Environ. Sci. Technol. 2008, 42, 9051–9057
Chemical Compositions of Subway Particles in Seoul, Korea Determined by a Quantitative Single Particle Analysis SUNNI KANG, HEEJIN HWANG, YOOMYUNG PARK, HYEKYOUNG KIM, AND CHUL-UN RO* Department of Chemistry, Inha University, 253, Yonghyun-dong, Nam-gu, Incheon 402-751, Korea
Received August 14, 2008. Revised manuscript received October 9, 2008. Accepted October 20, 2008.
A novel single particle analytical technique, low-Z particle electron probe X-ray microanalysis, was applied to characterize seasonal subway samples collected at a subway station in Seoul, Korea. For all 8 samples collected twice in each season, 4 major types of subway particles, based on their chemical compositions, are significantly encountered: Fe-containing; soilderived; carbonaceous; and secondary nitrate and/or sulfate particles. Fe-containing particles are generated indoors from wear processes at rail-wheel-brake interfaces while the others may be introduced mostly from the outdoor urban atmosphere. Fe-containing particles are the most frequently encountered with relative abundances in the range of 61-79%. In this study, it is shown that Fe-containing subway particles almost always exist either as partially or fully oxidized forms in underground subway microenvironments. Their relative abundances of Fe-containing particles increase as particle sizes decrease. Relative abundances of Fe-containing particles are higher in morning samples than in afternoon samples because of heavier train traffic in the morning. In the summertime samples, Fe-containing particles are the most abundantly encountered, whereas soil-derived and nitrate/sulfate particles are the least encountered, indicating the air-exchange between indoor and outdoor environments is limited in the summer, owing to the air-conditioning in the subway system. In our work, it was observed that the relative abundances of the particles of outdoor origin vary somewhat among seasonal samples to a lesser degree, reflecting that indoor emission sources predominate.
1. Introduction People spend most of their time indoors, either at home, in the workplace, or in transit, and concern over air quality of indoor microenvironments and its influence on public health is increasing. Among the various types of indoor microenvironments, underground subway stations have some unique characteristics in that they are somewhat closed spaces with strong indoor particle emission sources. Aerosol particles in underground subway stations may be generated mainly by the movement of trains and passengers. The aerosol particles can accumulate in this closed environment, resulting in high * Corresponding author tel: +82 32 860 7676; fax: +82 32 867 5604; e-mail:
[email protected]. 10.1021/es802267b CCC: $40.75
Published on Web 11/18/2008
2008 American Chemical Society
concentrations of indoor particulate matter (PM). Many people within the worldwide metropolitan areas commute using underground subway transportation and spend considerable time in the underground subway environment on a daily basis. Therefore, there has been increasing attention to air quality in the underground subway system because of its possible adverse influence on public health (1-4). Until now, several studies on air quality of worldwide underground subway systems have reported that the concentrations of particulate matter in subway systems are generally much higher than those in outdoor environments. In Prague, PM10 concentrations in underground subway stations are reported to be about 30-70% higher than those in the outdoor environment (5). The concentrations of daytime PM2.5 collected at 2 underground subway stations in Helsinki were 3-6 times higher than those at urban background and street canyon monitoring sites (6). A similar observation was also made for PM10 and PM2.5 samples collected in an underground station in central Stockholm, in which the PM concentrations were 5-10 times higher than those measured at a busy street in Stockholm (7). Higher PM concentrations in underground subway stations in Tokyo than in the aboveground were observed throughout the seasons (8). In addition to the elevated PM concentrations observed in underground subway stations, personal exposures to particulate matter were also reported more for commuters using underground subway systems than for those using other aboveground transportation systems; in the works performed in London, it was observed that personal exposure levels to PM2.5 were 3-8 times higher for commuters using the underground subway system than for those using a bicycle, bus, or car (9, 10). Similar observations were also made in other studies carried out in Toronto (11) and London (12, 13), where the personal exposure levels to PM for commuters using the underground subway system could be up to 40 times higher. Since PM emission sources in the underground subway stations are very different from those in aboveground urban environments, the chemical compositions of PM collected at worldwide underground subway systems are unique. Major chemical elements in PM in underground subway systems are Fe and Si, with Mn, Cr, Cu, Ca, and K elements also abundantly observed (1, 2, 6, 13-15). In particular, concentrations of Fe, Mn, and Cr in PM2.5 samples collected in underground subway stations of New York City were found to be more than 100 times higher than those in aboveground ambient samples, and concern over the potential adverse health effect of long-term exposures to Mn and Cr was mentioned (1). Indeed, a study reported that particles collected at a subway station were about 8 times more genotoxic and 4 times more likely to induce oxidative stress in cultured human lung cells than urban street particles (3). When the genotoxicity of different particle types collected from wood and pellet combustion, tire-road wear, an urban street, and a subway station was compared, the particles from the subway station were the most genotoxic among all particles tested (4). Another genotoxicity test for particles collected at underground stations in London showed that subway particles are comparable in toxicity to welding fume particles, although the concentrations at subway stations are below allowable workplace concentrations for iron oxide and thus a significant cumulative risk to the health of workers or commuters is improbable (2). As effectively summarized in a recent review article (16), some characteristic features of underground subway particles such as PM concentrations have been provided by a significant number of previous studies. However, detailed VOL. 42, NO. 24, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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information on chemical compositions of subway particles is still scarce with just a few of chemical elemental bulk analyses having been performed (2, 6, 8, 14, 15). Chemical compositions of subway particles can be more clearly elucidated by the application of single-particle analysis, since atmospheric subway particles are chemically and morphologically heterogeneous and the average composition and average aerodynamic diameter obtained from bulk analyses do not describe well the population of the particles. The single-particle analysis can provide detailed information on the sources of subway particles as well as their chemical compositions. Until now, only 1 single-particle analysis study on chemical compositions of subway particles has been published, in 1999 (13). In this work, subway particle samples were analyzed through use of computer-controlled scanning electron microscopy and energy dispersive X-ray detection, showing that the most abundant particles are Fe/Si-rich particles. In this work, particle samples collected at an underground subway station in Seoul, Korea were characterized by the application of a novel single-particle analytical technique, low-Z particle electron probe X-ray microanalysis (low-Z particle EPMA). The low-Z particle EPMA allows the determination of the concentration of low-Z elements such as carbon, nitrogen, and oxygen, as well as elements observed using conventional EPMA, in individual particles of micrometer size. Further, by the application of a quantification method, which employs Monte Carlo simulation combined with successive approximations, quantitative specification of the chemical compositions can be performed, and thus this technique can perform molecular analyses of individual particles (17-21). By the combined use of morphological and chemical information obtained from the low-Z particle EPMA, it was observed that particles collected at a subway station in Seoul are mostly from wear processes at railwheel-brake interfaces and indoor emission sources predominate in the underground subway microenvironments.
2. Experimental Section 2.1. Samples. Subway particle samples were collected at a platform of the “Hyehwa” subway station in Seoul, Korea. Seoul, the capital of Korea, is a densely populated megacity (population 10.3 million, area 605 km2). The Seoul subway system contains 8 lines with a total of 327 stations. According to statistics provided from the Seoul metro transportation center (http://www.seoulmetro.co.kr), approximately 6.7 million commuters use the Seoul subway system on a daily basis. The Hyehwa station is one of the busiest subway stations with 58,000 commuters per day. Ambient annual PM10 concentrations in Seoul were 58.0 µg/m3 in 2005 with overall PM10 concentrations at subway stations and the Hyehwa station reported to be 110.9 and 121.5 µg/m3, respectively, in 2004, indicating much higher PM10 concentrations at subway stations than in outdoor environment (http://www.seoulmetro.co.kr). Sampling was done at the platform of the Hyehwa station, twice every season during 2004-2005: December 16 and 17, 2004; May 3 and 5, July 4 and 6, and November 23 and 25, 2005. Particles were sampled on Ag foil using a 7-stage May cascade impactor (22). The May impactor has, at a 20 L/min sampling flow, aerodynamic cutoffs of 16, 8, 4, 2, and 1 µm for stages 1-5, respectively. Sampling durations varied in order to obtain a good loading of particles at the impaction slots. Since the number concentration of smaller particles in the air is higher than that of larger particles, it is necessary to collect smaller particles within a shorter sampling duration to avoid the collection of agglomerated particles. The collected samples were put in plastic carriers, sealed, and stored in a desiccator. 2.2. EPMA Measurements. The measurements were carried out using a Hitachi S-3500N environmental scanning 9052
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electron microscope equipped with an Oxford Link SATW ultrathin window EDX detector. The resolution of the detector was 133 eV for Mn KR X-rays. X-ray spectra were recorded under the control of EMAX Hitachi software. To achieve optimal experimental conditions, such as a low background level in the spectra and high sensitivity for low-Z element analysis, a 10 kV accelerating voltage was chosen. The beam current was 1.0 nA for all measurements. To obtain statistically enough counts in the X-ray spectra while limiting beam damage effects on sensitive particles, a typical measuring time of 10 s was used. A more detailed discussion on the measurement conditions is given elsewhere (17). Computercontrolled X-ray data acquisition for individual particles was carried out automatically in the point analysis mode, whereby the electron beam was focused at the center of each particle and X-rays were acquired while the beam remained fixed on this single spot. The localization of the particles was based on inverse backscattered electron contrast. Morphological parameters, such as diameter and shape factor, were calculated by an image processing routine. These estimated geometrical data were set as input parameters for the quantification procedure. Some 300 particles for each stage sample were analyzed totaling some 10,400 particles (1,300 particles for each sample). 2.3. Data Analysis. The net X-ray intensities for the elements were obtained by nonlinear least-squares fitting of the collected spectra using the AXIL program (23). The elemental concentrations of individual particles were determined from their X-ray intensities by the application of a Monte Carlo calculation combined with reverse successive approximations (19, 24). The quantification procedure provided results accurate within 12% relative deviations between the calculated and nominal elemental concentrations when the method was applied to various types of standard particles such as NaCl, Al2O3, CaSO4 · 2H2O, Fe2O3, CaCO3, and KNO3 (18-20). The low-Z particle EPMA can provide quantitative information on the chemical composition, and particles can be classified based on their chemical species. The classification procedure takes a substantial amount of time if done manually, because analysis of tens of thousands of pieces of particle data is required. Thus, an expert system that can determine chemical species from the elemental concentration data has been applied to these underground subway sample data. By applying the expert system, the time necessary for chemical speciation becomes significantly shorter, and detailed information on particle data can be saved and extracted when more information is needed for further analysis (20).
3. Results and Discussion Overall, some 10,400 particles for 8 day samples (1,300 particles for each sample) collected at the subway station platform were analyzed and classified on the basis of their chemical species. A typical secondary electron image obtained from a stage 4 sample collected on July 4, 2005 is shown in Figure 1, where the chemical species of each particle is noted together with its particle number. For all 36 particles in the image field, both elemental concentrations in atomic fraction and identified chemical species for the particles are shown in Table S1 of the Supporting Information. Previous studies for subway particles reported that Fe-containing particles are the most abundant in subway particles in their weight concentrations. Sitzmann et al. (13) reported that subway particle samples collected by commuters using the London Underground system contained the most Fe/Si-rich particles with 53% of the total number of particles. For PM2.5 subway samples collected at London Underground stations, iron oxide particles were the majority (67% in weight fraction) (2). Subway samples collected in New York, Helsinki, Tokyo, and Budapest also contained very high concentrations of
FIGURE 1. Typical secondary electron image for subway particles obtained from a stage 4 sample collected on July 4, 2005. iron, at 42% of PM2.5, 88-92% of PM2.5, 74% of SPM, 40-46% of PM10, respectively (1, 6, 8, 15). Indeed, most particles in Figure 1 are Fe-containing (29 of 36 particles). 3.1. Iron-Containing Subway Particles. Fe-containing subway particles are generated mainly from mechanical wear and friction processes at rail-wheel-brake interfaces and at the interface between a third rail providing electricity to subway trains and an electricity guide of trains. Wear and friction processes initially produce iron metal particles, and yet the surface of the primary particles must be active enough so that oxygen in the air can easily react on the metallic surface, resulting in the formation of iron oxides. Since particle surface area is larger for smaller particles, oxidized iron particles are more abundantly encountered in fine fractions, in stage 4 and 5 samples (the cutoff diameters are 2 and 1 µm for stages 4 and 5, respectively). In addition, Fe-containing particles internally mixed with Si, Ca, and C species are abundantly encountered more often in fine fractions. In our study, Fe-containing particles are mainly in the form of iron oxides, although some iron metal particles are also encountered. In mineralogy, iron oxide minerals are known to exist as FeO (wu ¨stite), Fe2O3 (hematite, maghemite), or Fe3O4 (magnetite). If Fe-containing subway particles are fully oxidized, the oxidation states of the particles can be identified using the low-Z particle EPMA as the contents of iron and oxygen of individual particles can be quantitatively determined. However, some Fe-containing particles must be partially oxidized because a wide range of Fe/O concentration ratios for these particles were observed, i.e., the Fe/O ratios in atomic fraction for the 29 Fe-containing particles in Figure 1 range between 0.56 and 3.95, whereas Fe/O ratios for FeO, Fe2O3, and Fe3O4 minerals are 1.0, 0.67, and 0.75, respectively. In this study, Fe-containing particles can be classified into 3 different types: iron metal (denoted as “Fe” in Figure 1), partially oxidized (denoted as “Fe/FeOx”), and fully oxidized (denoted as “FeOx”) Fe-containing particles. Iron metal particles are clearly distinguishable from oxidized iron particles because of both their high iron content (Fe/O ratios >3) and secondary electron images different from those of the iron oxide particles. Wear particles can be recognized from the irregular shape of their secondary electron images.
Iron metal particles were observed mostly in the sliced form, implying that they are scraps broken off from train wheels or rails (see Figure 2a). Because they are not generated by wear from wheel-rail-brake interfaces or a third rail, their surface is not active enough to be significantly oxidized. Still, it is worth noting that all iron metal particles contain a small amount of oxygen. In the secondary electron image, the iron metal particles look dark compared to bright particles that contain iron oxides. The brightness differences mainly result from the different secondary electron yields between conducting metal and insulating oxide particles whereby the insulating oxide particles can develop a negative voltage by the accumulation of primary electrons, resulting in a bright contrast due to a larger emission of secondary electrons by repulsion (25). In almost all cases, as shown in Figures 1 and 2, iron-oxide-containing particles agglomerate due to the coagulation of particles, indicating that they were generated at close places, such as the wheel-rail-brake or third rail-guide interfaces. Another possible explanation would be the existence of an attractive (magnetic) force between the iron oxide particles. A study reported that iron-oxide subway particles are in the form of magnetic Fe3O4, although determined by XRD for bulk subway particles (3). Particles with various sizes make an agglomerated lump, including very small nanosized particles, mostly on the surface of large Fe-containing particles (see particles (a), (b), and (c) in Figure 2b). Those nanosized particles are most probably formed by the condensation of gaseous iron species from the sparking between the third rail and the electricity guide of subway trains and by the coagulation thereafter. This generation mechanism of nanosize particles is similar to that of arc welding particles (26). The existence of gaseous iron species in the subway environment explains why almost all the subway particles contain minor iron species even for particles from outdoor environment such as soil-derived particles. Fe/O concentration ratios in atomic fraction for iron oxidecontaining particles ranged between 0.5 and 1.8, whereas those for iron metal particles were higher than 3. When the particles were fully oxidized, the Fe/O ratios had to be consistent with those for Fe3O4 or Fe2O3. When the ratio is significantly higher than 0.75, the particles are partially VOL. 42, NO. 24, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 2. Magnified Fe-containing particles shown in Figure 1. oxidized into mixtures of iron oxide and iron metal. In this study, particles with Fe/O lower than 0.85 are judged to be fully oxidized, considering the uncertainty (3% in atomic fraction (22 particles with >10%, see Table S1, Supporting Information). According to Korean Standard Database (http://www.standard.go.kr), cast irons used for rails and wheels contain C (0.5-0.7%), Si (0.1-0.35%), Mn (0.6-0.95%), P (
