Environ. Sci. Technol. 2005, 39, 699-706
Size and Composition of Airborne Particles from Pavement Wear, Tires, and Traction Sanding K A A R L E J . K U P I A I N E N * ,†,‡ A N D HEIKKI TERVAHATTU‡ Department of Biological and Environmental Sciences, University of Helsinki, P.O. Box 27, FIN-00014, Helsinki, Finland, and Nordic Envicon Oy, Koetilantie 3, FIN-00790 Helsinki, Finland M I K A R A¨ I S A¨ N E N Geological Survey of Finland, P.O. Box 96, FIN-02151 Espoo, Finland T I M O M A¨ K E L A¨ , M I N N A A U R E L A , A N D RISTO HILLAMO Finnish Meteorological Institute, Sahaajankatu 20 E, FIN-00810 Helsinki, Finland
Mineral matter is an important component of airborne particles in urban areas. In northern cities of the world, mineral matter dominates PM10 during spring because of enhanced road abrasion caused by the use of antiskid methods, including studded tires and traction sanding. In this study, factors that affect formation of abrasion components of springtime road dust were assessed. Effects of traction sanding and tires on concentrations, mass size distribution, and composition of the particles were studied in a test facility. Lowest particle concentrations were observed in tests without traction sanding. The concentrations increased when traction sand was introduced and continued to increase as a function of the amount of aggregate dispersed. Emissions were additionally affected by type of tire, properties of traction sand aggregate, and driving speed. Aggregates with high fragmentation resistance and coarse grain size distribution had the lowest emissions. Over 90% of PM10 was mineral particles. Mineralogy of the dust and source apportionment showed that they originated from both traction sand and pavement aggregates. The remaining portion was mostly carbonaceous and originated from tires and road bitumen. Mass size distributions were dominated by coarse particles. Contribution of fine and submicron size ranges were approximately 15 and 10% in PM10, respectively.
Introduction A significant fraction of urban PM10 is road dust, lifted from road surfaces by traffic (e.g., refs 1-4). Road dust is a complex mixture of mainly coarse particles released from several different sources, usually dominated by geological material * Corresponding author present address: International Institute for Applied Systems Analysis, Schlossplatz 1, A-2361 Laxenburg, Austria; phone: +43 2366 807 420; fax: +43 2366 71313; e-mail:
[email protected]. † University of Helsinki. ‡ Nordic Envicon Oy. 10.1021/es035419e CCC: $30.25 Published on Web 12/30/2004
2005 American Chemical Society
(5, 6). Road dust has been acknowledged as a dominant source of PM10 especially during spring in urban areas in, for example, Scandinavia, North America, and Japan (3, 7-10). The high proportion of road dust has been linked to the snowy winter conditions in the northern latitudes that make it necessary to use traction control methods. Traction control enhances the formation of mineral particles as wear products. A fraction of the particles is also deposited in snow and later released when snow melts. These abrasion particles are observed in high concentrations, especially during spring in urban areas with high volumes of traffic (3, 11). The methods for traction control include dispersion of traction sand aggregate on the road surface and equipping the tires with metal studs or a special rubber design. High particle concentrations have been linked to the use of studded tires in, for example, Japan and Norway (7, 8). Traction sanding has also been linked to an increased particle loading in urban air (9, 12, 13). The formation processes of the particles are the mechanical wear of the traction sand grains and the wear of road pavement both because of tires and by traction sand in a so-called “sandpaper effect” (12). This study is a part of a test series performed in a test facility to assess the factors that affect the abrasion components of springtime road dust. The first set of tests focused on PM10 and the effect of traction sanding on both emissions and mineralogy of the abrasion dust (12, 14). It was observed that traction sanding increases dust emissions from both traction sand and pavement. Earlier, Kanzaki and Fukuda (15) and Lindgren (16) concluded that sand should contribute to pavement wear but they present no measurement data. In this study, more data was collected about the impact of tire and traction sand on emissions and was used to estimate the contribution of the two mineral dust sources, pavement and traction sand aggregate. The focus was extended to study the mass size distribution of the particles, composition of particles in smaller size ranges, and fractions of organic and elemental carbon in the abrasion dust. The mass size distribution and composition of road dust collected from street locations has been studied by, for example, Rogge et al. (5), Chow et al. (6, 17), Puledda et al. (18), Silva et al. (19), and Pakkanen et al. (4, 20). The current article is, to the knowledge of the authors, the first to focus solely on the abrasion products and to include the effect of the antiskid methods and a discussion of the submicron mineral particles.
Experimental Section Test Facility. The study was conducted in a test facility, which made it possible to rule out dust contributions from sources other than abrasion of tires, pavement, and traction sand aggregates. It is difficult to estimate the proportions of the different mineral dust sources, because they often have a similar mineralogical composition (21). For these tests, aggregates (traction sand and pavement aggregate) with distinguishable mineralogy were chosen to be able to estimate the proportions from different dust sources. Point-counting method (1000 analysis points per one polished thin section) with polarizing microscope was used to determine the mineralogical composition of the chosen aggregates (see ref 14). Fractions of main minerals with chemical composition are shown in Table 1. Only the pavement aggregate contained the mineral hornblende, which was a tracer for the mineral dust from the pavement. The pavement and traction sand aggregates of these tests are commonly used in local road construction and maintenance. The grain size distribution of the traction sand aggregates was 2-5.6 mm but additionally the effect of a VOL. 39, NO. 3, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. Abundances (%) of Minerals and Carbonaceous Fractions in Traction Sand Aggregates, Pavement, and Tires traction sand aggregates minerals quartz K-feldspar plagioclase biotite hornblende compositeb other inorganic
Granite1
Granite2
Diabase
30 30 32 6
28 26 39 2
57 4
2
4
35 4
b
finer grained aggregate, 1-5.6 mm, with 40% of the mass smaller than 2 mm, on dust emissions was studied with Granite1. The design of the pavement was asphalt concrete with a grain size up to 11 mm. The facility as well as the mineralogy and physical properties of the aggregates used in the tests is presented also in Kupiainen et al. (12) and Ra¨isa¨nen et al. (14, 43). The work was performed during May 2002 in a road simulator (Figure 1) fitted with studded and friction tires. The road simulator was situated in a closed chamber (approximately 180 m3 in volume) equipped with cooling and ventilating systems. Before each test, the chamber was cooled to a temperature of 0-2 °C to represent the temperature in dry spring conditions. Relative humidity was 5075%. The road simulator had an electrically powered rotating axle with two wheels and adjustable rotating speed. The axel system was tuned to move sideways so that the driving space of the tires was 33 cm. The diameter of the test ring was 390 cm. The driving ring was surrounded with low walls to prevent the traction sand aggregate from flying off. The urban driving conditions vary very much and it is important to discuss how the test conditions relate to urban situations. The short distance between the wheels and the rotating motion result in a grinding effect that is rarely found in street conditions with vehicles traveling straightforward. In the test facility, abrasive wear dominates over fragmentation (14) and thus asphalt wear might be higher than in a straight motion. Also, relatively low speeds (15 and 30 km h-1) were used. These conditions might represent better the traffic situations in crossings and parking areas, which actually often are places with extensive traction control. The 9
tirea
chemical composition SiO2 KAlSi3O8 (Na,Ca)Al(Si,Al)Si2O8 K(Mg,Fe)3(Al,Fe)Si3O 10(OH,F)2 Ca2(Mg,Fe)4Al(Si7Al) O22(OH,F)2
13 29 58
EC OC CC
Composite of clinopyroxene[(Ca,Mg,Fe)2Si2O6 (augite)], olivine [(Mg,Fe)2SiO4], and
FIGURE 1. Test room with the rotating axle system in the foreground and the impactor inlets in the back.
700
3 3 33 0 38 11 7
Carbonaceous Fractions 0 5 0
elemental carbon organic carbon carbonate carbon a Source profile based on SPECIATE 3.2 (40, 41). cummingtonite-grunerite [(Mg,Fe)7Si8O22(OH)2].
pavement
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tests were run on dry surfaces so that the dust concentrations would not be suppressed. It is obvious that in real life wet surfaces might be at least as common during spring, because of precipitation, temperature variations, or use of road salt to melt ice from pavements. Wet conditions have been linked to increased wear of pavement (see, e.g., ref 15). Altogether 17 tests were performed: 13 with traction sand and 4 without. Each test lasted for 20-30 min. After each test, the dust was allowed to settle for 30-45 min, and the room was carefully vacuumed and ventilated. The effects of following variables on particle concentrations and composition were tested: amount (0, 2, and 4 liters) and type of traction sand aggregate (three different aggregates) and type of tire (friction and studded) and the driving speed (15 and 30 km h-1). The amounts of traction sand were chosen to represent “dirty” and “very dirty” road conditions found in springtime. The largest amount was chosen to represent extreme conditions when traction sand has accumulated on the pavement for a long time. All studied traction sand aggregates had high average hardness (ca. 6-6.5 in Mohs scale). However, there was variation in the resistance against fragmentation, which describes how well antiskid aggregate grains can resist breakage into smaller sizes. As tested by Ra¨isa¨nen et al. (43)), Granite1 had a lower fragmentation resistance than Diabase and Granite2, which were similar in that respect. The tires were commercially available passenger car winter tires equipped with (Nokian Hakkapeliitta 1, 175/70R13, studded tire) or without metal studs (Nokian Hakkapeliitta Q, 175/70R13, friction tire). The tire pressure was 2.0 bar and the weight on each tire was 300 kg. The design of the tests including tire type, aggregate type, and amount of traction sand dispersed is described with the results in Table 2. Particulate Sampling and Analysis. To estimate the size distribution of the aerosol, samples were collected with two virtual impactors (VI, PM2.5-10 and PM2.5), two 12 stage (0.04510.7 µm) cascade impactors (SDI) (22), and two high-volume samplers (Wedding & Associates Sample, TSP and PM10). The inlets of the virtual and cascade impactors can be seen in the background of Figure 1. All the inlets were situated similarly, approximately 1.5 m of the driving ring at a height of approximately 2.5 m. The VIs were equipped with quartz (Pallflex, type Tissuquartz 2500QAT-UP) and PTFE (Millipore, type FS3 µm) filters, the SDI with aluminum substrates and polycarbonate membranes (Whatman, type Nuclepore 800120), and the high-volume samplers with glass fiber filters (Munktell, type MG160). Background concentration was monitored before the tests and the average was less than 10% of the lowest concentration measured in the tests. Eight tests done in the first set of tests (see ref 12) were rerun to study the repeatability of the
TABLE 2. Test Descriptions and Results
tire
traction sand aggregate
amount of traction sand aggregate dm3
g m-2
TSPa (mg m-3)
PM 10a (mg m-3)
PM 2.5a (mg m-3)
PM10a (mg km-1)
PM 2.5a (mg km-1)
EC (% in PM10)
Granite1 Granite2 Diabase
Tests with Friction Tires (Nonstudded), Traction Sand Aggregate Grain Size 2-5.6 mm 0 0 1.9 0.44 0.05 11 1 1.4 2 880 7 2.5 0.18 60 4 0.2 2 933 4.6 1.5 0.11 36 3 0.3 2 1046 4.9 2.01 0.13 48 3 0.3
studded studded studded studded studded studded studded
Granite1 Granite1 Granite2 Granite2 Diabase Diabase
0 2 4 2 4 2 4
friction studded
Granite1 Granite1
2 2
865 865
Granite1
0 0 2
0 0 880
friction friction friction friction
friction studded studded a
Tests with Studded Tires, Traction Sand Aggregate Grain Size 2-5.6 mm 0 2.8 0.7 0.05 17 1 0.4 880 9.2 2.82 0.28 68 7 0.1 1760 26.3 1.12 27 0.1 933 6.5 1.77 0.13 43 3 0.1 1866 14 4.82 0.43 116 10 0.1 1046 2.79 0.20 67 5 0.1 2112 16.4 5.46 0.46 131 11 0.0
OC (% in PM10)
CCb (% in PM10)
8.3 5.9 6.3 6.4
0.4 0.4 0.4 0.6
3.0 3.9 2.6 6.2 2.8 3.1 2.8
0.1 0.3 0.3 0.4 0.3 0.3 0.3
Tests with Traction Sand Aggregate Grain Size 1-5.6 mm 11.8 4.51 0.44 108 11 16.9 6.48 3.47 155 83
0.2 0.1
4.0 2.8
0.5 0.3
Tests with Speed 30 km h-1 0.75 0.08 9 3.37 0.33 40 3.36 0.52 40
0.0 0.0 0.2
5.7 3.0 4.8
0.4 0.3 0.3
2.4 9.1 12.6
TSP ) high-volume sampler; PM10 and PM2.5 ) virtual impactor.
measurements and analyses. The first set measured PM10 concentrations with a similar high-volume sampler as used in this study. The concentrations in the first set were on average 97% (SD 18) of the ones observed now. This was considered to manifest a good repeatability. The morphology, composition, and mineralogy of the particles were determined from high-volume TSP- and PM10filters and the cascade impactor membranes based on individual particle analysis with SEM/EDX. A similar instrumentation has been used in several particle studies (23-28). The carbonaceous fractions elemental carbon (EC), organic carbon (OC), and carbonate carbon were determined from the VI-quartz filter samples with a thermal-optical analyzer (31, 32). SEM/EDX-samples were prepared by pressing a doublesided tape (Scotch Ruban Adhesive), attached to the SEMstub, onto the filter surface covered with particles. The samples were then coated with either carbon or chromium to make the sample surface conductive. The elemental composition of large sets of particles was studied with a scanning electron microscope (SEM, ZEISS DSM 962) coupled with an energy-dispersive X-ray microanalyzer (EDX, LINK ISIS with ZAF-4 measurement program). A minimum of two samples from each filter was made and coated with carbon (Agar SEM Carbon Coater). The accelerating voltage was 20 kV. The total X-ray count rate was calibrated to 1500 counts s-1 with cobalt. From each particle, an X-ray spectrum was collected with a preset time of 15 s. The morphology and chemistry of fine and submicron particles was studied in detail from the polycarbonate membranes of the cascade impactor. A field emission SEM (FESEM, JEOL JSM-6335F) coupled with an energy-dispersive X-ray microanalyzer (ED, LINK ISIS and INCA) was used for the analyses. Samples were prepared similarly as for conventional SEM, with Cr as the coating material. The acceleration voltage was 15 kV. In the elemental analyses, the ZAF correction method assumes flat samples. This is not the case with complex sized and shaped particles, and the atomic concentrations may therefore be biased (26). However, it is possible to estimate the presence of the main elements. As in Paoletti et al. (26),
b
2 8 12
Carbonate carbon.
the particle classes in this study were determined by distinguishing the presence and proportional concentrations of the typical elements. This classification could be made with good confidence. The elemental composition of 150 randomly selected particles from each hi-vol filter was analyzed and saved with ZAF-4. Additional observations about the shape and size were made. A representative number of particles depend on the complexity of the sample and the objectives of the study (29, 30). The accuracy of the classification was studied for similar samples by Kupiainen et al. (12) and it was concluded that a set of 100-150 particles gives reliable results for the purposes of this study. The presence of Al, Ca, Cl, Fe, K, Mg, Na, O, S, Si, and Ti for each particle was determined. If other elements were clearly observed, their presence was also recorded. Particles were classified according to the elemental composition. The elemental compositions of the particle classes were then compared, for example, with the elemental composition of the minerals in the aggregates to serve as a base in classifying and eventually determining the sources of particles in the PM samples. The carbonaceous fractions were treated as individual particle classes. The source contributions to ambient PM were calculated using the particle class balance analysis (36), which is a modification of the chemical mass balance method (CMB) and especially developed for individual particle data. The approach in both is to use the least-squares solution to a set of mass-balance equations that express the chemical composition at the receptor as a linear sum of products of source profile abundances and source contributions (37). The model assumptions for the particle class balance approach are that (a) composition of source emissions is constant, (b) components do not react with each other, and (c) only identified sources contribute to the receptor (36). These conditions were well fulfilled in the tests described in this paper. A more detailed description of the methodology can be found in Kim and Hopke (36), U.S. EPA (38), and the references therein. Latest CMB software (CMB8; 38) from U.S. EPA was used for the calculations. VOL. 39, NO. 3, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 3. PM10 mass size distribution for friction and studded tire with 15 km h-1 and without sanding aggregate (see Table 2). FIGURE 2. Correlation between PM10 and the amount of traction sand aggregate; the solid line shows the regression and correlation of aggregates with high fragmentation resistance. The sources contributing to the PM in the tests were estimated to be tires, traction sand aggregate, and pavement. Large amounts of dust were produced from these materials during the tests and thus the contribution of particles from the background air was assumed to be negligible. The chemical speciation of tire dust was taken from the SPECIATE3.2 source profile database (39) and is based on the information reported by Cass et al. (40) and Hildemann et al. (41). It was estimated to be mainly carbonaceous (87%, (40)) with 58% organic matter and 29% elemental carbon (41). The remaining 13% is composed of the inorganic material in the tire material (see refs 8, 40, and 41). The source profiles for the traction sand aggregates were received from the mineralogical analyses (Table 1). The average from the four tests without traction sanding serves as the source profile for pavement dust that includes apart from minerals of the pavement aggregate also, for example, bitumen. The abundances of minerals and carbonaceous fractions in traction sand aggregates, pavement, and tires are shown in Table 1.
Results and Conclusions Effect of Traction Sanding and Type of Tire on Concentrations. Table 2 shows the results of the individual tests, with TSP-, PM10-, and PM2.5-concentrations and PM10- and PM2.5emission factors per kilometer (calculated to represent emissions from four wheels) as well as results from the carbon measurements (EC, OC, and carbonate-C). The concentrations were affected by the type of the tire and the properties and dispersion amount of the traction sand aggregate as well as driving speed. The lowest concentrations and emission factors were measured with low speed (15 km h-1), nonstudded tires, and without traction sand on the pavement, and the highest were measured with high traction sand loads (2000 g m-2). The amount of traction sand and its properties were the most significant factor affecting the concentrations in these tests. The effect of tire type on the PM10 and PM2.5 concentrations was studied by comparing six pairs of similar measurements where only the tire type differed. Statistically significant differences were observed, with approximately 1.5 times higher concentrations with studded tires (Sign Test, PM10 p-value 0.016 and PM2.5 p-value 0.031). In the earlier study (12, 14), it was observed that the concentrations rose as a function of mass of traction sand dispersed. It was also suspected that the properties of the aggregates affect the concentrations (12). Evidence to support this hypothesis was found on the basis of the additional measurements made in 2002. Figure 2 demonstrates the effect of the dispersion amount and quality of the traction sand aggregate on the measured concentrations. These factors were studied by comparing the tests with same speed (15 km h-1), same tire (studded), and similar traction sand grain 702
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FIGURE 4. PM10 mass size distribution for friction and studded tire with 30 km h-1 and without sanding aggregate (see Table 2). size (2-5.6 mm). Figure 2 also includes data from the earlier test set from 2001 (see ref 12). Concentrations were 4-fold with approximately 1000 g m-2 of traction sand aggregate and 9-fold with 2000 g m-2, compared with the experiments without traction sanding (Table 2, Figure 2). The PM10 rose as a function of the amount of traction sand and the correlation was especially strong (R2 ) 0.934) for aggregates with a similar resistance against fragmentation (Figure 2). Taking into account the mass of traction sand dispersed, the aggregate with a low resistance against fragmentation (Granite1) gave higher concentrations than the ones with a better resistance and the effect became more significant with increasing traction sand load (Figure 2). Another significant property of the traction sand aggregates that affected the concentrations was grain size. The concentrations were approximately 2-fold with the aggregate with smaller sand grains (1-5.6 mm with 40% of the mass between 1 and 2 mm). Also, higher driving speed increased the concentrations, but the increase resulted from the longer traveling distance of the tire. Only in the studded tire test without traction sand was an increase in the emissions per traveled distance becaue of a higher speed observed (Table 2). Size of the Particles. Size of particles was studied to find out the possible contribution of the abrasion components in the fine and submicron size ranges. A comparison of the size ranges observed in the VI- and hi-vol measurements shows that the PM10 fractions were on average approximately 30% of TSP and PM2.5 was approximately 15% of PM10 (Table 2). More detailed size distribution was measured in six tests with cascade impactors (SDI). The PM10 concentrations measured with the SDIs agreed fairly well with the VImeasurements being on average 13% lower (SD 9). The comparison of size distributions in tests with studded and friction tires is shown in Figure 3 for 15 km h-1, Figure 4 for 30 km h-1, and Figure 5 for 15 km h-1 and 880 g m-2 traction sand. The particulate masses in several size ranges are also presented in tabular form in Table 3. Fine and submicron particles were present in all measurements. The size distribution evolved relatively uniformly with the mass concentrations. The submicron fraction (PM1.3)
FIGURE 5. PM10 mass size distribution for friction and studded tire with 15 km h-1 and 880 g m-2 of Granite1 (see Table 2).
TABLE 3. Concentrations Observed in Different Size Ranges [µg m-3] size range (µm) friction, no sand studded, no sand friction, no sand-30 kph studded, no sand-30 kph friction, 2l Granite1-15 kph studded, 2l Granite1-15 kph
