Road Dust Emissions from Paved Roads Measured Using Different Mobile Systems

Transcription

1 Journal of the Air & Waste Management Association ISSN: (Print) (Online) Journal homepage: Road Dust Emissions from Paved Roads Measured Using Different Mobile Systems Liisa Pirjola, Christer Johansson, Kaarle Kupiainen, Ana Stojiljkovic, Hans Karlsson & Tareq Hussein To cite this article: Liisa Pirjola, Christer Johansson, Kaarle Kupiainen, Ana Stojiljkovic, Hans Karlsson & Tareq Hussein (2010) Road Dust Emissions from Paved Roads Measured Using Different Mobile Systems, Journal of the Air & Waste Management Association, 60:12, , DOI: / To link to this article: Published online: 24 Jan Submit your article to this journal Article views: 565 View related articles Citing articles: 15 View citing articles Full Terms & Conditions of access and use can be found at

2 TECHNICAL PAPER ISSN: J. Air & Waste Manage. Assoc. 60: DOI: / Copyright 2010 Air & Waste Management Association Road Dust Emissions from Paved Roads Measured Using Different Mobile Systems Liisa Pirjola Metropolia University of Applied Sciences, Department of Technology; and Department of Physics, University of Helsinki, Helsinki, Finland Christer Johansson Department of Applied Environmental Science, Stockholm University; and Environment and Health Administration, Stockholm, Sweden Kaarle Kupiainen Nordic Envicon Oy; and Finnish Environment Institute, Helsinki, Finland Ana Stojiljkovic Nordic Envicon Oy, Helsinki, Finland Hans Karlsson Department of Applied Environmental Science, Stockholm University, Stockholm, Sweden Tareq Hussein Department of Physics, University of Helsinki, Helsinki, Finland; and Department of Physics, University of Jordan, Amman, Jordan ABSTRACT Very few real-world measurements of road dust suspension have been performed to date. This study compares two different techniques (referred to as Sniffer and Emma) to measure road dust emissions. The main differences between the systems are the construction of the inlet, different instruments for recording particulate matter (PM) levels, and different loads on the wheel axes (the weight of Sniffer was much higher than that of Emma). Both systems showed substantial small-scale variations of emission levels along the road, likely depending on-road surface conditions. The variations observed correlated quite well, and the discrepancies are likely a result of IMPLICATIONS In most developed countries, exhaust emissions from vehicles are controlled by legislation (e.g., European Union, U.S. Environmental Protection Agency); therefore, exhaust emissions are expected to decrease drastically in the future. However, suspended road dust will remain or become an even bigger problem because of an increasing number of vehicles in urban and rural areas. There is still a knowledge gap regarding the dominant mechanisms leading to road dust emissions, although resuspension of surface particle loading obviously plays an important role. This paper presents a comparison of two different state-of-the-art mobile systems and establishes quantitative relationships between PM emissions under different conditions. variations in dust load on the road surface perpendicular to the driving direction that cause variations in the measurements depending on slightly different paths driven by the two vehicles. Both systems showed a substantial influence on the emission levels depending on the type of tire used. The summer tire showed much lower suspension than the winter tires (one nonstudded and one studded). However, the relative importance of the nonstudded versus studded tire was rather different. For the ratio of studded/nonstudded, Emma shows higher values on all road sections compared with Sniffer. Both techniques showed increased emission levels with increasing vehicle speed. When the speed increased from 50 to 80 km hr 1, the relative concentrations increased by % depending on the tire type and dust load. However, for road sections that were very dirty, Sniffer showed a much higher relative increase in the emission level with the nonstudded tire. Sniffer s absolute concentrations were mostly higher than Emma s. Possible reasons for the differences are discussed in the paper. Both systems can be used for studying relative road dust emissions and for designing air quality management strategies. INTRODUCTION Road traffic and wood burning are the two main sources for PM 10 (mass concentration of particles 10 m in aerodynamic diameter) in urban areas in Europe. 1 For road traffic, emission inventories of PM 10 often exclude 1422 Journal of the Air & Waste Management Association Volume 60 December 2010

3 suspended road dust, although it makes a substantial contribution to PM 10 in urban areas. In Northern Europe, nonexhaust PM 10 emissions are particularly high in winter and spring because of several factors: Roads are being sanded and the use of studded tires is common in many urban areas. In other cities in Europe, 8,9 road dust suspension also contributes a large fraction of PM 10. In practice, quantification of real-world road dust emissions is complicated because of the many different factors that might affect the emissions, not least the use of antiskid methods (e.g., street sanding and studded tires). Sanding has been found to increase the PM 10 emissions because of (1) the addition of PM 10 contained in sand, (2) creation of PM 10 due to wear of sand granules, and (3) creation of PM 10 due to enhanced wear of the road surface It has been shown that PM 10 emissions are very different depending on the type of tire used, 12,14,15 with the use of studded tires enhancing road surface wear, which increases PM 10 concentrations, especially during dry road conditions. 3,12,15,16 Vehicle speed affects PM 10 emissions, especially if studded tires are used. 14,15,17 The quality of the pavement on the road is also an important factor. 12,15 De-icers such as calcium magnesium acetate and calcium chloride have been shown to reduce the amount of suspended road dust and have been used as dust suppressors because of their hygroscopic properties, thereby creating a wetted surface. 3 Road surface wetness influences PM 10 concentrations close to densely operated roads. 2,3,18 Despite the importance of road dust emissions, there are very few real-world measurements outside of those conducted in the United States. Fitz et al. 19 measured PM 10 emission factors from roadways using a trailer with sensors mounted in front and behind the vehicle in the well-mixed wake (SCAMPER: System of Continuous Aerosol Monitoring of Particulate Emissions from Roadways). Kuhns et al. 20 developed the on-road measurement system TRAKER (Testing Re-entrained Aerosol Kinetic Emissions from Roads) to quantify road dust emissions. Recently that system was applied to study road dust emissions around Lake Tahoe. 21 Hussein et al. 15 developed a similar system to TRAKER in Sweden (referred to as Emma), and Pirjola et al. 22 developed a different system in Finland (referred to as Sniffer), but no systematic comparative study has been performed to evaluate the possible errors, biases, limitations, and performance of these different methods. To gain further insight into the quantitative importance of those factors that influence road dust emissions, it is important to know that the methods used are adequate for its purpose. The primary objective of this work is to establish quantitative relationships between PM emissions, tire type, vehicle speed, and road conditions using the two different measurement techniques, Emma and Sniffer. METHODOLOGY Emma Mobile Laboratory and Instrumentation PM emissions using the Emma vehicle have been described previously by Hussein et al. 15 A Volkswagen van (LT35, 2002) was equipped with three metal tube inlets: Two inlets were mounted behind the front tires, and one inlet was mounted underneath the van. The inlet mounted underneath the van extended to sample ambient background air below the front bumper (Figure 1). The road dust emissions are assumed to be proportional to the increase in concentration behind the tire. The three inlet lines entered the van compartment through the underbody. The front inlet was 0.4 cm in diameter and 300 cm long; the flow rate in this inlet was 1.7 L/min. Both inlets behind the front tires were mounted symmetrically and they were 1.9 cm in diameter and 230 cm long; the inlet was 21 cm above the ground, 5 cm behind the tire, and 6.3 cm in (toward the center of the vehicle) from the outside edge of the tire (Figure 1). Figure 1. A schematic diagram 15 showing the sampling line setup behind the front tires of the Emma system. Note that both sampling lines are identical on the right and left sides. (a) Distance and location of the inlet behind the tire, (b) sampling lines between the inlet and the manifold, and (c) design of the manifold. Volume 60 December 2010 Journal of the Air & Waste Management Association 1423

4 Pirjola et al. Flow rates through these two inlets were 75 L/min. Therefore, the air speed at the inlet mouth was constant ( 4 m sec 1), allowing isokinetic sampling only under certain speed conditions. According to Etyemezian et al.,23 measurements behind the tires during motion indicated that, along the center of the tire, the air speed is not significantly influenced by the ambient wind direction at distances less than 10 cm from the tire. Each inlet behind the left and right tires fed into a 60-cm long torpedo-shaped manifold that had an inner diameter of 7.5 cm (Figure 1). A specially designed baffle ensured that the flow through the manifold was uniform and well mixed. A single carbon vane pump drew the necessary air through both of the manifolds. An inline valve/rotameter combination allowed for independent control of the flow rates through each of the manifolds. The manifold was used to distribute the sampled air to as many as five instruments. Particle sampling instruments were connected to the manifold via 100-cm long tygon tubes. PM concentrations were measured using three DustTraks (TSI model 8529) and a particle size analyzer (GRIMM Technologies, model 1.109, 31-channel dust spectrometer). The DustTraks simultaneously sampled (with 1-sec time resolution) the background air from the front inlet and behind the two front tires (left and right). The flow rate in each of the DustTraks was 1.7 L/min. The GRIMM analyzer was mounted to sample air behind the right tire. The GRIMM analyzer provides number size distributions of aerosol particles between and 32 m in diameter with 31 channels and 6-sec time resolution. However, in this work, only the DustTrak recordings are considered. A global positioning system receiver (Ashtek, ProMark GPS) provided the position and speed of the vehicle in 1-sec intervals. All of the data were collected and displayed by an on-board computer using the Labview program. Sniffer Mobile Laboratory and Instrumentation The Sniffer mobile laboratory provides measurements of traffic exhaust emissions under real driving conditions24 26 as well as measurements of nonexhaust particles.22 The instrumentation was set in a Volkswagen LT35 diesel van with a length of 5585 mm, a width of 1933 mm, a height of 2570 mm, and a maximum total weight of 3550 kg. Exhaust samples and background road dust samples can be collected through two different inlet systems opening toward the driving direction. One is situated above the windshield at a height of 2.4 m (main inlet), and the other is above the bumper at a height of 0.7 m (chasing inlet). The dust sample is collected from behind the left rear tire through a conical inlet with a surface area of m into a vertical tube with a diameter of 0.1 m. The lower edge of the conical inlet is 7 cm above the street surface, and the upper edge is as high as the geometry of the fender of the wheel allows. The width of the inlet is approximately 2 cm less than the width of the tire (or 1 cm less from each side), and the distance of the inlet from the tire is 5 cm (Figure 2a). A stainless steel tube (diameter of 0.1 m) runs through the rear part to the top of the van (Figure 2b). An electric engine located on the roof of the vehicle produces a constant flow rate of approximately 2000 L/min. A sampling air branch-off into the tube of m diameter was constructed for the particle mass monitors TEOM (tapered element oscillating microbalance; series 1400A, Rupprecht & Patashnick) and ELPI (electrical low-pressure impactor; Dekati Ltd.). The orifice of the smaller tube was located downward in the middle of the thicker tube to allow isokinetic sampling. The total flow rate is 13 L/min (3 L/min for TEOM and 10 L/min for ELPI). With this flow rate, a sampling cyclone (SAC-65, Dekati) gives a 9.2- m cut size for the PM. TEOM operates at a temperature of 50 C; however, road dust evaporation of semi-volatile aerosol material (e.g., ammonium nitrate and certain organic compounds) is negligible and does not cause any problems. TEOM was installed to save a 30-sec running average mass concentration every 10 sec. Particle number concentration and size distribution are measured by two ELPIs. One ELPI measures street dust particles behind the left rear tire, and the other measures background particles via the chasing inlet in front of the van. An ELPI with the electrical filter stage enables measurement of real-time particle number concentration and size distribution (1-sec time resolution) in the size range of 7 nm to 10 m (aerodynamic diameter) with 12 channels.27 To calculate the background PM10 from the number concentrations, a particle density of 2.6 g cm 3 was assumed,23 except for exhaust particles (particle sizes 0.6 m), for which a value of 1 g cm 3 was used. Recently, Sniffer was also equipped with two DustTraks (TSI, model Figure 2. (a) Conical inlet behind the left rear tire of Sniffer. (b) The branch-off of sampled air leading to the PM monitors. More details can be found in the text and in ref Journal of the Air & Waste Management Association Volume 60 December 2010

5 Sniffer also provides measurements of gaseous concentrations such as carbon monoxide (CO; model CO12M, Environnement S.A.), nitric oxide (NO), nitrogen oxides (NO x ;NO x NO nitrogen dioxide [NO 2 ]; model APNA 360, Horiba), and carbon dioxide (CO 2 ; model VA 3100, Horiba) via the main inlet or the chasing inlet in front of the van. A weather station on the roof at a height of 2.9 m provides meteorological parameters. Relative wind speed and direction are measured with an ultrasonic wind sensor (model WAS425AH, Vaisala). Temperature and relative humidity are measured with temperature and humidity probes (model HMP45A, Vaisala). Additionally, a global positioning system (model GPS V, Garmin) saves the van s speed and the driving route. Figure 3. (a) Time series of the TEOM and DustTrak mass concentrations in Helsinki. (b) DustTrak vs. TEOM concentrations. 8530), each of which can sample PM 10,PM 2.5 (mass concentration of particles 2.5 m in aerodynamic diameter), or PM 1 (mass concentration of particles 1 m in aerodynamic diameter) with a resolution time of 1 sec. Data Handling and Validation As mentioned above, the mobile laboratories use different instruments for PM monitoring. Sniffer measures PM concentrations with the TEOM, which is based on a gravimetric principle. The mass is collected on an exchangeable filter cartridge by monitoring the corresponding frequency changes of a tapered element. Changes in the frequency of oscillation are related to the mass of material accumulating on the filter. Emma uses DustTraks that are based on the optical properties of the particles. In the DustTraks, the light emitted from the laser diode is scattered by particles drawn through the unit in a constant stream. The particle mass concentration is calculated from the amount of the scattered light using a calibration factor. Because the instruments used here were factory calibrated using Arizona road dust and because the climate and fugitive dust source are vastly different in the deserts of the southwestern United States than in Scandinavia, simultaneous road dust measurements were performed with TEOM and DustTrak by Sniffer on a 20-km route in Helsinki (Figure 3). A linear relationship between the TEOM and DustTrak can be fitted with a slope of The results indicate that the DustTrak underestimates road dust emissions; therefore, the Emma recordings were multiplied by 3.7. To be able to compare the two techniques (Emma and Sniffer), the measurement data were treated in the same Figure 4. The tires used in this work: (a) summer, (b) nonstudded (studless winter tire), and (c) and studded tires. Volume 60 December 2010 Journal of the Air & Waste Management Association 1425

6 way: From the DustTrak data of Emma and the ELPI data of Sniffer, 30-sec running averages were calculated and saved every 10 sec. To minimize the influence of extreme values, medians over different road sections were further calculated. The number of data points with different vehicle speeds of 35, 50, 65, and 80 km hr 1 were 247, 164, 130, and 105, respectively. The background concentrations measured above the front bumper (by DustTrak for Emma and by ELPI for Sniffer) were subtracted from the measurements behind the tires to eliminate the dust and exhaust emissions from other vehicles on the road. Note that the median values of the background concentration were close to each other. Emma s and Sniffer s recorded values were paired by their position so that the closest values were treated as a pair. Because the number of recorded values was not always equal, some of the values were not paired ( 10% of the total number of recorded values). Such values were not used for developing the ratios, but they are included in other data analyses. Sniffer s positions were used to plot the ratio data on maps. Test Roads The measurements were performed on May 22 and 23, 2007 on a road situated approximately 30 km northeast from Stockholm, Sweden. The data on May 22 have been excluded from the data analysis because the road surfaces were wet in some places because of rain in the morning. The test road (road no. 269) is a countryside road carrying between 3000 and 4000 vehicles per day, with Figure 5. PM 10 in g m 3 measured by Emma and Sniffer behind the summer tire when driving north at (a) 35, (b) 50, (c) 65, and (d) 80 km hr Journal of the Air & Waste Management Association Volume 60 December 2010

7 Figure 6. Median PM 10 in g m 3 over the sections northward and southward by Sniffer. Also shown are the 25% and 75% values. the speed limit set at 70 or 90 km hr 1. The asphalt is stone mastic asphalt (SMA). Road sections with certain asphalt types that are commonly used in Stockholm are SMA and dense asphalt concrete (DAC). Normally SMA is used on densely operated roads and highways with high speed limits because this type of asphalt is more wear Figure 7. PM 10 concentrations in mg m 3 using summer tires on Emma and Sniffer on the three different sections of the test road for different vehicle speeds: (a) P1 north, (b) P1 south, (c) P2 north, (d) P2 south, (e) P3 north, and (f) P3 south. Volume 60 December 2010 Journal of the Air & Waste Management Association 1427

8 resistant to the use of studded tires than DAC. 28 The Nordic ball mill index (NBMI) is often used to describe the wear resistance of the stone material: low NBMI corresponds to harder stone material and thus higher wear resistance. 14,28 The test road used here has an NBMI of less than 9. The most wear-resistive roads have an index of less than 4. On the 10-km way north, three different parts of the road could easily be visually separated because of different colors of the asphalt. According to the Swedish Road Administration, which is responsible for the maintenance of this road, they are all of the same quality (SMA with NBMI 9), but they may have different ages in the interval of These sections are named P1, P2, and P3. On the right side of section P1 when driving north, there is a small unpaved connecting road that comes from a quarry. From the quarry, there were trucks driving north or south on the test road carrying a lot of sludge on their tires (hereafter called the quarry effect ). Tires and Vehicle Speeds Steady driving speeds of 50, 65, and 80 km hr 1 were tested for all tires and road sections. For the summer tire, 35 km hr 1 was also tested. The Sniffer vehicle is equipped with automatic constant speed control, so the variation of speed may be somewhat larger with Emma, which did not have automatic constant speed control. The test tires (Figure 4) included a summer tire (Nokian Z), a nonstudded winter tire (Nokian Hakkapeliitta Rsi; hereafter also referred to as the friction tire ), and a studded winter tire (Nokian Hakkapeliitta 4). All test tires were car tires and had dimensions 235/60R16. These designs were chosen for the tests because they are among the most sold brands in Scandinavia. The load on the measurement tire in Sniffer (1100 kg) exceeded the maximum load recommended by the tire manufacturer (900 kg). However, the maximum load was exceeded by less than 20% and therefore it should not affect the tires performance or road grip. In the case of Emma, the load per tire was 780 kg. For Emma the right Figure 8. PM 10 concentrations in mg m 3 using nonstudded tires (friction) on Emma and Sniffer on the three different sections of the test road for different vehicle speeds: (a) P1 north, (b) P1 south, (c) P2 north, (d) P2 south, (e) P3 north, and (f) P3 south Journal of the Air & Waste Management Association Volume 60 December 2010

9 tire is always the same a summer tire 17 (Dunlop SP LT8). Studless winter tires are lamellar nonstudded tires and have a softer tread compared with the summer tires. The tread is designed to enhance the tire grip during frozen and snowy street conditions. Studded tires have studs distributed in a certain order on the tire surface to further enhance the tire grip on icy road surfaces. For short, these tires will be referred to in the text by their type as studded, nonstudded, and summer tires. RESULTS AND DISCUSSION Along-Road Variations Figure 5 illustrates the variability of the Emma and Sniffer concentrations (hereafter also called emission levels because sampling is close to the emission source) behind the summer tires along the northbound road for the vehicle speeds of 35, 50, 65, and 80 km hr 1. To exclude the effects due to tire type, the summer tires have been chosen for this figure. All dots refer to the 10-sec values of 30-sec running averages. The PM 10 concentrations show large variations along the test road although the pavement stone material and stone size are similar. This exists independently of vehicle speed. Effects of the connecting unpaved road to the quarry (P1) are clearly seen on these maps because the concentrations increase drastically on section P2 when driving north. When driving to the south, increased concentrations can be observed on section P1. As an example, Figure 6 shows the median concentrations over the sections to both directions for Sniffer at 50-km hr 1 speed. Because of the quarry effect, the PM 10 concentrations on sections P2 and P3 were lower on the southbound lane compared with the northbound lane, opposite of section P1. Similar observations can be seen also for the studded and nonstudded tires. The reason for the large variations in emissions along the test road is not known. In terms of pavement stone material and stone size, they are similar. Visually, the road surface seems to be clean and homogeneous except for the section close to the connecting road to the quarry. The likely reason for the variability in emissions is uneven Figure 9. PM 10 concentrations in mg m 3 using studded tires on Emma and Sniffer on the three different sections of the test road for different vehicle speeds: (a) P1 north, (b) P1 south, (c) P2 north, (d) P2 south, (e) P3 north, and (f) P3 south. Volume 60 December 2010 Journal of the Air & Waste Management Association 1429

10 Figure 10. PM 10 concentrations in mg m 3 (a) behind the left front tire of Emma and (b) behind the left rear tire of Sniffer as a function of vehicle speed. Su, Fr, and St refer to summer, nonstudded, and studded tires, respectively. N north, S south. Also shown are the equations for the linear dependences and the correlation coefficients. distribution of accumulated PM. Obviously the accumulated dust in connection to the quarry has a drastic effect on the emissions. On the other hand, there may also be variations in the emissions across the road stretch (perpendicular to the driving direction). The vans were mostly driven in the center of the road lane, but occasionally the wheels may have run to the left or right of the center. Norman and Johansson 3 showed that suspension of PM may increase drastically if a vehicle is driving just outside of the driving lane. This is likely because of accumulated road dust. Speed Dependence Figures 7 9 summarize the effect on the PM road dust emissions measured with Sniffer and Emma using summer, nonstudded, and studded tires, respectively, on the three different sections of the test road and with three different speeds. Note that the absolute scales on the y-axes are different for different figures to clearly illustrate the geographic variability. The results are presented for the northbound and southbound lane separately and are averaged for different vehicle speeds. For Emma, the concentrations in front of the vehicle correspond to the background concentration (measured with DustTrak) and this is essentially the same during all measurements and much lower than the concentration behind the two tires. The concentration behind the right tire (a Dunlop SP LT8) is always much lower than behind the left tire for all tested tires. For the summer tire, northbound and southbound directions show quite similar emissions with Emma and Sniffer (Figure 7). For the nonstudded and studded tires, the concentrations along sections P2 and P3 are consistently lower in the southbound direction (Figures 8 and 9, respectively). Note that the influence of road dust due to traffic going to and from the quarry is clearly seen as higher concentrations along section P1. To the northbound direction, higher concentrations are detected in section P2 consistently with the quarry effect; additionally, the Sniffer concentrations for the nonstudded tires are high in section P3. When considering speed dependence in Figures 7 9, it is found that higher concentrations are measured behind the tires as the speed of the vehicle increases. This is in good agreement with Hussein et al. 15 and references therein. However, when going southward with summer tires, the speed dependence is less pronounced (Figure 7). Emma and Sniffer show similar qualitative behavior. The mean value of the median concentrations over the sections in both directions for each vehicle speed were calculated (Figure 10). The relative increases in the concentrations going from 50 to 80 km hr 1 are given in Table 1. Comparing Sniffer to Emma, the relative growth is almost similar for the studded tires, the summer tire going south, and the nonstudded tire going north; however, the relative growth is 47% greater for the nonstudded tire going south and 19% smaller for the summer tire going north. The concentrations for the summer tires were very low. Tire Effects To decrease the effects of road variations, the ratios of the concentrations for studded/summer, nonstudded/summer, and studded/nonstudded tires were calculated for each section and vehicle speed for Emma and Sniffer (Table 2). The studded tire gives a factor 2 28 higher concentration as compared with the summer tire, as expected. In most cases (except section P1 northbound at 50 and 80 km hr 1 ), Emma shows higher ratios of studded/ summer concentrations as compared with Sniffer. For nonstudded/summer, Sniffer shows higher ratios except for P3 southbound at 65 km hr 1 and P1 southbound at 50 km hr 1. For studded/nonstudded, Emma shows higher ratios on all sections. The range of the ratios shows somewhat smaller variation with Sniffer compared with Emma; for example, for Emma studded/summer is in the range of 5 28, whereas for Sniffer it is Although Emma and Sniffer clearly indicate large differences in emission levels depending on the type of tire Table 1. Concentration ratios of 80 to 50 km hr 1 for Sniffer and Emma when driving north or south. 80/50 Su N Su S Fr N Fr S St N St S Sniffer Emma Notes: Su, Fr, and St refer to summer, nonstudded, and studded tires, respectively. N north, S south Journal of the Air & Waste Management Association Volume 60 December 2010

11 Table 2. Ratios between concentrations behind the St/Su, Fr/Su, and St/Fr tires for different road sections at 50, 65, and 80 km hr 1. P1 N P1 S P2 N P2 S P3 N P3 S Speed (km/hr) Ratio Sniffer Emma Sniffer Emma Sniffer Emma Sniffer Emma Sniffer Emma Sniffer Emma 50 St/Su Fr/Su St/Fr St/Su Fr/Su St/Fr St/Su Fr/Su St/Fr used (Figures 7 9), linear dependence can be observed when comparing the tire ratios of Emma and Sniffer (Figure 11). The data points are the same as given in Table 2. The slopes with 95% confidence levels and the equations in Figure 11 might be useful in comparing Emma and Sniffer results in the future. Gustavsson et al. 14 compared PM 10 generation due to studded and nonstudded tires in a road test facility in a laboratory. They found ratios of studded/nonstudded tires varying from 60 to 100 (i.e., much higher than Emma and Sniffer). On the other hand, Kupiainen et al. 12 showed in a study using a road simulator similar to the one used by Gustavsson et al. 14 ratios of approximately 1.7 at 15 km hr 1 and approximately 5 at 30 km hr 1. Several different reasons for this difference are discussed by Gustavsson et al., 14 including different temperatures and pavements. In the laboratory measurements, the PM 10 generation due to direct road wear is more important compared with the suspension of accumulated road dust particles. In the field, a larger fraction of the PM 10 generation is likely due to the suspension of accumulated PM. This may affect the ratio of studded versus nonstudded tires because tires with studs cause much larger road Figure 11. The x- and y-axes refer to the ratios of the concentrations behind the nonstudded and summer tires (Fr/Su), studded and summer tires (St/Su), and studded and nonstudded tires (St/Fr) measured by Emma and Sniffer, respectively. Also displayed are the equations (S Sniffer, E Emma). The slopes with 95% confidence levels were , , and for the ratios of Fr/Su, St/Su, and St/Fr, respectively. wear than nonstudded tires. Nonstudded tires may be more efficient at suspending PM 10 from the road surface than studded tires, as discussed in the next paragraph. Possible Reasons for Differences between Emma and Sniffer Results One possible reason for the differences in Emma and Sniffer results is that Sniffer samples the emitted PM behind the left rear tire, whereas Emma samples behind the front tires, which allow steering, thus possibly affecting the suspension concentrations. VW LT35 has rear-wheel drive, which means that approximately 60% of the vehicle weight (i.e., 2200 kg) burdens the rear axle, whereas the weight on the front axle is only 1360 kg. Additionally, Sniffer had even more weight on the rear axle than Emma because of the several instruments, battery systems, and extra tires that were used in this study. Sniffer s larger axle weight might have affected the road grip of the measurement tires used in this study and as a consequence increased the emissions behind the measurement tire compared with Emma. The influence of the load can be expected to be different for the different tires. On the other hand, the extent of the dispersion plume behind the tires may vary depending on the suspension processes and turbulence induced by the tire and the vehicle. The Emma inlet system is more sensitive to such processes than Sniffer because Sniffer samples a larger fraction of the suspended material, which tends to mask the effect of vehicle-induced turbulence. It is not known how much the front tire induced suspension and the vehicle body turbulence below the van can affect the PM concentrations measured behind the rear tire. However, Pirjola et al. 22 found that the emitted concentrations at 6 7 cm from the tire to the middle of the van and at altitudes of 7 30 cm from the road surface were very low (Figure 3 in Pirjola et al. 22 ), which indicates that the effect of the front tires and the whole vehicle body on the back tire concentration level is not significant because the dispersion plume of suspended particles from behind the front tire may be transported outside of the van before it reaches the rear tire. To resolve this issue, simultaneous measurements with two similar aged studded, nonstudded, and summer tires behind the back and front left tire are needed. Another possible reason for the differences in Emma and Sniffer results is that Sniffer and Emma had a different inlet Volume 60 December 2010 Journal of the Air & Waste Management Association 1431

12 Table 3. Comparison of the average tire ratios collected through the Sniffer conical inlet and the Emma-type inlet behind the Sniffer s left rear tire. Speed (km/hr) Sniffer Inlet Emma-Type Inlet St/Su Fr/Su St/Fr St/Su Fr/Su St/Fr construction and sample flow rate. As mentioned in the methodology section, Emma has a single tube 1.9 cm in diameter and 5 cm behind the front tire at an altitude of 21 cm from the road surface (flow rate of 75 L/min), whereas Sniffer has a conical inlet with a surface area of 340 cm 2 also 5 cm behind the back tire trying to catch the most of the suspended particles (flow rate of 2000 L/min). As shown in Pirjola et al., 22 the concentrations through the similar inlet size and location as Emma but behind the left rear tire are approximately 76 90% of the concentrations measured by the conical inlet, depending on the tire type and vehicle speed. This work has applied these percentages to the tire ratios (studded/nonstudded, studded/summer, and friction/summer) averaged over all road sections and directions for vehicle speeds of 50 and 80 km hr 1. Table 3 shows that with the Emma type of inlet behind the left back tire, the concentration ratio of studded to nonstudded tires would be 5 7% higher, of studded to summer tires approximately 5 10% smaller, and of nonstudded to summer tires approximately 10 15% smaller than with the Sniffer inlet system. The average ratios of Sniffer compared with the Emma type would be 1.07, 1.13, and 0.94, respectively, and are too far from the slopes mentioned in Figure 12 (0.48, 1.79, and 0.25, respectively). Consequently, the different inlet size and location cannot explain the measured PM 10 differences. An interesting feature is found when comparing Figure 10a for Emma and Figure 10b for Sniffer. As the vehicle speed increases, the mass concentrations increase for both vans but the increase with the nonstudded tire is clearly larger for Sniffer compared with Emma, even larger than the increase with studded tires. This indicates that along with the increasing vehicle speed for Sniffer, the suspension of PM behind the back nonstudded tire enhances relatively more than that behind the studded tire. Emma did not see this effect behind the front tire. The reason for this is still unclear; however, the different weights on the tires as well as the driving tire of Sniffer might be factors. The earlier results measured by Sniffer show that as long as the road surface is clean ( gm 3 ), studded tires generate higher concentrations than the nonstudded and summer tires (Figure 12). This is connected to primary emissions due to road wear caused by the steel studs. However, under dirty conditions, suspension of accumulated material becomes more important and may even be greater than primary emissions. Because of the suction pad effect, nonstudded tires cause greater PM 10 concentrations than studded tires. The reason is that nonstudded tires have more tread lamellas and are composed of softer rubber material than studded tires. When the lamellas touch the road surface, air between the lamellas is pressed out, and when the lamellas get unfastened, air is sucked between the lamellas. Loose PM is consequently lifted from the road surface and suspended in the ambient air. CONCLUSIONS Two different techniques to measure road dust emissions have been compared. The main difference between the systems is how measurements are done: behind front or rear wheels, the construction of the inlet, and type of instruments used for recording PM levels. This paper has focused on studying the relative changes in emission levels due to type of tire, vehicle speed, and road surface conditions. Both systems showed substantial small-scale variations of emission levels along the road, likely depending on road surface conditions. Road surface conditions were not quantified or characterized, but substantially higher emission levels were seen in connection with a crossing of an unpaved road leading to a quarry, from which several trucks were seen to deposit large amounts of sand onto the paved road being studied. Sand from the unpaved road leading to the quarry and around the quarry was carried on the tires of the trucks and deposited on the paved test road. Most of the material was deposited close to the road junction near the quarry. This effect was seen independently of the system and the tires used. Both systems showed a substantial influence on the emission levels depending on the type of tire used. The summer tire showed much lower suspension than the winter tires (a nonstudded tire and a studded tire). However, the relative importance of the nonstudded versus studded tire was rather different when the two techniques are compared. For the ratio of studded/nonstudded, Emma shows higher values compared with Sniffer on all sections. Both techniques showed increased emission levels with increasing vehicle speed. For the studded tire, Emma indicated an approximately 60% increase in emission levels on average when the vehicle speed increased from 50 to 80 km hr 1, whereas Sniffer indicated an 80% increase. The corresponding percents for the nonstudded tire and the summer tire were 105 and 110% for Emma, and 170 and 75% for Sniffer, respectively. Figure 12. Comparison of PM 10 emissions using different tires on the basis of the measurements performed in Helsinki by Sniffer. The data consist of nearly simultaneous recordings by the summer, nonstudded, and studded tires averaged over a 1.5-km road. The data are sorted with ascending nonstudded tire measurements. The x- and y-axes give the measurement number and the PM 10 concentration, respectively Journal of the Air & Waste Management Association Volume 60 December 2010

13 The Sniffer absolute concentrations were mostly higher than those of Emma. The main reason might be due to the instruments because the simultaneous measurements by Dust- Trak and TEOM showed that DustTrak underestimates road dust PM 10 by a factor of 3 4. The other possible reason might be different weights on the tires behind which the measurements were made (Sniffer has much higher weight compared with Emma). This might also explain the different speed dependence observed by the systems. In conclusion, the authors believe that both systems can be used for studying relative variations, but more work is needed if absolute emission factors should be measured. Emission factors include road wear, resuspension, and to some extent brake wear. If the streets are not sanded, then the stone material is due to the studded tire wear. If only nonstudded tires are used in a city, then resuspension of sand and other sources of road dust PM dominates. ACKNOWLEDGMENTS The Swedish Road Administration financed the measurements using the Swedish Emma system. The Ministry of Environment of Finland financially supported the Finnish Sniffer measurements. REFERENCES 1. Transboundary Particulate Matter in Europe: Status Report 2009; European Monitoring and Evaluation Programme: 2009; available at (accessed 2010). 2. Omstedt, G.; Johansson, C.; Bringfelt, B. A Model for Vehicle-Induced Non-Tailpipe Emissions of Particles along Swedish Roads; Atmos. Environ. 2005, 39, Norman, M.; Johansson, C. Studies of Some Measures to Reduce Road Dust Emissions from Paved Roads in Scandinavia; Atmos. Environ. 2006, 40, Johansson, C.; Norman, M.; Gidhagen, L. Spatial and Temporal Variations of PM 10 and Particle Number Concentrations in Urban Air; Environ. Monit. Assess. 2006, 127, Pohjola, M.A.; Kousa, A.; Kukkonen, J.; Härkönen, J.; Karppinen, A.; Aarnio, P.; Koskentalo, T. The Spatial and Temporal Variation of Measured Urban PM 10 and PM 2.5 in the Helsinki Metropolitan Area; Water Air Soil Pollut. 2002, 2, Laakso, L.; Hussein, T.; Aarnio, P.; Komppula, M.; Hiltunen, V.; Viisanen, Y.; Kulmala, M. Diurnal and Annual Characteristics of Particle Mass and Number Concentrations in Urban, Rural and Arctic Environments in Finland; Atmos. Environ. 2003, 37, Areskoug, H.; Johansson, C.; Alesand, T.; Hedberg, E.; Ekengren, T.; Vesely, V.; Wideqvist, U.; Hansson, H.-C. Concentrations and Sources of PM 10 and PM 2.5 in Sweden; ITM Report no. 110; Stockholm University: Stockholm, Sweden, 2004; available at rapporter.html (accessed 2010). 8. Lenschow, P.; Abraham, H.-J.; Kutzner, K.; Lutz, M.; Preuss, J.-D.; Reichenbächer, W. Some Ideas about the Sources of PM 10 ; Atmos. Environ. 2001, 35(Suppl 1), Harrison, R.M.; Deacon, A.R.; Jones, M.R. Sources and Processes Affecting Concentrations of PM 10 and PM 2.5 Particulate Matter in Birmingham (UK); Atmos. 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Properties and Toxicological Effects of Particles from the Interaction Between Tyres, Road Pavement and Winter Traction Material; Sci. Total Environ. 2008, 393, Hussein, T.; Johansson, C.; Karlsson, H.; Hansson, H.-C. Factors Affecting Non-Tailpipe Aerosol Particle Emissions from Paved Roads: On-Road Measurements in Stockholm, Sweden; Atmos. Environ. 2008, 42, Kupiainen, K.; Tervahattu, H.; Räisänen, M. Experimental Studies about the Impact of Traction Sand on Urban Road Dust Composition; Sci. Total Environ. 2003, 308, Hagen, L.O.; Larssen, S.; Schaug, J. Speed Limit in Oslo Effect on Air Quality of Reduced Speed on RV4 (in Norwegian); NILU OR 41/2005; Norwegian Institute for Air Research: Kjeller, Norway, Gertler, A.; Kuhns, H.; Abu-Allaban, M.; Damm, C.; Gillies, J.; Etyemezian, V.; Clayton, R.; Proffitt, D. A Case Study of the Impact of Winter Road Sand/Salt and Street Sweeping on Road Dust Re-Entrainment; Atmos. Environ. 2006, 40, Fitz, D.R.; Bumiller, K.; Etyemezian, V.; Kuhns, H.; Nikolich, G. Measurement of PM 10 Emission Rate from Roadways in Las Vegas, Nevada Using a SCAMPER Mobile Platform with Real-Time Sensors. Presented at the 14th International Emission Inventory Conference, April 12 14, 2005, Las Vegas, NV, Kuhns, H.; Etyemezian, V.; Landwehr, D.; MacDougall, C.; Pitchford, M.; Green, M. Testing Re-Entrained Aerosol Kinetic Emissions from Roads (TRAKER): A New Approach to Infer Silt Loading on Roadways; Atmos Environ. 2001, 35, Zhu, D.; Kuhns, H.; Scott, S.; Gillies, J.; Etyemezian, V.; Gertler, A. Fugitive Dust Emissions from Paved Road Travel in the Lake Tahoe Basin; J. Air & Waste Manage. Assoc. 2009, 59, ; doi: / Pirjola, L.; Kupiainen, K.J.; Perhoniemi, P.; Tervahattu, H.; Vesala, H. Non-Exhaust Emission Measurement System of the Mobile Laboratory SNIFFER; Atmos. Environ. 2009, 43, Etyemezian, V.; Kuhns, H.; Gillies, J.; Green, M.; Pitchford, M.; Watson, J. 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Prediction Models for Pavement Wear and Associated Costs; SE ; Swedish National Road and Transport Research Institute: Linköping, Sweden, About the Authors Dr. Liisa Pirjola is a principal lecturer at the Metropolia University of Applied Sciences, Department of Technology, and a docent (adj. prof.) at the Department of Physics at the University of Helsinki. Prof. Christer Johansson is with the Department of Applied Environmental Science at Stockholm, University and Environment and Health Administration, Stockholm. Hans Karlsson is with the Department of Applied Environmental Science at Stockholm University. Dr. Kaarle Kupiainen is with Nordic Envicon Oy and the Finnish Environmental Institute. Ana Stojiljkovic is with Nordic Envicon Oy. Docent (adj. prof.) Tareq Hussein is with the University of Helsinki Department of Physics and assistant prof. at the University of Jordan, Faculty of Science, Department of Physics. Please address correspondence to: Liisa Pirjola, Metropolia University of Applied Sciences, Department of Technology, PO Box 4000, FI Helsinki, Finland; phone: ; fax: ; liisa.pirjola@metropolia.fi. Volume 60 December 2010 Journal of the Air & Waste Management Association 1433