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Particulate matter emission by a vehicle running on unpaved road
ARTICLE IN PRESS
Atmospheric Environment 42 (2008) 3899–3905 www.elsevier.com/locate/atmosenv
Technical note
Particulate matter emission by a vehicle running on unpaved road David Scott Williamsa, Manoj K. Shuklaa,, Jim Rossb a
Department of Plant and Environmental Sciences, New Mexico State University, MSC 3Q, P.O. Box 30003, Las Cruces, NM 88003, USA b Department of Entomology Plant Pathology and Weed Science, New Mexico State University, Las Cruces, NM 88003, USA Received 4 August 2007; received in revised form 5 February 2008; accepted 5 February 2008
Abstract The particulate matter (PM) emission from unpaved roads starts with the pulverization of surface material by the force of the vehicle, uplifting and subsequent exposure of road to strong air currents behind the wheels. The objectives of the project were to: demonstrate the utility of a simple technique for collecting suspended airborne PM emitted by vehicle running on an unpaved road, determine the mass balance of airborne PM at different heights, and determine the particle size and elemental composition of PM. We collected dust samples on sticky tapes using a rotorod sampler mounted on a tower across an unpaved road located at the Leyendecker Plant Sciences Research Center, Las Cruces, NM, USA. Dust samples were collected at 1.5, 4.5 and 6 m height above the ground surface on the east and west side of the road. One rotorod sampler was also installed at the centre of the road at 6 m height. Dust samples from unpaved road were mostly (70%) silt and clay-sized particles and were collected at all heights. The height and width of the PM plume and the amount of clay-sized particles captured on both sides of the road increased with speed and particle captured ranged from 0.05 to 159 mm. Dust particles between PM10 and PM2.5 did not correlate with vehicle speed but particles pPM2.5 did. Emission factors estimated for the total suspended PM were 10147 g km1 at 48 km h1 and 11062 g km1 at 64 km h1 speed, respectively. The predominant elements detected in PM were carbon, aluminum and silica at all heights. Overall, sticky tape method coupled with electron microscopy was a useful technique for a rapid particle size and elemental characterization of airborne PM. r 2008 Elsevier Ltd. All rights reserved. Keywords: Particulate matter; Unpaved road; Rotorod; Sticky tape; Emission; Moisture content; Silt and clay
1. Introduction Unpaved roads produce as much as 10 million tons of particulate matter (PM) each year in the United States (Ferguson, 1999). In New Mexico, Corresponding author. Tel.: +1 505 646 2324; fax: +1 505 646 6041. E-mail address: [email protected] (M.K. Shukla).
1352-2310/$ - see front matter r 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.atmosenv.2008.02.003
like most arid regions, a large amount of PM is generated from vehicles traveling on unpaved rural roads, which can significantly affect air quality on a local scale (Pinnick et al., 1985). Most unpaved roads consist of graded and compacted roadbed usually created from the parent material. The rolling wheels of the vehicles impart a force to the surface that pulverizes the roadbed material and ejects particles from the shearing force as well as by
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D.S. Williams et al. / Atmospheric Environment 42 (2008) 3899–3905
the turbulent wake (EPA, 2006). However, little is known about the quantity, composition, particle size, extinction characteristic, fluxes and transport distances of fugitive dust from unpaved roads and their contribution to PM10 exceedances (Pinnick et al., 1985; Kuhn et al., 2001; Veranth et al., 2003). Dust emission rates from unpaved roads are a function of the silt loadings or particle size distribution (Cowherd et al., 1974; Kuhn et al., 2005), vehicle speed (Etyemezian et al., 2003; Gillies et al., 2005), size of the vehicle (Gillies et al., 2005) and moisture content of the road dust (Etyemezian et al., 2003). Dust particles emitted by vehicular traffic consist of aggregates of fine clay to quartz particles and can be several orders of magnitude different in the size distribution (Pinnick et al., 1985). Knowledge of the variability of the individual factors associated with physical properties of the unpaved road and their role on PM emissions is critical to develop accurate air quality standards and models. At the same time, there is a need to develop cost-effective sensors, which can collect emission data to quantify the spatial variations, composition and spread of airborne PM. These samplers should be cost-effective, collect enough samples for simultaneous chemical and physical analysis, and the samples should be representative of the PM generated by the vehicle. Thus, the
objectives of this project were to (i) demonstrate the utility of a simple method (rotorod and sticky tapes), (ii) carry out the mass balance of airborne PM, and (iii) determine the elemental composition and particle size distribution. The hypothesis for the research was that increasing vehicular speed would result in greater dispersion of finer-sized PM. 2. Methods and materials 2.1. Experimental site and setup The unpaved road for the PM emission experiment was located at Leyendecker Plant Sciences Research Center (PSRC) of New Mexico State University, Messiah Valley, NM, USA (321110 35.8400 N and 1061440 08.7500 W). Soils of the area are classified as Glendale (fine-silty, mixed, calcareous, thermic typic Torrifluvents)-Harkey (coarsesilty, mixed, calcareous, thermic typic Torrifluvents). The alluvium is modified by wind and Aeolian material. According to the USDA textural classification, the dominant soil type of the study area was clay loam. Climate of the experimental area was classified as arid with mean annual temperature ranging from 19 to 20 1C and mean annual precipitation from 180 to 230 mm (Bulloch and Neher, 1980).
Fig. 1. Photograph of the experimental set up at the Plant Sciences Research Station, Las Cruces, NM, USA. Rotorods and sticky tape were installed at east (E), west (W) and top (T) of the tower at 1.5, 4.5 and 6 m height, respectively.
ARTICLE IN PRESS D.S. Williams et al. / Atmospheric Environment 42 (2008) 3899–3905
Two towers were erected on either side (east– west) of the unpaved road and four rotorods consisting of a constant speed motor, U-rods (Sampling Technologies, 1989), and glass slides with double-sided sticky tapes (STR tape 0.076 mm thick; Shinto Paint Company Ltd.) were placed at 1.5 and 4.5 m above the ground surface on east and west side of the road (E1.5, E4.5, W1.5 and W4.5; Fig. 1). In addition, three rotorods 4.5 m apart were suspended at a height of 6 m across the unpaved road (TE6, TM6 and TW6). Wind speed and direction, relative humidity and other meteorological variables were measured using a standard portable meteorological instrument (Davis Instruments, model Vantage Pro). The road was isolated from other traffic approximately an hour prior to the start and until the end of experiments. Two experiments were conducted using a Chevrolet pick-up truck (approximately 2300 kg; 1.8 m high on the rear; 4-wheel drive) at two speeds (48 and 64 km h1). Dust samples were collected for a total of 6 min after the vehicle passed underneath the tower and total volume of air sampled was determined as the product of rpm of rotorods, circumference of the sampling area, and length (2.9470.18 cm) and width (0.870.0 cm) of sticky tape. The sticky tapes were weighed before and after the experiment to obtain the total amount of aerosol retained and was used to calculate the concentration of aerosol at different heights and speeds. The quantity of total suspended particulate matter emission (PE) from unpaved road (lb per vehicle mile travel) was estimated by following equation (EPA, 2006) and results were subsequently
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converted to g per vehicle km travel: PE ¼
kðs=12Þa ðS=30Þd C ðM=0:5Þc
(1)
where k, a, c and d are constants and were obtained as 6.0; 1; 0.3; and 0.3, respectively, for total suspended PM from public road; s is average silt content of unpaved road (%); S is mean vehicle speed (mph); and C is emission factor (0.00047 lb per vehicle mile travel (EPA, 2006). 2.2. Physical properties of dust on unpaved road At the end of the experiment, four road samples were collected from locations separated by 10 m on the unpaved road and particle size distribution was obtained using the standard hydrometer method (Gee and Bauder, 1986). Moisture content of the dust particles collected from unpaved road was also determined by oven drying the sample at 105 1C. At each sampling location, three penetration tests were performed using a standard soil penetrometer to a depth of 5, 10 and 15 cm and a soil compaction tester (Dickey-John Corporation) with a 1.27 cm tip to a depth of 5 cm. 2.3. Electron microscope and image analysis Images of the PM samples retained on sticky tapes were created using a scanning electron microscope (HITACHI, S-3400N) (Fig. 2). A total of 54 micrographs were created for the two experiments at three different heights. One particle (largest size) on the micrograph was selected at each height on
Fig. 2. Raw image generated by electron microscope showing the dust particles, smudges and bubbles on the sticky tape at east (E), west (W) and top (T) of the tower at 1.5, 4.5 and 6 m height, respectively.
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east tower and magnified to 25 or 50 mm. Energy dispersive X-ray spectroscopy (EDX; Thermo electron Noran System six) at a voltage of 25 kV determined the elemental composition of the particle at each location. Each electron micrograph was analyzed using ImageJ and Microsoft Paint. Each image was altered to a binary image and bubbles (see Fig. 2), scratches and smears were removed from the image. Once all the visually desired particles were within the threshold range, each image was processed into binary images. The particle area, standard deviation, integrated density, perimeter, and limit to threshold were checked using particle analyzer and the area was calculated as the number of pixels forming the 8-neighbor connected particle. The pixel range varied from 0 to infinity to account for all particle sizes from fine clay to coarse-grained sand and possibly aggregates. 3. Results and discussion 3.1. Physical properties of soil from unpaved road Sand content of the unpaved road ranged from 19% to 39%, silt content from 39% to 56% and clay content from 22% to 26%. Silt and clay contents constituted for about 72% of all primary particles (Table 1). Moisture content varied between 2% and 4% indicating that road was fairly dry during the experiment. The penetration resistance varied between 15.5 and 28.1 kg cm2 (CV ¼ 18%) at a depth of 5 cm. Number of blows required to penetrate to a depth of 5, 10 and 15 cm ranged from 18 to 45, 19 to 67 and 27 to 65, respectively, and displayed large variability with CV ranging from 32% to 45% despite the low variability of moisture
content of road dust. The large variability of penetration resistance indicated that apart from moisture content, compaction likely played an important role for PM emission from unpaved roads. 3.2. Mass balance from rotorod data The Exp-1 (48 km h1) was started at 7:20 AM and atmospheric conditions recorded 2-m west of the tower showed that average wind direction during the test was 1651, average wind velocity was 1.6 km h1 and the relative humidity was 42%. The Exp-2 (64 km h1) was started at 8:05 AM with average wind direction of 1501, average wind velocity of 2.4 km h1 and the average relative humidity of 20%. Height of the turbulent wake behind the vehicle as seen by the unaided eyes ranged from 2 m above ground for 48 km h1 to 3.5 m for 64 km h1 speeds (1.1 and 1.9 times the vehicle height). Thus, injection height seemed to be a function of the vehicle speed. In contrast, Gillies et al. (2005) reported that height of turbulent wake was 1.7 times the height of the vehicle. Injection height is important because increasing injection height decreases the possibility of a particle being deposited within a given time. The revolution per minute (rpm) of the rotorods displayed low variability (CV ¼ 5%) with a mean rpm of 1662783, so the errors due to differences in rpm were low. For Exp-1, the amount of dust particles collected on sticky tapes at E1.5 and E4.5 were higher than those collected at TE6, TM6 and TW6 or at W1.5 and W4.5 (Fig. 3). As vehicular speed increased to 64 km h1 more dust particles were collected on the sticky tapes at TE6, TM6 and TW6 with maximum amount of dust collected by
Table 1 Descriptive statistics for physical property data from the unpaved road (n ¼ 12) Parameter
Sand (%)
Silt (%)
Clay (%)
Moisture content (%)
Number of blows at 5 cm
Number of blows at 10 cm
Number of blows at 15 cm
Compaction at 5 cm (kg cm2)
Mean SE Median Stdev Kurtosis Skewness Minimum Maximum
27.8 3.4 28.6 7.7 0.3 0.3 18.6 38.6
47.6 2.8 46.0 6.3 0.2 0.0 39.0 56.0
24.6 0.7 25.4 1.6 1.7 0.5 22.4 26.4
3.0 0.0 4.0 0.01 0.08 1.18 2.0 4.0
26.6 4.1 23.0 10.7 0.2 1.1 18.0 45.0
35.3 6.1 32.0 16.0 2.4 1.4 19.0 67.0
40.3 4.8 38.0 12.8 1.9 1.3 27.0 65.0
20.5 1.1 19.3 3.8 0.3 1.0 15.5 28.1
Size range 2–0.02 mm is sand, 0.02–0.002 mm is silt, o0.002 mm is clay, SE is standard error and Stdev is standard deviation.
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the sticky tape at TM6. During these experiments changes in wind speed, direction and relative humidity were low. Thus, our hypothesis was accepted that the primary driving mechanism for the vertical and horizontal spread of the dust plume was the increasing vehicle speed. Total volumes sampled at different locations during an experiment were nearly similar and was 1.1970.06 m3 for Exp-1 and 1.1670.08 m3 for Exp-2. The PM concentration on sticky tapes was much higher at E1.5 (0.870.3 mg m3) and E4.5 m height (0.770.2 mg m3) than at 6 m height (0.270.07 mg m3) for Exp-1. As speed increased to 64 km h1, the PM concentration also increased at TE6, TM6 and TW6 (average 0.370.1 mg m3 at 48 km h1 and 0.87 0.5 mg m3 at 64 km h1). For Exp-2, PM concentration recorded was similar at east and west tower at 1.5 and 4.5 m height (0.2870.6 and 0.297 0.002 48 km/h
Dust Weight (g)
0.0016
64 km/h
0.0012 0.0008 0.0004 0 E1.5
E4.5
TE6
TM6
TW6
W4.5
W1.5
Fig. 3. Analysis of sticky tapes for determining average and standard deviations of amount of dust particles in grams (Y-axis) on sticky tapes at different heights above ground surface and different vehicle speeds at east (E), west (W) and top (T) of the tower at 1.5, 4.5 and 6 m height, respectively.
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0.2 mg m3) suggesting that with increasing vehicle speed width of the dust plume also increased. Increases in dust particle emission with increasing speeds are also reported by Kuhn et al. (2005). Dyck and Stukel (1976) have also reported a linear relationship among dust emission, vehicle speed and silt content. 3.3. Elemental composition and mass balance from electron microscope The sample micrographs of dust particles (Fig. 2) showed a variety of shapes (Pinnick et al., 1985) and size of particles retained on the sticky tape ranged from 0.05 to 159 mm. The dominant elements detected at E1.5 and TM6 m heights were carbon, oxygen, aluminum, and silica additionally calcium, potassium, magnesium and iron were also detected at E4.5 (Fig. 4). These elements were expected as the unpaved road essentially consisted of entisol containing large amounts of calcium, aluminum, iron and silica. The highest PM concentration expressed as total number of particles per unit area of the slide was 275 for Exp-1 and 150 for Exp-2 at E1.5 m height. Thus, different speeds showed similar response to the PM emission from unpaved road. The larger concentration of particles on the east tower was expected, as wind direction throughout the experiment was eastward. The PM concentration was higher at greater vehicular speed indicating that increasing vehicle speed increased the width and height of the dust plume. Similar observations were also reported by Gillies et al. (2005). In general, dust
Fig. 4. The elemental composition of particles at each of the three locations at E4.5 for a vehicle speed of 64 km h1.
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particles between PM10 and PM2.5 did not correlate with vehicle speed but particles pPM2.5 did (Fig. 5). Since some of the particles were larger than PM10, the emission factor estimated using Eq. (1) for the total suspended PM was 10,147 and 11,062 g vehicle1 km1 travel for 48 and 64 km h1 vehicle speeds, respectively. Road dust emissions from unpaved roads are generally reported to be based on their silt loadings (o70 mm; EPA, 2006). The particle size distribution of the airborne PM on sticky tapes showed that the majority of the collected particles were clay and silt (Fig. 6). On an average more clay sized particles became airborne at higher speed supporting our hypothesis. On the other hand, more silt-sized particles were retained on sticky tapes at the lower of the two speeds. Such a variation was consistent with the large silt (48%) and clay (25%) contents as well as compaction of road sample. There was a small amount of airborne very fine and fine sand found on sticky tapes; however, particles coarser than fine sand were not detected although dust samples collected from the unpaved roads contained almost equal amounts of sand and clay sized particles. This could suggest that larger-sized particles were settling out of the suspension before reaching the tower located on either side of the road. It is also likely that these particles were too
1800 Number of Particles
1600 48 km/h
1400
64 km/h
1200 1000 800 600 400 200 0 E1.5
E4.5
TE6
TM6
TW6
W4.5
W1.5
PM10 < particles > PM2.5
B 4000 48 km/h
Number of Particles
3500
64 km/h
3000 2500 2000 1500 1000 500 0 E1.5
E4.5
TE6
TM6
TW6
W4.5
W1.5
Particles < PM2.5
Fig. 5. Total number of particles for size ranges of (A) PM10pparticles 4PM2.5 and (B) particles pPM2.5 at east (E), west (W) and top (T) of the tower at 1.5, 4.5 and 6 m height, respectively.
8000 7000
No. of Particles
6000 5000 48 km/h
4000
64 km/h
3000 2000 1000 0 Clay
Very Fine Silt
Silt
Very Fine Sand
Fine Sand
Medium Sand
Coarse Sand
Very Coarse Sand
48 km/h
4988
0
7425
253
30
0
0
0
64 km/h
6613
1
3168
28
5
0
0
0
Texture and Distribution of Particles
Fig. 6. Particle size distribution of the collected PM at both vehicular speeds. Note size range 2.0–1.0 mm is very coarse sand, 1.0–0.5 mm is coarse sand, 0.50–0.25 mm is medium sand, 0.025–0.10 mm is fine sand, 0.10–0.05 mm is very fine sand, 0.05–0.001 mm is silt, 0.001–0.002 mm is very fine silt, and o0.002 mm.
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large to stay attached onto the sticky tapes. Overall, this study demonstrated the usefulness of the sticky tapes for determining the PM concentrations, particle size distribution and the elemental composition of particles. More experiments using different vehicles, vehicle speeds and roads should be carried out to fully understand the potential of sticky tapes for mapping and characterizing airborne PM from unpaved roads. 4. Conclusions Both silt and clay-sized particles were retained on the sticky tapes at all three heights. The particle size analysis also showed that unpaved road dust contained more than 70% of silt and clay-sized particles. Increasing vehicle speed increased the amount of clay-sized particles retained as well as the height and width of the dust plume. The emission factors estimated for the total suspended PM were 10,147 g km1 at 48 km h1 and 11,062 g km1 at 64 km h1. The sample micrographs of dust particles showed a variety of shapes and the size of particles retained on the sticky tapes ranged from 0.05 to 159 mm. The amount of particles between PM10 and PM2.5 did not correlate with vehicle speed but particles pPM2.5 size did. The elemental analysis of the dust particles at each of the three sampling locations on east tower showed carbon, aluminum and silica as major elements present at all three heights. Overall this study demonstrated the utility of rotorod-sticky tapes coupled with electron microscopy for characterizing airborne PM. Acknowledgments This research was supported by the Southwest Center for Environmental Research and Policy (SCERP). Authors thank New Mexico State University Agriculture Experiment Station, Las Cruces, NM 880005 for the support. Special thanks are due to Dr. John Mexal for editing the manuscript, and
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Dr. Ghoshroy and Mr. Rami Al Khatib for the help and assistance in analyzing the slides in the electron microscopic Laboratory.
References Bulloch, H.E., Neher, R.E., 1980. Soil survey of Dona Ana county area New Mexico. United States Department of Agriculture, Soil Conservation Service, vol. 177. Cowherd, C., Jr et al., 1974. Development of emission factors for fugitive dust sources. EPA-450/3-74-037, U.S. EPA, Research Triangle Park, NC, June. Dyck, R.J., Stukel, J.J., 1976. Fugitive dust emissions from trucks on unpaved roads. Environment Science and Technology 10, 1046–1048. EPA, 2006. Unpaved Roads. /http://www.epa.gov/ttn/chief/ ap42/ch13/final/c13s0202.pdfS. Available on Web on June 2007. Etyemezian, V.J., Kuhn, H., Gillies, M., Green, M., Pitchtord, M., Watson, J., 2003. Vehicle based road dust emissions measurements (I): methods and calibration. Atmospheric Environment 37, 4559–4571. Ferguson, J.F., Downs, H. W., Pfost, D. L., 1999. Fugitive dust: nonpoint sources. Agriculture MU Guide. University of Missouri-Columbia, /http://extension.missouri.edu/explore / agguides/agengin/g01885.htmS. Available on Web on October 2, 2007. Gee, G.W., Bauder, J.W., 1986. Particle size analysis. In: Klute, A. (Ed.), Methods of Soil Analysis. Part 1, second ed. ASA Monograph No. 9. Madison, WI, pp. 337–382. Gillies, J., Etyemezian, V., Kuhn, H., Nikolic, D., Gillette, D.A., 2005. Effect of vehicle characteristics on unpaved road dust emissions. Atmospheric Environment 39, 2341–2347. Kuhn, H., Etyemezian, V., Landwehr, D., MacDougall, C., Pitchford, M., Green, M., 2001. Testing re-entrained aerosol kinetic emissions from roads (TRAKER): a new approach to infer silt loading on roadways. Atmospheric Environment 35, 2815–2825. Pinnick, R.G., Fernandez, G., Hinds, B.D., Bruce, C.W., Schaefer, R.W., 1985. Dust generated by vehicular traffic on unpaved roadways: size and infrared extinction. Aerosol Science and Technology 4, 99–121. Sampling Technologies, L., 1989. Operating Instructions for the Rotorod Sampler. Sampling Technologies Inc., Minnetonka, MI, USA. Veranth, M.J., Pardyjak, E.R., Seshadri, G., 2003. Vehicle generated fugitive dust transport analytic models and field study. Atmospheric Environment 37, 2295–2303.
Atmospheric Environment 42 (2008) 3899–3905 www.elsevier.com/locate/atmosenv
Technical note
Particulate matter emission by a vehicle running on unpaved road David Scott Williamsa, Manoj K. Shuklaa,, Jim Rossb a
Department of Plant and Environmental Sciences, New Mexico State University, MSC 3Q, P.O. Box 30003, Las Cruces, NM 88003, USA b Department of Entomology Plant Pathology and Weed Science, New Mexico State University, Las Cruces, NM 88003, USA Received 4 August 2007; received in revised form 5 February 2008; accepted 5 February 2008
Abstract The particulate matter (PM) emission from unpaved roads starts with the pulverization of surface material by the force of the vehicle, uplifting and subsequent exposure of road to strong air currents behind the wheels. The objectives of the project were to: demonstrate the utility of a simple technique for collecting suspended airborne PM emitted by vehicle running on an unpaved road, determine the mass balance of airborne PM at different heights, and determine the particle size and elemental composition of PM. We collected dust samples on sticky tapes using a rotorod sampler mounted on a tower across an unpaved road located at the Leyendecker Plant Sciences Research Center, Las Cruces, NM, USA. Dust samples were collected at 1.5, 4.5 and 6 m height above the ground surface on the east and west side of the road. One rotorod sampler was also installed at the centre of the road at 6 m height. Dust samples from unpaved road were mostly (70%) silt and clay-sized particles and were collected at all heights. The height and width of the PM plume and the amount of clay-sized particles captured on both sides of the road increased with speed and particle captured ranged from 0.05 to 159 mm. Dust particles between PM10 and PM2.5 did not correlate with vehicle speed but particles pPM2.5 did. Emission factors estimated for the total suspended PM were 10147 g km1 at 48 km h1 and 11062 g km1 at 64 km h1 speed, respectively. The predominant elements detected in PM were carbon, aluminum and silica at all heights. Overall, sticky tape method coupled with electron microscopy was a useful technique for a rapid particle size and elemental characterization of airborne PM. r 2008 Elsevier Ltd. All rights reserved. Keywords: Particulate matter; Unpaved road; Rotorod; Sticky tape; Emission; Moisture content; Silt and clay
1. Introduction Unpaved roads produce as much as 10 million tons of particulate matter (PM) each year in the United States (Ferguson, 1999). In New Mexico, Corresponding author. Tel.: +1 505 646 2324; fax: +1 505 646 6041. E-mail address: [email protected] (M.K. Shukla).
1352-2310/$ - see front matter r 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.atmosenv.2008.02.003
like most arid regions, a large amount of PM is generated from vehicles traveling on unpaved rural roads, which can significantly affect air quality on a local scale (Pinnick et al., 1985). Most unpaved roads consist of graded and compacted roadbed usually created from the parent material. The rolling wheels of the vehicles impart a force to the surface that pulverizes the roadbed material and ejects particles from the shearing force as well as by
ARTICLE IN PRESS 3900
D.S. Williams et al. / Atmospheric Environment 42 (2008) 3899–3905
the turbulent wake (EPA, 2006). However, little is known about the quantity, composition, particle size, extinction characteristic, fluxes and transport distances of fugitive dust from unpaved roads and their contribution to PM10 exceedances (Pinnick et al., 1985; Kuhn et al., 2001; Veranth et al., 2003). Dust emission rates from unpaved roads are a function of the silt loadings or particle size distribution (Cowherd et al., 1974; Kuhn et al., 2005), vehicle speed (Etyemezian et al., 2003; Gillies et al., 2005), size of the vehicle (Gillies et al., 2005) and moisture content of the road dust (Etyemezian et al., 2003). Dust particles emitted by vehicular traffic consist of aggregates of fine clay to quartz particles and can be several orders of magnitude different in the size distribution (Pinnick et al., 1985). Knowledge of the variability of the individual factors associated with physical properties of the unpaved road and their role on PM emissions is critical to develop accurate air quality standards and models. At the same time, there is a need to develop cost-effective sensors, which can collect emission data to quantify the spatial variations, composition and spread of airborne PM. These samplers should be cost-effective, collect enough samples for simultaneous chemical and physical analysis, and the samples should be representative of the PM generated by the vehicle. Thus, the
objectives of this project were to (i) demonstrate the utility of a simple method (rotorod and sticky tapes), (ii) carry out the mass balance of airborne PM, and (iii) determine the elemental composition and particle size distribution. The hypothesis for the research was that increasing vehicular speed would result in greater dispersion of finer-sized PM. 2. Methods and materials 2.1. Experimental site and setup The unpaved road for the PM emission experiment was located at Leyendecker Plant Sciences Research Center (PSRC) of New Mexico State University, Messiah Valley, NM, USA (321110 35.8400 N and 1061440 08.7500 W). Soils of the area are classified as Glendale (fine-silty, mixed, calcareous, thermic typic Torrifluvents)-Harkey (coarsesilty, mixed, calcareous, thermic typic Torrifluvents). The alluvium is modified by wind and Aeolian material. According to the USDA textural classification, the dominant soil type of the study area was clay loam. Climate of the experimental area was classified as arid with mean annual temperature ranging from 19 to 20 1C and mean annual precipitation from 180 to 230 mm (Bulloch and Neher, 1980).
Fig. 1. Photograph of the experimental set up at the Plant Sciences Research Station, Las Cruces, NM, USA. Rotorods and sticky tape were installed at east (E), west (W) and top (T) of the tower at 1.5, 4.5 and 6 m height, respectively.
ARTICLE IN PRESS D.S. Williams et al. / Atmospheric Environment 42 (2008) 3899–3905
Two towers were erected on either side (east– west) of the unpaved road and four rotorods consisting of a constant speed motor, U-rods (Sampling Technologies, 1989), and glass slides with double-sided sticky tapes (STR tape 0.076 mm thick; Shinto Paint Company Ltd.) were placed at 1.5 and 4.5 m above the ground surface on east and west side of the road (E1.5, E4.5, W1.5 and W4.5; Fig. 1). In addition, three rotorods 4.5 m apart were suspended at a height of 6 m across the unpaved road (TE6, TM6 and TW6). Wind speed and direction, relative humidity and other meteorological variables were measured using a standard portable meteorological instrument (Davis Instruments, model Vantage Pro). The road was isolated from other traffic approximately an hour prior to the start and until the end of experiments. Two experiments were conducted using a Chevrolet pick-up truck (approximately 2300 kg; 1.8 m high on the rear; 4-wheel drive) at two speeds (48 and 64 km h1). Dust samples were collected for a total of 6 min after the vehicle passed underneath the tower and total volume of air sampled was determined as the product of rpm of rotorods, circumference of the sampling area, and length (2.9470.18 cm) and width (0.870.0 cm) of sticky tape. The sticky tapes were weighed before and after the experiment to obtain the total amount of aerosol retained and was used to calculate the concentration of aerosol at different heights and speeds. The quantity of total suspended particulate matter emission (PE) from unpaved road (lb per vehicle mile travel) was estimated by following equation (EPA, 2006) and results were subsequently
3901
converted to g per vehicle km travel: PE ¼
kðs=12Þa ðS=30Þd C ðM=0:5Þc
(1)
where k, a, c and d are constants and were obtained as 6.0; 1; 0.3; and 0.3, respectively, for total suspended PM from public road; s is average silt content of unpaved road (%); S is mean vehicle speed (mph); and C is emission factor (0.00047 lb per vehicle mile travel (EPA, 2006). 2.2. Physical properties of dust on unpaved road At the end of the experiment, four road samples were collected from locations separated by 10 m on the unpaved road and particle size distribution was obtained using the standard hydrometer method (Gee and Bauder, 1986). Moisture content of the dust particles collected from unpaved road was also determined by oven drying the sample at 105 1C. At each sampling location, three penetration tests were performed using a standard soil penetrometer to a depth of 5, 10 and 15 cm and a soil compaction tester (Dickey-John Corporation) with a 1.27 cm tip to a depth of 5 cm. 2.3. Electron microscope and image analysis Images of the PM samples retained on sticky tapes were created using a scanning electron microscope (HITACHI, S-3400N) (Fig. 2). A total of 54 micrographs were created for the two experiments at three different heights. One particle (largest size) on the micrograph was selected at each height on
Fig. 2. Raw image generated by electron microscope showing the dust particles, smudges and bubbles on the sticky tape at east (E), west (W) and top (T) of the tower at 1.5, 4.5 and 6 m height, respectively.
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east tower and magnified to 25 or 50 mm. Energy dispersive X-ray spectroscopy (EDX; Thermo electron Noran System six) at a voltage of 25 kV determined the elemental composition of the particle at each location. Each electron micrograph was analyzed using ImageJ and Microsoft Paint. Each image was altered to a binary image and bubbles (see Fig. 2), scratches and smears were removed from the image. Once all the visually desired particles were within the threshold range, each image was processed into binary images. The particle area, standard deviation, integrated density, perimeter, and limit to threshold were checked using particle analyzer and the area was calculated as the number of pixels forming the 8-neighbor connected particle. The pixel range varied from 0 to infinity to account for all particle sizes from fine clay to coarse-grained sand and possibly aggregates. 3. Results and discussion 3.1. Physical properties of soil from unpaved road Sand content of the unpaved road ranged from 19% to 39%, silt content from 39% to 56% and clay content from 22% to 26%. Silt and clay contents constituted for about 72% of all primary particles (Table 1). Moisture content varied between 2% and 4% indicating that road was fairly dry during the experiment. The penetration resistance varied between 15.5 and 28.1 kg cm2 (CV ¼ 18%) at a depth of 5 cm. Number of blows required to penetrate to a depth of 5, 10 and 15 cm ranged from 18 to 45, 19 to 67 and 27 to 65, respectively, and displayed large variability with CV ranging from 32% to 45% despite the low variability of moisture
content of road dust. The large variability of penetration resistance indicated that apart from moisture content, compaction likely played an important role for PM emission from unpaved roads. 3.2. Mass balance from rotorod data The Exp-1 (48 km h1) was started at 7:20 AM and atmospheric conditions recorded 2-m west of the tower showed that average wind direction during the test was 1651, average wind velocity was 1.6 km h1 and the relative humidity was 42%. The Exp-2 (64 km h1) was started at 8:05 AM with average wind direction of 1501, average wind velocity of 2.4 km h1 and the average relative humidity of 20%. Height of the turbulent wake behind the vehicle as seen by the unaided eyes ranged from 2 m above ground for 48 km h1 to 3.5 m for 64 km h1 speeds (1.1 and 1.9 times the vehicle height). Thus, injection height seemed to be a function of the vehicle speed. In contrast, Gillies et al. (2005) reported that height of turbulent wake was 1.7 times the height of the vehicle. Injection height is important because increasing injection height decreases the possibility of a particle being deposited within a given time. The revolution per minute (rpm) of the rotorods displayed low variability (CV ¼ 5%) with a mean rpm of 1662783, so the errors due to differences in rpm were low. For Exp-1, the amount of dust particles collected on sticky tapes at E1.5 and E4.5 were higher than those collected at TE6, TM6 and TW6 or at W1.5 and W4.5 (Fig. 3). As vehicular speed increased to 64 km h1 more dust particles were collected on the sticky tapes at TE6, TM6 and TW6 with maximum amount of dust collected by
Table 1 Descriptive statistics for physical property data from the unpaved road (n ¼ 12) Parameter
Sand (%)
Silt (%)
Clay (%)
Moisture content (%)
Number of blows at 5 cm
Number of blows at 10 cm
Number of blows at 15 cm
Compaction at 5 cm (kg cm2)
Mean SE Median Stdev Kurtosis Skewness Minimum Maximum
27.8 3.4 28.6 7.7 0.3 0.3 18.6 38.6
47.6 2.8 46.0 6.3 0.2 0.0 39.0 56.0
24.6 0.7 25.4 1.6 1.7 0.5 22.4 26.4
3.0 0.0 4.0 0.01 0.08 1.18 2.0 4.0
26.6 4.1 23.0 10.7 0.2 1.1 18.0 45.0
35.3 6.1 32.0 16.0 2.4 1.4 19.0 67.0
40.3 4.8 38.0 12.8 1.9 1.3 27.0 65.0
20.5 1.1 19.3 3.8 0.3 1.0 15.5 28.1
Size range 2–0.02 mm is sand, 0.02–0.002 mm is silt, o0.002 mm is clay, SE is standard error and Stdev is standard deviation.
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the sticky tape at TM6. During these experiments changes in wind speed, direction and relative humidity were low. Thus, our hypothesis was accepted that the primary driving mechanism for the vertical and horizontal spread of the dust plume was the increasing vehicle speed. Total volumes sampled at different locations during an experiment were nearly similar and was 1.1970.06 m3 for Exp-1 and 1.1670.08 m3 for Exp-2. The PM concentration on sticky tapes was much higher at E1.5 (0.870.3 mg m3) and E4.5 m height (0.770.2 mg m3) than at 6 m height (0.270.07 mg m3) for Exp-1. As speed increased to 64 km h1, the PM concentration also increased at TE6, TM6 and TW6 (average 0.370.1 mg m3 at 48 km h1 and 0.87 0.5 mg m3 at 64 km h1). For Exp-2, PM concentration recorded was similar at east and west tower at 1.5 and 4.5 m height (0.2870.6 and 0.297 0.002 48 km/h
Dust Weight (g)
0.0016
64 km/h
0.0012 0.0008 0.0004 0 E1.5
E4.5
TE6
TM6
TW6
W4.5
W1.5
Fig. 3. Analysis of sticky tapes for determining average and standard deviations of amount of dust particles in grams (Y-axis) on sticky tapes at different heights above ground surface and different vehicle speeds at east (E), west (W) and top (T) of the tower at 1.5, 4.5 and 6 m height, respectively.
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0.2 mg m3) suggesting that with increasing vehicle speed width of the dust plume also increased. Increases in dust particle emission with increasing speeds are also reported by Kuhn et al. (2005). Dyck and Stukel (1976) have also reported a linear relationship among dust emission, vehicle speed and silt content. 3.3. Elemental composition and mass balance from electron microscope The sample micrographs of dust particles (Fig. 2) showed a variety of shapes (Pinnick et al., 1985) and size of particles retained on the sticky tape ranged from 0.05 to 159 mm. The dominant elements detected at E1.5 and TM6 m heights were carbon, oxygen, aluminum, and silica additionally calcium, potassium, magnesium and iron were also detected at E4.5 (Fig. 4). These elements were expected as the unpaved road essentially consisted of entisol containing large amounts of calcium, aluminum, iron and silica. The highest PM concentration expressed as total number of particles per unit area of the slide was 275 for Exp-1 and 150 for Exp-2 at E1.5 m height. Thus, different speeds showed similar response to the PM emission from unpaved road. The larger concentration of particles on the east tower was expected, as wind direction throughout the experiment was eastward. The PM concentration was higher at greater vehicular speed indicating that increasing vehicle speed increased the width and height of the dust plume. Similar observations were also reported by Gillies et al. (2005). In general, dust
Fig. 4. The elemental composition of particles at each of the three locations at E4.5 for a vehicle speed of 64 km h1.
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particles between PM10 and PM2.5 did not correlate with vehicle speed but particles pPM2.5 did (Fig. 5). Since some of the particles were larger than PM10, the emission factor estimated using Eq. (1) for the total suspended PM was 10,147 and 11,062 g vehicle1 km1 travel for 48 and 64 km h1 vehicle speeds, respectively. Road dust emissions from unpaved roads are generally reported to be based on their silt loadings (o70 mm; EPA, 2006). The particle size distribution of the airborne PM on sticky tapes showed that the majority of the collected particles were clay and silt (Fig. 6). On an average more clay sized particles became airborne at higher speed supporting our hypothesis. On the other hand, more silt-sized particles were retained on sticky tapes at the lower of the two speeds. Such a variation was consistent with the large silt (48%) and clay (25%) contents as well as compaction of road sample. There was a small amount of airborne very fine and fine sand found on sticky tapes; however, particles coarser than fine sand were not detected although dust samples collected from the unpaved roads contained almost equal amounts of sand and clay sized particles. This could suggest that larger-sized particles were settling out of the suspension before reaching the tower located on either side of the road. It is also likely that these particles were too
1800 Number of Particles
1600 48 km/h
1400
64 km/h
1200 1000 800 600 400 200 0 E1.5
E4.5
TE6
TM6
TW6
W4.5
W1.5
PM10 < particles > PM2.5
B 4000 48 km/h
Number of Particles
3500
64 km/h
3000 2500 2000 1500 1000 500 0 E1.5
E4.5
TE6
TM6
TW6
W4.5
W1.5
Particles < PM2.5
Fig. 5. Total number of particles for size ranges of (A) PM10pparticles 4PM2.5 and (B) particles pPM2.5 at east (E), west (W) and top (T) of the tower at 1.5, 4.5 and 6 m height, respectively.
8000 7000
No. of Particles
6000 5000 48 km/h
4000
64 km/h
3000 2000 1000 0 Clay
Very Fine Silt
Silt
Very Fine Sand
Fine Sand
Medium Sand
Coarse Sand
Very Coarse Sand
48 km/h
4988
0
7425
253
30
0
0
0
64 km/h
6613
1
3168
28
5
0
0
0
Texture and Distribution of Particles
Fig. 6. Particle size distribution of the collected PM at both vehicular speeds. Note size range 2.0–1.0 mm is very coarse sand, 1.0–0.5 mm is coarse sand, 0.50–0.25 mm is medium sand, 0.025–0.10 mm is fine sand, 0.10–0.05 mm is very fine sand, 0.05–0.001 mm is silt, 0.001–0.002 mm is very fine silt, and o0.002 mm.
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large to stay attached onto the sticky tapes. Overall, this study demonstrated the usefulness of the sticky tapes for determining the PM concentrations, particle size distribution and the elemental composition of particles. More experiments using different vehicles, vehicle speeds and roads should be carried out to fully understand the potential of sticky tapes for mapping and characterizing airborne PM from unpaved roads. 4. Conclusions Both silt and clay-sized particles were retained on the sticky tapes at all three heights. The particle size analysis also showed that unpaved road dust contained more than 70% of silt and clay-sized particles. Increasing vehicle speed increased the amount of clay-sized particles retained as well as the height and width of the dust plume. The emission factors estimated for the total suspended PM were 10,147 g km1 at 48 km h1 and 11,062 g km1 at 64 km h1. The sample micrographs of dust particles showed a variety of shapes and the size of particles retained on the sticky tapes ranged from 0.05 to 159 mm. The amount of particles between PM10 and PM2.5 did not correlate with vehicle speed but particles pPM2.5 size did. The elemental analysis of the dust particles at each of the three sampling locations on east tower showed carbon, aluminum and silica as major elements present at all three heights. Overall this study demonstrated the utility of rotorod-sticky tapes coupled with electron microscopy for characterizing airborne PM. Acknowledgments This research was supported by the Southwest Center for Environmental Research and Policy (SCERP). Authors thank New Mexico State University Agriculture Experiment Station, Las Cruces, NM 880005 for the support. Special thanks are due to Dr. John Mexal for editing the manuscript, and
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Dr. Ghoshroy and Mr. Rami Al Khatib for the help and assistance in analyzing the slides in the electron microscopic Laboratory.
References Bulloch, H.E., Neher, R.E., 1980. Soil survey of Dona Ana county area New Mexico. United States Department of Agriculture, Soil Conservation Service, vol. 177. Cowherd, C., Jr et al., 1974. Development of emission factors for fugitive dust sources. EPA-450/3-74-037, U.S. EPA, Research Triangle Park, NC, June. Dyck, R.J., Stukel, J.J., 1976. Fugitive dust emissions from trucks on unpaved roads. Environment Science and Technology 10, 1046–1048. EPA, 2006. Unpaved Roads. /http://www.epa.gov/ttn/chief/ ap42/ch13/final/c13s0202.pdfS. Available on Web on June 2007. Etyemezian, V.J., Kuhn, H., Gillies, M., Green, M., Pitchtord, M., Watson, J., 2003. Vehicle based road dust emissions measurements (I): methods and calibration. Atmospheric Environment 37, 4559–4571. Ferguson, J.F., Downs, H. W., Pfost, D. L., 1999. Fugitive dust: nonpoint sources. Agriculture MU Guide. University of Missouri-Columbia, /http://extension.missouri.edu/explore / agguides/agengin/g01885.htmS. Available on Web on October 2, 2007. Gee, G.W., Bauder, J.W., 1986. Particle size analysis. In: Klute, A. (Ed.), Methods of Soil Analysis. Part 1, second ed. ASA Monograph No. 9. Madison, WI, pp. 337–382. Gillies, J., Etyemezian, V., Kuhn, H., Nikolic, D., Gillette, D.A., 2005. Effect of vehicle characteristics on unpaved road dust emissions. Atmospheric Environment 39, 2341–2347. Kuhn, H., Etyemezian, V., Landwehr, D., MacDougall, C., Pitchford, M., Green, M., 2001. Testing re-entrained aerosol kinetic emissions from roads (TRAKER): a new approach to infer silt loading on roadways. Atmospheric Environment 35, 2815–2825. Pinnick, R.G., Fernandez, G., Hinds, B.D., Bruce, C.W., Schaefer, R.W., 1985. Dust generated by vehicular traffic on unpaved roadways: size and infrared extinction. Aerosol Science and Technology 4, 99–121. Sampling Technologies, L., 1989. Operating Instructions for the Rotorod Sampler. Sampling Technologies Inc., Minnetonka, MI, USA. Veranth, M.J., Pardyjak, E.R., Seshadri, G., 2003. Vehicle generated fugitive dust transport analytic models and field study. Atmospheric Environment 37, 2295–2303.