential attributable to street sweeping. Maps of the test streets are shown in Fig. 3. For each street, at least three passes with the TRAKER vehicle were made immediately before and after the sweeper. The one exception, Quarter Horse, was pre-surveyed on 22 July 2001 and swept and postsurveyed two days later. All streets tested were in residential neighborhoods. The sweeper used in these tests was the Elgin Whirlwind vacuum sweeper.
Unpaved road dust emission potentials and precipitation data are presented in Fig. 5. For each day the loop was sampled, the emission potential was averaged over the entire length of the unpaved road. The black circles correspond to the emission potential in [g/vkt]/[m/s]. Observations during the winter season indicated that paved roads remained visibly wet for no more than 6 h after a rain shower of less than 3.5 mm. Unlike paved roads, unpaved roads did not dry out within hours of a precipitation event. The upper panel of Fig. 5 shows a time series of unpaved road emission potential and hourly precipitation from the winter sampling period. Most rain events during the study period were light (o5 mm). The emission potential was found to be closely related to the period since the last rainfall
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Fig. 3. Map of summer, 2001 street sweeper test areas in Boise, ID. The hatched lines are the areas where the sweeper tests occurred. The top panel shows an urban residential neighborhood and the bottom panel shows a rural residential neighborhood.
(Fig. 5b). The regression curve shown in Fig. 5b has a slope of 1.2 ([g/vkt]/[m/s])/(day without rain). Although not shown by this experiment, it is expected that as the road continues to dry the emission potential will reach a maximum value. The unpaved road section shown in Fig. 5 was not resampled in the summertime when conditions were dryer. Other unpaved roads in the Treasure Valley had summertime emission potentials ranging from 3.1 to 14.2 [g/vkt]/[m/s] with an average of 8.6 [g/vkt]/[m/s]. If we assume that 8.6 [g/vkt]/[m/s]
represents the dry state of the unpaved road from Fig. 5, then we may conclude tentatively that rain may reduce unpaved road dust emissions for up to one week after a precipitation event. These results are likely to vary with soil type, initial soil moisture levels, solar radiation intensity, relative humidity, and ADT. US E.P.A.’s emission factor guidance document AP-42 (Midwest Research Institute, 1993; Midwest Research Institute, 1998) suggests the approximation that emissions from unpaved roads
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Emission Potential (g/vkt)/(m/s)
1.6 Collectors Interstates Minor Arterials Principal Arterials Residentials
1.4 1.2 1 0.8 0.6 0.4 0.2
7/22/2001
7/20/2001
7/19/2001
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7/12/2001
3/17/2001
3/15/2001
3/10/2001
3/6/2001
3/3/2001
3/1/2001
2/27/2001
0
Sample Date
Fig. 4. Time series of road dust emission potentials. The double vertical lines separate the winter and summer portions of the study.
should be set to zero for all days when the total daily rainfall exceeds 0.25 mm. The results of the TRAKER loop indicate that unpaved road emissions may require adjustments compared to the dry road emission rate up to one week following the day that rainfall was reported. AP-42 accounts for the effects of rain emission using a soil moisture content term for unpaved roads. Due to the expense and effort associated with collecting representative soil moisture measurements, this parameter is seldom available for road dust emissions estimation. It may be possible to improve upon the AP-42 method for estimating unpaved road emissions by employing precipitation time series to infer emission factors in place of the soil moisture content 5.3. Effects of road sanding and sweeping on paved road dust emissions 5.3.1. Winter road sanding experiment Fig. 2 shows the locations and results of the winter sanding/sweeper tests. Because Chinden Street (left panels) was also on the TRAKER loop, this location was sampled on five different occasions prior to the experiment. The figure shows that prior to the sanding/ sweeping test, the emission potential for the three test sections from Chinden varied less than 15%. Ten minutes after sand application, no significant change was detected in the emission potential from the road surface with the exception of the vacuum swept portion of Chinden. The emission potential from this section dropped from 0.47 to 0.27 [g/vkt]/[m/s]. This initial drop in potential is probably due to moisture
from the sand on the road; the first section on Chinden received the sand that had been sitting at the bottom of the truck hopper overnight and was probably wetter than sand that was applied later in the test (i.e. on road Sections 2 and 3). At 2.5 h after sand application, the emission potential had increased at all test sections with respect to the baseline value prior to sanding. At the Chinden test area, the vacuum swept section S1 emission potential increased by 26%, the mechanically swept section S2 emission potential increased by 42%, and the unswept section S3 emission potential increased by 46%. At the Rose Hill/Franklin test area; emission potentials increased 69% on the vacuum swept section, 63% on the mechanically swept section, 61% on the unswept section. Approximately 8 h after the initial sanding and sweeping, the emission potentials from all sections of both roads had returned to within 15% of their pretreated levels. At this time, the travel lanes of all road sections were clear of visible sand. However much of the sand in the unswept sections had migrated to the untraveled portions of the road (i.e. shoulders and center turn lanes). Sand was not visible on the sections where the road had been swept using either a vacuum or mechanical sweeper. Traffic counter data from Rose Hill on 03/16/01 indicated that 40% of the measured ADT or B2000 cars traveling at 35 mph (16 m/s or 56 kph) passed over the road in the first 8 h. Similarly, on Chinden it is estimated that 2000–2500 cars traveling at 45 mph (20 m/s or 72 kph) passed over each lane in the first 8 h.
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10
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9
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Unpaved Road Emission Potential (g/vkt)/(m/s)
Hourly Precipitaton (mm)
8
2 0.5 1 0.0
0
2/18/2001
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2/28/2001
3/5/2001
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3/20/2001
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Unpaved Road Emission Potential (g/vkt)/(m/s)
10 9 8 7 6 5 4 3 EP (g/vkt)/(m/s) = 1.2 t (days) R2 = 0.94
2 1 0 0
2
4
6
8
10
Days Since Last Precipitation Fig. 5. Plot of emission potentials of unpaved road on TRAKER loop and daily precipitation. The upper panel shows the time series of the average emission potential and hourly precipitation events. The lower panel compares the measured emission potential and the length of time since the last rain event.
The results of the winter sanding and sweeping experiment indicate that the direct impacts of road sanding on PM10 emissions are short lived lasting no more than 8 h or 2500 vehicle passes. While both the vacuum and mechanical sweepers did an excellent job collecting the visible sand on the roads, the systems tested were ineffective at removing the source of the PM10 road dust particles. The application of sand initially increased PM10 emissions from the roads, though only for a short time after application. On unswept sections, sand was transported to the shoulders
within a few hours of application. On the short timescale of these experiments, it was not possible to investigate whether or not sand blown to the side of the road can serve as a long-term reservoir for subsequent PM10 emissions. 5.3.2. Summer street-sweeping experiment Street sweeper performance in summer was consistent with the wintertime tests. Fig. 6 shows the average emission potential for each road segment and heading before and after sweeping. The results from these tests
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Before Sweeping After Sweeping
1.6
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1.4 1.2 1 0.8 0.6 0.4 0.2
Wilcomb South
Wilcomb North
Quarter Horse West
Quarter Horse East
Ponderosa West
Ponderosa East
Ponderay South
Ponderay North
Clark West
Clark East
0
Fig. 6. Comparison of emission potentials from 5 streets in Ada County before and after street sweeping.
are counterintuitive. If the street sweeper is removing suspendable material from the roadway, the emission potential should decrease after sweeping. Instead, the emission potential is observed to increase by up to 40% after sweeping. The average emission potential increase measured on all five roads is 16%. The reason for the increase in emission potential after sweeping is unclear. It is possible that material is displaced from the curb onto the street as the sweeper passes over the road. Another possibility is that particles trapped in the cracks and pits of the road may be redistributed across the road surface by the sweeper. Those particles would then be available for suspension by tires and wakes of passing vehicles.
6. Discussion The TRAKER road dust measurement system is a new tool that can be used to evaluate temporal changes in emissions and the effectiveness of control strategies. Repeated operation of the TRAKER on a fixed route indicated that paved road dust emissions decreased by up to 50% over a period of 3 weeks during the winter. Moreover, data from the TRAKER indicate winter rainfall events suppress unpaved road emissions for a week or longer. These results have important implications for the accuracy of road dust emission models. Road dust emission factor surrogates (e.g. silt loading or TRAKER signal) are generally measured only once for an urban area if at all. Dynamic variations in road dust emissions limit the ability of a small set of measurements to accurately simulate the magnitude and variations of emission factors. Although it may not be practical to collect year round road dust surrogate data for all urban areas with PM10 issues, a quantitative understanding of the processes that
produce road dust could be obtained to more accurately simulate urban road dust emissions. For example, the simplified particulate model (SIMPTM) simulates storm water quality from paved roads based on rainfall, material accumulation rates on road surfaces, and street sweeping practices (Sutherland and Jelen, 1995). SIMPTM was calibrated in several cities with measurements of storm water composition as well as the loading of material on the street surface. The material on the street surface is typically sieved to obtain information about the size distribution of particles on the road with the smallest size fraction being all particles less than 75 mm. Using the structure and inputs of the SIMPTM model, road dust emission simulation may be improved to capture much of the variability observed in the seasonal TRAKER measurements. Additional information would be needed to calibrate a PM10 road dust model based on SIMPTM since the storm water quality simulation is based on much larger particle measurements of sieved material. Control of PM10 road dust emissions can be achieved by minimizing the deposition of material onto roadways and maximizing the removal efficiency of the PM10 sized material already on the roads by road cleaning equipment. TRAKER results have demonstrated that although the sweepers tested effectively capture large material (e.g. sand, trash, and leaf debris), the mechanical and vacuum sweepers tested are ineffective for reducing PM10 road dust emissions in the short term. Results indicate that PM10 emissions immediately after sweeping increase by up to 40% rather than decrease. This is consistent with previous work indicating that sweepers are more effective for surface removal of large particles than for particles with diameters o75 mm (Pitt, 1979). At present, it may be premature to conclude that street sweeping has no effect on the urban scale PM10
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emission inventory. Little is known about the evolution of particle size distributions on the road surface as car and truck tires come in contact with the surface loading material. If street sweeping can remove particles that may evolve into PM10, then sweeping may have a beneficial effect on air quality over the long term. Thus, it is possible that the removal of comparatively large grains of geologic material reduces the total amount of PM10 that would otherwise be available for emission in the long-term. This mechanism, not examined by the experiments presented here, should be studied further since it may have important implications for the effectiveness of street sweeping programs in PM10 emission reduction. Additional study is needed to determine whether long term emission reduction can be associated with street sweeping. The application of sand for traction control on dry roads was found to increase PM10 emissions by up to 75% 2.5 h after application. The dust emission effects of sanding were short lived and emissions returned to the pre-sanding levels within 8 h. The rapid removal rate of the sanding material from the road surface suggests the that the levels of loading on street surfaces are dynamic. If fine material loading on the road (i.e. PM10 sized particles) exhibit a similar behavior, the loading of suspendable material must be recharged quickly or else there would be negligible reservoir of fine material on the road. The paved road PM10 material on the road surface likely exists at an equilibrium with a balance of deposition and emission processes. Deviations from this equilibrium appear to be short lived (on the order of several hours) on typical urban roads. The source of PM10 material on roads is not well quantified. Understanding all processes that control this equilibrium level may one-day result in control strategies that effectively reduce both deposition and emissions from paved roads.
Acknowledgements This work was completed under contract with the Idaho Department of Environmental Quality, Boise, ID (C041). We would like to thank the staff of the Ada County Highway District for their assistance conducting the street sweeping and sanding experiments. We would also like to thank Ross Dodge and Mary Ann Waldinger of the Community Planning Association of Southwest Idaho for providing relevant traffic demand and forecasting model results. Thanks to Mark Kinter (TENNANT CO) for valuable comments and suggestions.
References Chow, J.C., Watson, J.G., Egami, R.T., Frazier, C.A., Lu, Z., Goodrich, A., Bird, A., 1990. Evaluation of regenerative-air vacuum street sweeping on geological contributions to PM10. Journal of the Air and Waste Management Association 40 (8), 1134–1142. Clark, D.E., Cobbins, W.C., 1963. Removal effectiveness of simulated dry fallout from paved areas by motorized vacuumized street sweepers. Report prepared by US Naval Radiological Defense Laboratory, USNRDL-TR-745. Condor, S., 1989. Street cleaning report: 24-city survey and city of portland cost allocation and funding analysis. Report prepared by City of Portland Bureau of Environmental Services. Cowherd, C., Cuscino, T.A., Small, M., 1982. Cost effectiveness of road dust controls. Report prepared by US Environmental Protection Agency, EPA-600/9-82-021, Research Triangle Park, NC. Cuscino, T.A., Muleski, G.E., Cowherd, C., 1983. In Proceedings of the Symposium on Iron and Steel Pollution Abatement Technology for 1982 US Environmental Protection Agency, Research Triangle Park. Etyemezian, V., Kuhns, H., Gillies, J., Green, M., Pitchford, M., Watson, J., 2003a. Vehicle based road dust emissions measurements (I): Methods and Calibration. Atmospheric Environment, this issue (doi:10.1016/S1352-2310(03)00528-4). Etyemezian, V., Kuhns, H., Gillies, J., Chow, J., Hendrickson, K., McGown, M., Pitchford, M., 2003b. Vehicle based road dust emissions measurements (III): Effect of speed, traffic volume, location, and season on PM10 road dust emissions in the Treasure Valley, ID. Atmospheric Environment, this issue (doi:10.1016/S1352-2310(03)00530-2). Midwest Research Institute, 1993. Emission factor documentation for AP-42, Section 13.2.1, Paved roads. Report prepared by Midwest Research Institute, MRI 9712-44, Kansas City, MO. Midwest Research Institute, 1998. Emission factor documentation for AP-42, Section 13.2.2 unpaved roads. Report prepared by Midwest Research Institute, 9712–44. PEDCo-Environmental Specialists, 1981. Study of street cleaning impact on particulate levels in Kansas City, Kansas. Report prepared by PEDCo Environmental, 68-02-2535, Portland, OR. Pitt, R., 1979. Demonstration of nonpoint pollution abatement through improved street cleaning practices. Report prepared by Woodward-Clyde Consultants, PB80-108988, San Francisco, CA. Sartor, J.D., Boyd, G.B., van Horn, W.H., 1972. How effective is your street sweeping? APWA Reporter, 18–23. Seton, J., 1983. Portland road dust demonstration project— Final report prepared by Odell Inc., Portland, OR. Sutherland, R.C., Jelen, S.L., 1995. In: James, W. (Ed.), Advances in Modeling the Management of Stormwater Impacts. pp. 1–14. CHI (Chapter 1).
Atmospheric Environment 37 (2003) 4573–4582
Vehicle-based road dust emission measurement—Part II: Effect of precipitation, wintertime road sanding, and street sweepers on inferred PM10 emission potentials from paved and unpaved roads H. Kuhnsa,*, V. Etyemezianb, M. Greenb, Karin Hendricksonc, Michael McGownc, Kevin Bartond, Marc Pitchforde a
Desert Research Institute, Division of Atmospheric Sciences, 2215 Raggio Parkway, Reno, NV 89512-1095, USA b Desert Research Institute, Las Vegas, NV, USA c Idaho Department of Environmental Quality, Boise, ID, USA d Ada County Highway District, Boise, ID, USA e National Oceanic and Atmospheric Administration, Las Vegas, NV, USA Received 30 August 2002; received in revised form 4 February 2003; accepted 13 June 2003
Abstract Testing Re-Entrained Kinetic Emissions from Roads (TRAKER) is a new technique to infer paved and unpaved road dust PM10 emission potentials based on particulate matter (PM) measurements made onboard a moving vehicle. Light scattering instruments mounted in front and behind the vehicle’s tires measure the differential particle concentration of dust suspended by the vehicle’s tire in contact with the road surface. Through empirical regressions relating the differential concentration (i.e. TRAKER signal) with the vehicle speed and the downwind flux of PM10 particles from the road, an equation is derived to infer the speed independent road dust emission potential from the measured TRAKER signal. Measurements from TRAKER offer a new perspective on the processes that affect road dust emissions. The system was used to investigate temporal changes in emission potentials from paved roads in both the winter and summer in the Treasure Valley in Southwest Idaho. During the 3-week wintertime sampling period, the residential road dust PM10 emission potential decreased by B50%. Summertime PM10 emission potentials were similar to those observed at the end of the winter sampling and showed no upward or downward trends. Wintertime unpaved road emissions increased consistently with the number of days since the last rainfall. Measurement of road dust emission potentials after road sanding on dry roads indicated a 75% increase in PM10 emissions after 2.5 h. This effect was short lived and emission potentials returned to their pre-sanding levels within 8 h of the sand application. Street sweeping with mechanical and vacuum sweepers was found to offer no measurable reduction in PM10 emission potentials. On several roads, the PM10 emission potentials actually increased immediately after vacuum sweeping. Long term effects of street sweeping on road dust emissions were not evaluated as part of this study and may offer some overall reduction in PM emissions from paved roads. r 2003 Elsevier Ltd. All rights reserved. Keywords: TRAKER; Road dust; Emissions; Road sanding; Street sweeping; Paved road; Unpaved road; PM10
1. Introduction *Corresponding author. Tel.: +1-775-674-7111; fax: +1775-674-7060. E-mail address: [email protected] (H. Kuhns).
This is the second paper in a three part series describing the results of paved and unpaved road dust
1352-2310/$ - see front matter r 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S1352-2310(03)00529-6
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emission experiments using the Testing Re-Entrained Kinetic Emissions from Roads (TRAKER) system. TRAKER is a vehicle equipped with real time particle monitors mounted in front of the vehicle and behind the front tires to measure the concentration of road dust suspended from the road surface while the vehicle is in motion. The vehicle configuration and the calibration of the TRAKER with respect to horizontal particle flux measurements are described in the first paper of the series (Etyemezian et al., 2003a). This paper discusses how the TRAKER system was used to evaluate the effects of precipitation, road sanding for traction control, and street sweeping on road dust emission potentials in the Treasure Valley, Idaho. The third paper in the series (Etyemezian et al., 2003b) describes how dust emission potentials vary with road characteristics such as the posted speed limit, volume of traffic, and geographic setting. Particulate matter (PM) emission inventories for most urban areas indicate that paved and unpaved road dust are the largest source categories. For example, for the 2001 base year in the state of California, paved and unpaved road dust emissions account for 17% and 27% of the total estimated PM10 emissions, respectively. Despite the importance of geologic material to the total PM10 and PM2.5 emission budgets, limited quantitative information is known about how road dust emissions change throughout the seasons and what controls, if any, can be effectively implemented to reduce their overall levels. More detailed emission factors are required as input into increasingly sophisticated computer models to simulate the transport and dispersion of emissions within urban and regional air sheds. Knowledge of the effectiveness of emission control strategies is needed to develop practical strategies for reducing ambient pollutant concentrations. This type of information is necessary for states to develop credible State Implementation Plans (SIP’s) to demonstrate how new or existing regulations will bring ambient air quality into attainment of US national standards. With respect to street sweeping, one published study found no discernable differences in airborne geologic PM10 concentrations measured either during or after one 10- and one 7-day long street sweeping campaigns in a Reno, Nevada neighborhood (Chow et al., 1990). Other studies attempting to quantify road dust emission reductions as a result of sweeping have had only limited success since ambient PM10 concentrations are influenced by meteorology and other emission sources that add large uncertainties to what appears to be a small effect (Cowherd et al., 1982; Cuscino et al., 1983; PEDCo-Environmental Specialists, 1981; Seton, 1983). Street sweepers have been studied in the context of road dust impacts on storm water quality. Rainfall has been found to increase material picked up by routine street sweeping in Portland, Oregon (Condor, 1989;
Sutherland and Jelen, 1995). Wintertime (rain season) sweeper pickups were up to 500% greater than summertime sweeper pickups, perhaps because less material is washed onto roads during the summer. Although sweepers effectively pick up visible material from the road surface, some studies have shown that both mechanical and vacuum sweepers are not very effective at collecting small particles. Sweeper pick up efficiencies for particles less than 44 mm range from 3% to 15% (Clark and Cobbins, 1963; Sartor et al., 1972). This paper uses data from a field study conducted in the Treasure Valley, ID to investigate the seasonality of inferred PM10 road dust emission potentials, the relative effectiveness of mechanical and vacuum sweepers in reducing emissions, and the temporal response of emission potential from unpaved road dust after rainfall.
2. Experimental methods The methods of TRAKER operation and calibration are described in detail in a companion paper (Etyemezian et al., 2003b). The inference of PM10 emission potential from the TRAKER measurement is reviewed here. TRAKER directly measures PM10 concentrations in front of and behind a vehicle’s tires while the vehicle is in motion. The difference in concentration between the two particle sensors was empirically related to the speed of the vehicle by the equation T ¼ TT TB ¼ as3 ;
ð1Þ
where TT is the aerosol concentration at the vehicle tire, TB is the background aerosol concentration measured through the vehicle’s front bumper, T is the difference in concentrations (i.e. TRAKER signal in mg/m3), and s is the speed of the vehicle. The variable a is a characteristic of the road being sampled. Experiments using towers to measure the horizontal flux of PM10 particles downwind of an unpaved road found an empirical power relationship between the TRAKER signal T and the PM10 emission factor (in grams per vehicle kilometer traveled) measured at 9 and 50 m downwind of the road: EFðg=vktÞ ¼ 8:8ðsg ¼ 1:5ÞT 0:3370:06 ;
ð2Þ
where EF is the PM10 emission factor. The geometric standard error (sg ) of the regression coefficient (8.8) was 1.5. Note that range of emission factors used for the calibration, 10–150 g/vkt, is generally one or two orders of magnitude higher than the typical emission factors for paved roads. Also, the vehicle speeds used to establish the relationship in Eq. (2) ranged from 2 to 8 m/s (5– 15 mph or 8–23 kph) on unpaved roads. Measurements downwind of unpaved roads indicate that PM10 emission factors increase linearly with vehicle speed. By dividing the PM10 emission factor by the vehicle speed,
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the resultant term (i.e. the emission potential in (g/vkt)/ (m/s)) is solely a property of the suspendable material on road and is independent of the travel speed of the vehicle operating on that road. This paper explores how the PM10 emission potential changes with season, precipitation, road sanding, and road sweeping. The precision of the TRAKER signal pertains to its ability to repeatedly measure consistent values over identical conditions. Repeated measurements of the TRAKER signal (T in Eq. (1)) on a 500 m stretch of paved road in Treasure Valley, Idaho and a 1200 m stretch of paved road in El Paso, Texas resulted in relative precision values between 10% and 20% for speeds greater than 10 m/s (20 mph or 36 km/h) and precisions between 20% and 85% for slower speeds. Since the emission potential is proportional to the cube root of the TRAKER signal, the relative precision of the emission potential is approximately one-third of the relative TRAKER signal precision (i.e. o7% for speeds greater than 10 m/s and o30% for speeds less than 10 m/s). The accuracy of the TRAKER measurement can be defined as its ability to infer the true PM10 emission potential from a road. The downwind horizontal flux of PM10 from the road is generally assumed to represent the ‘‘true’’ PM10 emissions from the road. However the precision of the flux measurement ranges from 10% to
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nearly 50% (Etyemezian et al., 2003a, Table 1). Although the accuracy of the emission potential based on the TRAKER signal cannot be better than the precision of the horizontal PM10 flux measurements, a precise TRAKER signal may be able to resolve relative differences in emission potentials that are less than the relative precision of the flux measurement itself. Thus, it is reasonable to state that the relative precision of the TRAKER emission potential (7–30%) is less than the relative accuracy of the emission potential (10–50%).
3. Temporal measurements of road dust variation To investigate temporal changes in road dust emission potentials, a 50 km loop was selected near Boise, Idaho spanning a variety of road types (interstate, arterial, collector, local, and unpaved) in both urban and rural areas. The loop was surveyed with the TRAKER 7 times during the winter season (26 February 2001 to 17 March 2001) and 5 times during the summer season (10 July 2001 to 28 July 2001). A map of the loop is shown in Fig. 1. Paved road sampling took place only when the road was visibly dry. The TRAKER loop included a 1 km section of private unpaved road that was sampled during the winter season only. The effect of precipitation
1 mile
Fig. 1. Loop of streets surveyed 7 times during the winter and 5 times during summer 2000 using the TRAKER vehicle in the Boise area in southwest Idaho.
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on unpaved road emission potentials was examined by comparing precipitation and relative humidity measurements collected at the Boise Airport to TRAKER measurements from this unpaved section.
4. Road sanding and sweeping experiments 4.1. Wintertime road sanding experiment The winter road sanding experiment took place on the morning of 15 March 2001 (Thursday) at two different locations in the Boise area. The first test location was on the rightmost eastbound lane on Chinden Road between 50th and 42nd St (Fig. 2). In this area, Chinden is a principle arterial with commercial zoning on both sides of the street. The road has four traffic lanes and a turn lane in the middle. The shoulder of the road is paved and at least 1 m wide in areas where there are no ingress– egress points such as driveways and intersections. Traffic counts obtained by the Ada County Highway District on 24 September 1997 (Wednesday) were 14,192 vehicles
Chinden Road (6000 ADT per lane; 45 mph)
per day in one direction or approximately 7000 per lane. The local traffic demand and forecasting model calculated year 2001 average daily traffic (ADT) of 23,392 in both directions or approximately 6000 ADT per lane. The standard error of the regression of modeled ADT versus actual traffic counts in Ada county is B2000 ADT or 30% of the average modeled ADT. Sections 1–3 were 500, 600, and 600 m long, respectively. The posted speed limit on this road is 45 mph (20 m/s or 72 kph). The second location was the westbound lane on Rose Hill/Franklin Road between Owyhee and Orchard. Rose Hill Road turns into Franklin Road west of Roosevelt. This road section has 2 traffic lanes and a turn lane. It is located in a residential neighborhood with curbing on both sides. Between 16 March 2001 4:00 (Friday) and 18 March 2001 5:00 (Sunday), traffic counters were deployed on the westbound section of Rose Hill 60 m west of Owyhee. Total one-way counts were measured at 5700 on Friday and 4200 on Saturday with a median speed of 33 mph (15 m/s or 53 kph). Modeled 2001 ADT for the road was 17,910 in both directions or approximately 9000 ADT per lane. The posted speed limit on
Rose Hill/Franklin Road (9000 ADT per lane; 35 mph)
Section 1
Section 2 Section 3 Section 2
1 0.9
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Fig. 2. Maps of street sanding and sweeping locations. The upper panels show the maps of the sanding and sweeping locations. The lower panels show the time series of PM10 emission potentials for each test section before and after treatment.
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Rose Hill/Franklin is 35 mph (16 m/s or 56 kph). Since ADT was not measured throughout the week, the Friday and Saturday measurements may not accurately reflect the weekly average ADT on Rose Hill. Lengths of Sections 1–3 were 400, 400, and 800 m, respectively. Both roads were surveyed twice with the TRAKER vehicle early on the morning of 15 March 2001. After initial surveying, a sand truck was operated at 5 mph (2.2 m/s or 8 kph) over all sections of both roads. The rate of sand flow was measured at 2.0 kg/s. The swath of sand thrown from the truck was estimated to span 6 m in diameter. Visual observation of the sand on the test sections indicated that the truck did not uniformly disperse the sand across the lane. The sand in the truck was wet and had a tendency to clump as it was applied. While the sand deposits were not uniform in the across-lane direction, there is no basis for presuming that any test section received more or less sand than the others. Immediately after sanding, a vacuum sweeper (Elgin Whirlwind) began operating on Section 1 and a mechanical broom sweeper (Johnson model HSD) began operating on Section 2. Sweeper operators were instructed to follow routine sweeping procedures to collect all visible sand within their respective sections. The mechanical broom sweeper uses a broom to lift material from the street surface onto a conveyer belt. The material is then delivered to a collection hopper. The vacuum sweeper uses a gutter broom to loosen dirt and debris from the road surface and direct it to a vacuum nozzle that sucks it into a hopper. Section 3 was used as a control and was not swept after sanding. Once the sand had been swept from Sections 1 and 2, the TRAKER vehicle resurveyed the test sections. TRAKER surveys were repeated at several intervals after sweeping to evaluate how emissions from the three test sections evolve over time.
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5. Results 5.1. Seasonal changes in paved and unpaved road dust emissions Data from the TRAKER loop are shown in Fig. 4 as a time series of emission potentials segregated by road class. The figure is split into a winter and a summer portion with the double vertical line delineating the two sampling seasons. Several trends are readily apparent. First, the figure shows that high-speed roads such as interstates are likely to have lower emission potentials than lower speed roads such as those in residential neighborhoods. This result is consistent with what Etyemezian et al. (2003b) found for Southwestern Idaho on the whole. The second observation is that emission potentials decreased steadily over time during the winter sampling period. The largest relative reductions were observed on the residential streets. We hypothesize that the steady decrease over the 18-day sampling period in winter is due to the slow removal of geologic material associated with an earlier event. The geologic material may have been due to road sanding operations or trackout of mud from dirt lots or driveways. Since precipitation and low temperatures are more prevalent in winter, dirt lots and roads are wet for longer periods of time than in summer. Vehicles leaving muddy areas may carry out substantial amounts of mud on the tires and body unto the paved road network. Third, summer emission potentials were lower than those for winter and were also relatively constant over the 10 days of measurements. This indicates that by the time the summer study had begun, excess wintertime geologic material associated with winter conditions had been removed from Ada County roads.
4.2. Summertime street sweeping experiment
5.2. Effects of precipitation on unpaved road dust emissions
During the summer, tests were conducted between 22 July 2001 and 24 July 2001. These tests were intended to evaluate the effectiveness of street sweeping during periods when roads are not impacted by sanding (i.e. spring through fall). The TRAKER vehicle surveyed several roads before and after sweeping to evaluate the relative change in emission potential attributable to street sweeping. Maps of the test streets are shown in Fig. 3. For each street, at least three passes with the TRAKER vehicle were made immediately before and after the sweeper. The one exception, Quarter Horse, was pre-surveyed on 22 July 2001 and swept and postsurveyed two days later. All streets tested were in residential neighborhoods. The sweeper used in these tests was the Elgin Whirlwind vacuum sweeper.
Unpaved road dust emission potentials and precipitation data are presented in Fig. 5. For each day the loop was sampled, the emission potential was averaged over the entire length of the unpaved road. The black circles correspond to the emission potential in [g/vkt]/[m/s]. Observations during the winter season indicated that paved roads remained visibly wet for no more than 6 h after a rain shower of less than 3.5 mm. Unlike paved roads, unpaved roads did not dry out within hours of a precipitation event. The upper panel of Fig. 5 shows a time series of unpaved road emission potential and hourly precipitation from the winter sampling period. Most rain events during the study period were light (o5 mm). The emission potential was found to be closely related to the period since the last rainfall
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Fig. 3. Map of summer, 2001 street sweeper test areas in Boise, ID. The hatched lines are the areas where the sweeper tests occurred. The top panel shows an urban residential neighborhood and the bottom panel shows a rural residential neighborhood.
(Fig. 5b). The regression curve shown in Fig. 5b has a slope of 1.2 ([g/vkt]/[m/s])/(day without rain). Although not shown by this experiment, it is expected that as the road continues to dry the emission potential will reach a maximum value. The unpaved road section shown in Fig. 5 was not resampled in the summertime when conditions were dryer. Other unpaved roads in the Treasure Valley had summertime emission potentials ranging from 3.1 to 14.2 [g/vkt]/[m/s] with an average of 8.6 [g/vkt]/[m/s]. If we assume that 8.6 [g/vkt]/[m/s]
represents the dry state of the unpaved road from Fig. 5, then we may conclude tentatively that rain may reduce unpaved road dust emissions for up to one week after a precipitation event. These results are likely to vary with soil type, initial soil moisture levels, solar radiation intensity, relative humidity, and ADT. US E.P.A.’s emission factor guidance document AP-42 (Midwest Research Institute, 1993; Midwest Research Institute, 1998) suggests the approximation that emissions from unpaved roads
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Emission Potential (g/vkt)/(m/s)
1.6 Collectors Interstates Minor Arterials Principal Arterials Residentials
1.4 1.2 1 0.8 0.6 0.4 0.2
7/22/2001
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7/12/2001
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3/15/2001
3/10/2001
3/6/2001
3/3/2001
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Sample Date
Fig. 4. Time series of road dust emission potentials. The double vertical lines separate the winter and summer portions of the study.
should be set to zero for all days when the total daily rainfall exceeds 0.25 mm. The results of the TRAKER loop indicate that unpaved road emissions may require adjustments compared to the dry road emission rate up to one week following the day that rainfall was reported. AP-42 accounts for the effects of rain emission using a soil moisture content term for unpaved roads. Due to the expense and effort associated with collecting representative soil moisture measurements, this parameter is seldom available for road dust emissions estimation. It may be possible to improve upon the AP-42 method for estimating unpaved road emissions by employing precipitation time series to infer emission factors in place of the soil moisture content 5.3. Effects of road sanding and sweeping on paved road dust emissions 5.3.1. Winter road sanding experiment Fig. 2 shows the locations and results of the winter sanding/sweeper tests. Because Chinden Street (left panels) was also on the TRAKER loop, this location was sampled on five different occasions prior to the experiment. The figure shows that prior to the sanding/ sweeping test, the emission potential for the three test sections from Chinden varied less than 15%. Ten minutes after sand application, no significant change was detected in the emission potential from the road surface with the exception of the vacuum swept portion of Chinden. The emission potential from this section dropped from 0.47 to 0.27 [g/vkt]/[m/s]. This initial drop in potential is probably due to moisture
from the sand on the road; the first section on Chinden received the sand that had been sitting at the bottom of the truck hopper overnight and was probably wetter than sand that was applied later in the test (i.e. on road Sections 2 and 3). At 2.5 h after sand application, the emission potential had increased at all test sections with respect to the baseline value prior to sanding. At the Chinden test area, the vacuum swept section S1 emission potential increased by 26%, the mechanically swept section S2 emission potential increased by 42%, and the unswept section S3 emission potential increased by 46%. At the Rose Hill/Franklin test area; emission potentials increased 69% on the vacuum swept section, 63% on the mechanically swept section, 61% on the unswept section. Approximately 8 h after the initial sanding and sweeping, the emission potentials from all sections of both roads had returned to within 15% of their pretreated levels. At this time, the travel lanes of all road sections were clear of visible sand. However much of the sand in the unswept sections had migrated to the untraveled portions of the road (i.e. shoulders and center turn lanes). Sand was not visible on the sections where the road had been swept using either a vacuum or mechanical sweeper. Traffic counter data from Rose Hill on 03/16/01 indicated that 40% of the measured ADT or B2000 cars traveling at 35 mph (16 m/s or 56 kph) passed over the road in the first 8 h. Similarly, on Chinden it is estimated that 2000–2500 cars traveling at 45 mph (20 m/s or 72 kph) passed over each lane in the first 8 h.
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10
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9
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8
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Unpaved Road Emission Potential (g/vkt)/(m/s)
10 9 8 7 6 5 4 3 EP (g/vkt)/(m/s) = 1.2 t (days) R2 = 0.94
2 1 0 0
2
4
6
8
10
Days Since Last Precipitation Fig. 5. Plot of emission potentials of unpaved road on TRAKER loop and daily precipitation. The upper panel shows the time series of the average emission potential and hourly precipitation events. The lower panel compares the measured emission potential and the length of time since the last rain event.
The results of the winter sanding and sweeping experiment indicate that the direct impacts of road sanding on PM10 emissions are short lived lasting no more than 8 h or 2500 vehicle passes. While both the vacuum and mechanical sweepers did an excellent job collecting the visible sand on the roads, the systems tested were ineffective at removing the source of the PM10 road dust particles. The application of sand initially increased PM10 emissions from the roads, though only for a short time after application. On unswept sections, sand was transported to the shoulders
within a few hours of application. On the short timescale of these experiments, it was not possible to investigate whether or not sand blown to the side of the road can serve as a long-term reservoir for subsequent PM10 emissions. 5.3.2. Summer street-sweeping experiment Street sweeper performance in summer was consistent with the wintertime tests. Fig. 6 shows the average emission potential for each road segment and heading before and after sweeping. The results from these tests
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Before Sweeping After Sweeping
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1.4 1.2 1 0.8 0.6 0.4 0.2
Wilcomb South
Wilcomb North
Quarter Horse West
Quarter Horse East
Ponderosa West
Ponderosa East
Ponderay South
Ponderay North
Clark West
Clark East
0
Fig. 6. Comparison of emission potentials from 5 streets in Ada County before and after street sweeping.
are counterintuitive. If the street sweeper is removing suspendable material from the roadway, the emission potential should decrease after sweeping. Instead, the emission potential is observed to increase by up to 40% after sweeping. The average emission potential increase measured on all five roads is 16%. The reason for the increase in emission potential after sweeping is unclear. It is possible that material is displaced from the curb onto the street as the sweeper passes over the road. Another possibility is that particles trapped in the cracks and pits of the road may be redistributed across the road surface by the sweeper. Those particles would then be available for suspension by tires and wakes of passing vehicles.
6. Discussion The TRAKER road dust measurement system is a new tool that can be used to evaluate temporal changes in emissions and the effectiveness of control strategies. Repeated operation of the TRAKER on a fixed route indicated that paved road dust emissions decreased by up to 50% over a period of 3 weeks during the winter. Moreover, data from the TRAKER indicate winter rainfall events suppress unpaved road emissions for a week or longer. These results have important implications for the accuracy of road dust emission models. Road dust emission factor surrogates (e.g. silt loading or TRAKER signal) are generally measured only once for an urban area if at all. Dynamic variations in road dust emissions limit the ability of a small set of measurements to accurately simulate the magnitude and variations of emission factors. Although it may not be practical to collect year round road dust surrogate data for all urban areas with PM10 issues, a quantitative understanding of the processes that
produce road dust could be obtained to more accurately simulate urban road dust emissions. For example, the simplified particulate model (SIMPTM) simulates storm water quality from paved roads based on rainfall, material accumulation rates on road surfaces, and street sweeping practices (Sutherland and Jelen, 1995). SIMPTM was calibrated in several cities with measurements of storm water composition as well as the loading of material on the street surface. The material on the street surface is typically sieved to obtain information about the size distribution of particles on the road with the smallest size fraction being all particles less than 75 mm. Using the structure and inputs of the SIMPTM model, road dust emission simulation may be improved to capture much of the variability observed in the seasonal TRAKER measurements. Additional information would be needed to calibrate a PM10 road dust model based on SIMPTM since the storm water quality simulation is based on much larger particle measurements of sieved material. Control of PM10 road dust emissions can be achieved by minimizing the deposition of material onto roadways and maximizing the removal efficiency of the PM10 sized material already on the roads by road cleaning equipment. TRAKER results have demonstrated that although the sweepers tested effectively capture large material (e.g. sand, trash, and leaf debris), the mechanical and vacuum sweepers tested are ineffective for reducing PM10 road dust emissions in the short term. Results indicate that PM10 emissions immediately after sweeping increase by up to 40% rather than decrease. This is consistent with previous work indicating that sweepers are more effective for surface removal of large particles than for particles with diameters o75 mm (Pitt, 1979). At present, it may be premature to conclude that street sweeping has no effect on the urban scale PM10
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emission inventory. Little is known about the evolution of particle size distributions on the road surface as car and truck tires come in contact with the surface loading material. If street sweeping can remove particles that may evolve into PM10, then sweeping may have a beneficial effect on air quality over the long term. Thus, it is possible that the removal of comparatively large grains of geologic material reduces the total amount of PM10 that would otherwise be available for emission in the long-term. This mechanism, not examined by the experiments presented here, should be studied further since it may have important implications for the effectiveness of street sweeping programs in PM10 emission reduction. Additional study is needed to determine whether long term emission reduction can be associated with street sweeping. The application of sand for traction control on dry roads was found to increase PM10 emissions by up to 75% 2.5 h after application. The dust emission effects of sanding were short lived and emissions returned to the pre-sanding levels within 8 h. The rapid removal rate of the sanding material from the road surface suggests the that the levels of loading on street surfaces are dynamic. If fine material loading on the road (i.e. PM10 sized particles) exhibit a similar behavior, the loading of suspendable material must be recharged quickly or else there would be negligible reservoir of fine material on the road. The paved road PM10 material on the road surface likely exists at an equilibrium with a balance of deposition and emission processes. Deviations from this equilibrium appear to be short lived (on the order of several hours) on typical urban roads. The source of PM10 material on roads is not well quantified. Understanding all processes that control this equilibrium level may one-day result in control strategies that effectively reduce both deposition and emissions from paved roads.
Acknowledgements This work was completed under contract with the Idaho Department of Environmental Quality, Boise, ID (C041). We would like to thank the staff of the Ada County Highway District for their assistance conducting the street sweeping and sanding experiments. We would also like to thank Ross Dodge and Mary Ann Waldinger of the Community Planning Association of Southwest Idaho for providing relevant traffic demand and forecasting model results. Thanks to Mark Kinter (TENNANT CO) for valuable comments and suggestions.
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