Sources and Characteristics of Particulate Matter in

International Journal of

Environmental Research and Public Health Article

Sources and Characteristics of Particulate Matter in Subway Tunnels in Seoul, Korea Yongil Lee 1,2 , Young-Chul Lee 3 , Taesung Kim 2 , Jin Seok Choi 4 and Duckshin Park 1, * 1 2 3 4

*

Korea Railroad Research Institute (KRRI), 176 Cheoldobakmulkwan-ro, Uiwang-si 16105, Korea; [email protected] Mechanical Engineering, Sungkyunkwan University, 2066 Seobu-ro, Suwon-si 16419, Korea; [email protected] Department of BioNano Technology, Gachon University, 1342 seongnamdae-ro, Seongnam-si 13120, Korea; [email protected] Analysis Center for Research Advancement, Korea Advanced Institute of Science and Technology (KAIST), 291 Daehak-ro, Yuseong-gu, Daejeon-si 34141, Korea; [email protected] Correspondence: [email protected]; Tel.: +82-31-460-5367

Received: 27 September 2018; Accepted: 31 October 2018; Published: 12 November 2018

Abstract: Hazards related to particulate matter (PM) in subway systems necessitate improvement of the air quality. As a first step toward establishing a management strategy, we assessed the physicochemical characteristics of PM in a subway system in Seoul, South Korea. The mean mass of PM10 and PM2.5 concentrations (n = 13) were 213.7 ± 50.4 and 78.4 ± 8.8 µg/m3 , with 86.0% and 85.9% of mass concentration. Chemical analysis using a thermal–optical elemental/organic carbon (EC–OC) analyzer, ion chromatography (IC), and inductively coupled plasma (ICP) spectroscopy indicated that the chemical components in the subway tunnel comprised 86.0% and 85.9% mass concentration of PM10 and PM2.5 . Fe was the most abundant element in subway tunnels, accounting for higher proportions of PM, and was detected in PM with diameters >94 nm. Fe was present mostly as iron oxides, which were emitted from the wheel–rail–brake and pantograph–catenary wire interfaces. Copper particles were 96–150 nm in diameter and were likely emitted via catenary wire arc discharges. Furthermore, X-ray diffraction analysis (XRD) showed that the PM in subway tunnels was composed of calcium carbonate (CaCO3 ), quartz (SiO2 ), and iron oxides (hematite (α-Fe2 O3 ) and maghemite-C (γ-Fe2 O3 )). Transmission electron microscopy images revealed that the PM in subway tunnels existed as agglomerates of iron oxide particle clusters a few nanometers in diameter, which were presumably generated at the aforementioned interfaces and subsequently attached onto other PM, enabling the growth of aggregates. Our results can help inform the management of PM sources from subway operation. Keywords: characteristics; particulate matter; source identification; subway tunnel; air quality

1. Introduction Subway systems relieve traffic congestion in metropolitan areas as an environmentally friendly means of transportation [1–5]; however, human exposure to air pollutants in subway systems is a concern [6]. Particulate matter (PM) exerts detrimental effects on health. In 2013, the specialized cancer agency of the World Health Organization (WHO), the International Agency for Research on Cancer (IARC), announced that it classified outdoor air pollution as carcinogenic to humans (Group 1) [7]. For instance, a 10 µg/m3 decrease in PM10 concentration resulted in an 8.36% drop in the all-cause mortality rate in Beijing, China [8], and the mortality risk estimated in a Dutch cohort study for PM2.5 was 6% per 10 µg/m3 for natural-cause mortality [9].

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Moreover, when PM is inhaled by the respiratory tract, smaller-diameter PM can penetrate the Int. J. Environ. Res. Public Health 2018, 15, 2534 2 of 17 lung (alveolus) [10]. PM in subway systems is more harmful than other sources, e.g., wood combustion, traffic (vehicles), and roadways, due to redox active iron on the surface of the subway particles [11,12].when Specifically, in the by literature [13], magnetite (Fe3O4) was observed the human Moreover, PM is inhaled the respiratory tract, smaller-diameter PM canin penetrate the brain. lung (alveolus) [10]. PM in subway systems is more harmful than other sources, e.g., wood combustion, Seoul Metro Company installed screen (PSDs) all stations, a feature trafficThe (vehicles), and roadways, due to redoxplatform active iron on thedoors surface of theatsubway particles [11,12]. which is also implemented in other nations. PSDs reduce in thethe risk of accidents Specifically, inbeing the literature [13], magnetite (Fe3 O4 ) was observed human brain. and increase the operation of heating, ventilation, air-conditioning [14,15].a As the The Seoul efficiency Metro Company installed platformand screen doors (PSDs) systems at all stations, feature installation PSDsimplemented results in the of thePSDs platform from the tunnel, the PM on the which is alsoofbeing in isolation other nations. reduce the risk of accidents and level increase platform significantly reduced [15]; however, the accumulation of PM in the tunnel operationisefficiency of heating, ventilation, and air-conditioning systems [14,15]. As subway the installation worsens the air quality therein [16]. of PSDs results in the isolation of the platform from the tunnel, the PM level on the platform is The PMreduced in subway tunnels originates from operation of subway the subway, significantly [15]; however, the accumulation of PM in the tunnelventilation, worsens theand air re-suspension settled PM [2,16–19]. Many studies reported high levels of PM in subway tunnels quality thereinof [16]. and determined its chemical [6].operation Metals are major constituents in subway The PM in subway tunnelscomposition originates from of the subway, ventilation, of andPM re-suspension tunnels principally Fe [16,22–24]. field tunnels studies,and wear PM with of settled[16,20–23], PM [2,16–19]. Many studies reportedAccording high levels to of previous PM in subway determined peak diameters of 100, 350, 300–700 nmmajor is emitted during mechanical braking [3]. In[16,20–23], addition, its chemical composition [6].and Metals are the constituents of PM in subway tunnels wear PM with peak diameters of 6.98 and 165.5field nmstudies, is emitted from wheel–rail interface during principally Fe [16,22–24]. According to previous wear PMthe with peak diameters of 100, 350, electrical braking Thisduring PM in mechanical subway tunnels could penetrate subway cabin viadiameters heating, and 300–700 nm is [2]. emitted braking [3]. In addition,the wear PM with peak ventilation, and air-conditioning (HVAC) systems interface and outflow via electrical the ventilation in PM the of 6.98 and 165.5 nm is emitted from the wheel–rail during brakingsystem [2]. This tunnel into tunnels the city.could penetrate the subway cabin via heating, ventilation, and air-conditioning in subway Thesystems secondary (SO 42–,ventilation NO3–, andsystem NH4+) in the PMtunnel are photochemically (HVAC) andpollutants outflow via the into the city. converted from 2 − − + gasesThe andsecondary comprisepollutants 2.4% of PM 10 and 3.7% of PM 2.5 . In particular, SO 42– is generated by secondary (SO4 , NO3 , and NH4 ) in PM are photochemically converted from 2 − – chemical theofatmosphere [25,26], whereas NO 3 is generated by combustion of fossil gases andreactions comprisein 2.4% PM10 and 3.7% of PM . In particular, SO is generated by secondary 2.5 4 fuels [27].reactions When thein subway service is inactive, a diesel motor is operatedby to combustion maintain theoftunnel, chemical the atmosphere [25,26], whereas NO3 − car is generated fossil – 2– and 3 and SOthe 4 are transported the outdoor airmotor through ventilation system [2,16,28–32]. fuelsNO [27]. When subway servicefrom is inactive, a diesel carthe is operated to maintain the tunnel, − 2− are chemical Information properties and sources of PM the in ventilation subway tunnels needed to and NO transported from the outdoor air through systemis[2,16,28–32]. 3 and SOon 4 the facilitate an effective management strategy and thereby improve the air quality. In this study, we Information on the chemical properties and sources of PM in subway tunnels is needed to facilitate characterized the PM in subway beneath Seoul,the Korea. an effective management strategytunnels and thereby improve air quality. In this study, we characterized the PM in subway tunnels beneath Seoul, Korea. 2. Materials and Methods 2. Materials and Methods 2.1. Study Area 2.1. Study Area In 2016, the Seoul metropolitan subway transported 7.4 million passengers daily. The Seoul InCompany 2016, the operates Seoul metropolitan subway 7.4 millionoperate passengers The Seoul Metro subway lines 1 to 8, transported and other companies line 9,daily. the airport line, Metro Company operates subway lines 1 to 8, and other companies operate line 9, the airport line, and the Bundang line. Line 4 is connected to the Danggogeo station in Seoul and the Oido station in and the Geyonggi-do, Bundang line. consisting Line 4 is connected to the Danggogeo station in Seoul and theunderground Oido station Ansan, of an overground and underground section. The in Ansan, Geyonggi-do, consisting of an overground underground The underground section runs from Beomgye to Chongshin Universityand station and from section. Sinyongsan to Ssangmun section runs from Beomgye to Chongshin University station and from Sinyongsan to Ssangmun station. station. The subway tunnels in line 4 consist of natural and mechanical ventilation systems, as The subway tunnels in line 4 consist of natural and mechanical ventilation systems, as shown Figure 1. shown Figure 1. When the mechanical ventilation system is not in operation, it can use natural When the mechanical ventilation system is not in operation, it can use natural ventilation. ventilation.

Figure 1. 1. The The natural natural and and mechanical mechanical ventilation ventilation systems systems in in line Figure line 4. 4.


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The number of trains per one hour The number of trains per one hour

We analyzed the PM in a subway tunnel at the M station (60,000 passengers/day) and S station analyzed the PM in a subway tunnel at the station passengers/day) S station (27,000We passengers/day), which handle 490 trains perM day with(60,000 eight carriages (Figure and 2). Figure 3 Wesampling analyzed the PM in athe subway tunnel at the M station (60,000 passengers/day) and station 3 (27,000 passengers/day), which handle 490 trains per day with eight carriages (Figure 2).S Figure shows positions on platform equipped with PSDs. (27,000 which 490equipped trains perwith day PSDs. with eight carriages (Figure 2). Figure 3 showspassengers/day), sampling positions on thehandle platform shows sampling positions on the platform equipped with PSDs. 50 S station M station S station M station

50 40 40 30 30 20 20 10 10 0 0

5h

6h

5h

6h

7h 7h

8h 8h

9h 9h

10h 11h 12h 13h 14h 15h 16h 17h 18h 19h 20h 21h 22h 23h 24h 10h 11h 12h 13h 14h 15h 16h 17h 18h 19h 20h 21h 22h 23h 24h

Figure 2. Number of trains at the M station and S station on weekdays. Figure 2. Number of trains at the M station and S station on weekdays. Figure 2. Number of trains at the M station and S station on weekdays.

Figure 3. Positions of the sampling sites at the M (A) and S (B) stations. Figure 3. Positions of the sampling sites at the M (A) and S (B) stations. Figure 3. 2.2. Collection and Analysis ofPositions Samples of the sampling sites at the M (A) and S (B) stations.

2.2. Collection and Analysis of Samples 1 lists measurement devices and sampling periods. To analyze the inorganic, ionic, 2.2. Table Collection andthe Analysis of Samples Table 1 lists the measurement devices and sampling periods. To analyze the inorganic, ionic, and carbonaceous components of PM, samples were collected in the subway tunnels at the M station Table 1 lists the measurement devices and sampling periods. To analyze the inorganic, ionic, and of PM, samples were collected the subway at the 47 M mm station fromcarbonaceous 15 May to 9 components June 2017. PM samples were collected oninquartz filters tunnels (QMA filter, in and carbonaceous components ofYork, PM, samples were collected in the subway tunnels at the station from 15 May 9 June 2017.New PM samples collected quartz filters filter, 47Mmm in diameter; PalltoCorporation, NY,were USA) using a on mini-volume air (QMA sampler (MiniVol TAS, from 15 Pall MayCorporation, to 9 June 2017. PMUSA) samples were using collected on quartz filters (QMA(MiniVol filter, 47TAS, mm diameter; New NY, a mini-volume air sampler 5in 5 L/min; Airmetrics, Eugene, OR,York, andUSA) Zefluor filters (polytetrafluoroethylene membrane filter, diameter; Pall Corporation, New York, NY, USA) using a mini-volume air sampler (MiniVol TAS, L/min; Airmetrics, OR, USA)New andYork, Zefluor membrane filter, 5 47 mm in diameter; Eugene, Pall Corporation, NY,filters USA)(polytetrafluoroethylene using a low-volume air sampler (PMS-104, Airmetrics, Eugene, OR, USA) and Zefluor filters (polytetrafluoroethylene membrane filter, 47L/min; mm in diameter; Pall Corporation, New York, NY, USA) using a low-volume air collected sampler 16.7 L/min; APM, Bucheon, Korea). For the X-ray diffraction (XRD) analysis, samples were 47 mm in16.7 diameter; Pall Corporation, New York, NY, USA) using a (XRD) low-volume airsamples sampler (PMS-104, L/min; APM, Bucheon, Korea). For the X-ray diffraction analysis, from the bottom of the subway tunnels at the S station. The quartz and Zefluor filters were changed (PMS-104, 16.7 L/min; APM, Bucheon, Korea). For the X-ray diffraction (XRD)and analysis, samples were from the bottom of the subway tunnels at the S station. The samples quartz Zefluor filters daily collected at 5:00 p.m. Lastly, for the transmission electron microscopy analysis, were collected in werechanged collecteddaily fromatthe bottom of the subway tunnels at theelectron S station.microscopy The quartzanalysis, and Zefluor filters were 5:00 p.m. Lastly, for the transmission samples the subway tunnels on 18 April 2017. werecollected changed daily at 5:00 p.m. Lastly, the transmission electron microscopy analysis, samples were thechemical subway tunnels on 18for April 2017. To analyzeinthe composition and morphology of PM, an aluminum foil filter (CFG-225, were in the subwaycomposition tunnels on 18 April 2017. Tocollected analyze the chemical and morphology PM, an aluminum foil(ELPI, filter (CFG-225, 25 mm in diameter; Dekati, Kangasala, Finland) and electricaloflow-pressure impactor 10 L/min; Toinanalyze the chemical compositionFinland) and morphology of PM, an aluminumimpactor foil filter(ELPI, (CFG-225, 25 mm diameter; Dekati, Kangasala, and electrical low-pressure 10 Dekati, Kangasala, Finland) were used to collect PM samples from 15 May to 17 May 2017. The size 25 mm in diameter; Dekati, Kangasala, Finland) and electrical low-pressure impactor (ELPI, 10 L/min; Dekati, Finland) used to collect ELPI+ PM samples 15 May toThe 17 May distribution of Kangasala, PM was also storedwere in high-resolution mode from (HR-ELPI+). floor 2017. dust L/min; Dekati, Kangasala, Finland) used to collect PM samples from 15 May to 17 The Mayfloor 2017. The sizesubway distribution of PM alsowere stored in high-resolution (HR-ELPI+). in the tunnels waswas collected and passed through a ELPI+ 1-mm mode (18 mesh) filter to identify The in size distribution of PMwas wascollected also stored inpassed high-resolution ELPI+ mode (HR-ELPI+). The floor dust the subway tunnels and through a 1-mm (18 mesh) filter to identify crystalline material. dust in the subway tunnels was collected and passed through a 1-mm (18 mesh) filter to identify crystalline material. Zefluor filters were weighed before and after sampling using an analytical balance (sensitivity: crystalline material. Zefluor filters were weighedat before andtemperature after sampling an analytical balance (sensitivity: 0.001 mg) after being equilibrated constant andusing humidity for three days in an electronic Zefluor filters were weighed before and after sampling using an analytical balance (sensitivity: 0.001 mg) after being equilibrated at constant temperature and humidity for three days in an ◦ desiccator. Quartz filters were heated to 850 C for >2 h to remove organic matter before sampling. 0.001 mg) after being equilibrated at constant temperature and humidity for three daysbefore in an electronic desiccator. Quartz filters were heated to 850 C for >2 h to remove organic matter After PM sampling, the Zefluor filters were cut in half using ceramic scissors. Each half was electronic desiccator. Quartz filters were heated to 850 C for >2 h to remove organic matter before sampling. preprocessed for analysis of the inorganic and ionic components. Preprocessing for the inorganic sampling. After PM sampling, the Zefluor filters werecoupled cut in half using ceramic scissors. Each half was component analysis was done using inductively plasma (ICP) spectroscopy, and complied After PM sampling, thethe Zefluor filters were cutcomponents. in half usingPreprocessing ceramic scissors. Each half was preprocessed for analysis of inorganic and ionic for the inorganic with the preprocessing standard of the Clean Water Act of the United States. The sample was placed preprocessed for analysis of the inorganic and coupled ionic components. Preprocessing forand thecomplied inorganic component analysis was done using inductively plasma (ICP) spectroscopy, in a perfluoroalkoxy polymer resin liner and mixed with 7 mL of 61% nitric acid and 3 mL of 35% component analysis was done using inductively plasma (ICP) spectroscopy, and complied with the preprocessing thethen Clean Watercoupled Act the United The placed hydrochloric acid. Thestandard mixture of was subjected to of a pressure ofStates. 150 psi forsample 10 minwas to extract with the preprocessing standard of the Clean Water Act of the United States. The sample was placed

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inorganic components. The extracted solution was filtered (number 5B filter paper, 110 mm; Advance MFS), diluted with 50 mL of ultrapure water, and stored at 4 ◦ C until required. The Fe, Ni, Ba, Pb, V, Cr, Cu, Zn, Mn, and Al contents were analyzed using ICP atomic emission spectrometry (ICP-AES; ICPE-9000, Shimadzu, Japan). Table 1. Measurement devices and sampling periods.

Item

Low-volume air sampler (PMS-104)

Mini-volume air sampler (MiniVol TAS)

Filter

Zefluor

Particle Diameter

PM10 and PM2.5

Flow Rate (L/min)

Analysis Instruments

Sampling Period and Site

16.7

Ion compound (IC), Metal compound (ICP-AES)

15 May to 9 June 2017 (n = 13) M station, Seoul

5.0

Organic and Elemental Carbon (Carbon analyzer)

Quartz

PM10 and PM2.5

Electrical low-pressure impactor (ELPI)

Aluminum foil

14 stages (D50 1 : 10, 5.3, 3.6, 2.5, 1.6, 0.94, 0.60, 0.38, 0.25, 0.15, 0.094, 0.054, 0.030, and 0.016 µm)

10.0

Low-volume air sampler PMS-104

Zefluor

PM10

16.7

Zipper bag

Floor dust under 1 mm

Sieve (mesh 18: Φ = 1.00 mm)

-

Morphology and Chemical by size distribution (SEM and TEM/EDX)

Chemical form (XRD)

15 May to 17 May 2017 M station, Seoul

18 April 2018 S station, Seoul Three times within 15 May to 9 June 2017 S station, Seoul

1

D-Values (D50 ) are the intercepts for 50% of the cumulative mass. PM—particulate matter; ICP-AES—inductively coupled plasma atomic emission spectrometry; EDX—energy-dispersive X-ray spectroscopy; XRD—X-ray diffraction; SEM—scanning electron microscopy; TEM—transmission electron microscopy.

For analysis of the water-soluble ion content, the filter paper was soaked in 20 mL of ultrapure water, and ions were extracted using an ultrasonic extractor. To prevent clogging of the ion chromatography (IC) column, the extracted solution was filtered through a 0.45-µm SCA cellulose acetate syringe filter (CHMLAB Group, Barcelona, Spain); the filtered solution was stored in a 60-mL narrow-mouth bottle (Nalgene, Waltham, MA, USA) at 4 ◦ C. The water-soluble ion contents of the extracts were analyzed by IC (861 Advanced Compact IC; Metrohm, Herisau, Switherland). The NO3 − , SO4 2− , and Cl− anions were analyzed using MetroSep A Supp 5 and MetroSep RP 2 Guard columns, and the NH4 + , Na+ , K+ , Mg2+ , and Ca2+ cations were analyzed using MetroSep C 4 and RP 2 guard columns. A thermal–optical elemental/organic carbon (EC–OC) analyzer (Sunset Laboratory Inc., Parsippany, NJ, USA) was used to determine the organic carbon (OC) and elemental carbon (EC) contents following a thermal/optical transmittance (TOT) protocol [33]. The sample was heated to 900 ◦ C in five steps in an He atmosphere to volatilize the OC and EC (Table 2). To evolve the EC and pyrolyzed OC, which were removed in the second part of the analysis, the sample was first cooled to 550 ◦ C and heated in 2% O2 /98% He to 910 ◦ C in five steps. The analyzer utilizes laser transmission to correct for OC charring. The EC was determined as the C evolved after filter transmittance returned to the initial value.


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Table 2. NIOSH Method 5040 parameters. Program Activity

Carbon

Oven purge 1st ramp 2nd ramp 3rd ramp 4th ramp Cooling for EC

OC1 OC2 OC3 OC4 and CC CC

Stabilize temp He/O2 1st ramp 2nd ramp 3rd ramp 4th ramp 5th ramp

PC and EC PC and EC

External standard, calibration and cool down

Carrier Gas

Ramp Time (s)

Program Temperature (◦ C)

He

10 60 60 60 90 30

Ambient 315 475 615 870 0

He/O2

45 45 45 45 45 120

550 625 700 775 850 910

Calibration gas and He/O2

120

0

EC

-

OC—organic carbon; CC—carbonate carbon; PC—pyrolytic carbon; EC—elemental carbon.

PM morphology was characterized by scanning electron microscopy (SEM; S-4700; Hitachi, Tokyo, Japan) and transmission electron microscopy (TEM; Talos F200X; FEI, Hillsboro, OR, USA). Energy-dispersive spectroscopy (EDX) was used to analyze the chemical composition of PM. PM samples on grids were sputter-coated with Pt prior to SEM. For TEM and EDX analysis, the filters were placed in tubes, mixed with 20 mL of deionized water, dispersed using an ultrasonic extractor, and transferred to a TEM grid (CF200 Cu-C film, 200 mesh; EMS, Hatfield, PA, USA) using a pipette. For the XRD analysis (SmartLab; Rigaku, Austin, TX, USA), a 1-mm sieve was used to filter dust collected from the floor of subway tunnels. Quality assurance of the ICP-AES, IC, and C analyses was performed using standard solutions. Accuracy was checked by calculating the relative error, and precision was checked by calculating the relative standard deviations (RSDs) and coefficients of variation of triplicate determinations. For the C analysis, the RSDs were calculated by analyzing the same sample in triplicate, as no standards that enable OC and EC to be distinguished are available. 2.3. Estimated Carbonate by Ion Balance Atmospheric PM consists of cations (Na+ , NH4 + , K+ , Mg2+ , and Ca2+ ), anions (CO3 2− , Cl− , NO3 − , and SO4 2− ), and trace amounts of organic acids and metal cations. We used the ion balance to evaluate the acid–base balance. We were unable to evaluate the carbonate contents of PM by IC. Therefore, we estimated the carbonate contents of PM as the difference between the contents of all other anions and cations, as shown in Equation (1) [34]. CO3 2− = (Na+ + NH4 + + K+ + Mg2+ + Ca2+ ) − (Cl− + NO3 − + SO4 2− ).

(1)

The normal concentration of carbonate estimated by ion balance was converted into mass concentration, and the carbonate carbon (CC) concentrations were subtracted from the organic carbon (OC) concentrations to avoid overlap with the carbonate estimated from the ion balance in Equation (1) [29].

The normal concentration of carbonate estimated by ion balance was converted into mass concentration, and the carbonate carbon (CC) concentrations were subtracted from the organic carbon (OC) concentrations to avoid overlap with the carbonate estimated from the ion balance in Equation (1) [29].


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3. Results and Discussion 3. Results and Discussion 3.1. PM Mass 3.1. PM Mass In Seoul, air-quality monitoring stations (AQMS) are operated at 25 points, and their data are In Seoul, Thus, air-quality monitoring stations (AQMS) are operated at 25and points, and their data are open-access. for the comparison of mass concentration outdoors in subway tunnels, we open-access. for2.5the comparison mass to concentration and in subway period. tunnels, collected PM10Thus, and PM from the AQMS of nearest the M stationoutdoors during the measurement we collected PM10 and PM from the PM AQMS nearest to the M during the measurement period. The mean masses of2.5PM 10 and 2.5 concentrations (n station = 13) were 213.7 ± 50.4 and 78.4 ± 8.8 3 3 The mean masses of PM and PMand (n outdoor = 13) were 213.7 ± 50.4 µ g/m in the subway tunnels and1044.0 ± 13.9 22.2 ± 9.5 µ g/m in air at the AQMS (n =and 13), 2.5 concentrations 3 in the subway tunnels and 44.0 ± 13.9 and 22.2 ± 9.5 µg/m3 in outdoor air at 78.4 ± 8.8 µg/m respectively (Figure 4). The mass concentrations of PM10 and PM2.5 in the subway tunnel were 4.8the (n =greater 13), respectively 4). Thein mass concentrations of PM in the subway andAQMS 2.5-fold than their(Figure counterparts outdoor air. The PM levels in PM the2.5subway tunnels 10 and tunnel werethe 4.8-WHO and 2.5-fold greater than their(PM counterparts air. PM25levels subway exceeded air-quality guidelines 10, 24 h, 50inµoutdoor g/m3; PM 2.5,The 24 h, µ g/min3)the [35]. In the 3 ; PM , 24 h, 25 µg/m3 ) [35]. tunnels air-quality guidelines (PM , 24 h, 50 µg/m subwayexceeded tunnels, the the WHO PM10 mass concentration was 2.5-fold greater than that of PM 2.5 . As shown in 10 2.5 In the subway tunnels, the PM mass concentration was 2.5-fold greater than that of PM2.5 . As shown Figure 4, the concentration in10the subway tunnel increased as the concentration outdoors increased. in Figure 4, the concentration the subway tunnel increased as the concentration outdoors increased. For comparisons between theintunnel and outdoor concentrations of PM, an independent t-test was For comparisons tunnel and outdoor concentrations of PM, an independent used according tobetween the datathe distribution with the SPSS 24 software (IBM Corporation, New t-test York,was NY, used according to the data distribution the SPSS software Corporation, New York, NY, USA). The independent t-test resulted with in t-values of 24 15.7 and 11.7(IBM for PM 10 and PM2.5 , respectively, USA). The independent t-test resulted in t-values of 15.7 and 11.7significant for PM10 and PM2.5 , respectively, and p-values lower than 0.001 for both, indicating a highly difference between the and p-values lower 0.001 for both, indicating a highly significant difference between the subway subway tunnel andthan outdoors. tunnel and outdoors.

Figure Figure4.4.Particulate Particulatematter matter(PM) (PM)mass massconcentration concentrationin inthe thesubway subwaytunnel tunnelatatthe theM Mstation. station.

PM10 originates from the wheel–rail–brake pad interface due to mechanical wear caused by the PM10 originates from the wheel–rail–brake pad interface due to mechanical wear caused by the structural characteristics of the subway tunnels [5,29,36]. According to Park et al. [16], 67.7% of PM10 structural characteristics of the subway tunnels [5,29,36]. According to Park et al. [16], 67.7% of PM10 originates from the wheel–rail–brake and catenary interfaces in subway tunnels. In addition, the PM originates from the wheel–rail–brake and catenary interfaces in subway tunnels. In addition, the PM concentration is influenced by train wind, the frequency of train operation, and tunnel cleaning [19]. concentration is influenced by train wind, the frequency of train operation, and tunnel cleaning [19]. The daily PM10 concentrations in the subway tunnel were quite different. As the measurement was only The daily PM10 concentrations in the subway tunnel were quite different. As the measurement was conducted on weekdays, it could be considered that the PM10 in the subway tunnel was diluted due to only conducted on weekdays, it could be considered that the PM10 in the subway tunnel was diluted the operation of mechanical ventilation [37]. The PM10 concentrations in subway tunnels were reported to be 232–338 µg/m3 in Seoul and 51–470 µg/m3 in other countries [16,21,38–40]. For comparison, the mean PM10 mass concentration at the M station in 2014 was 184 µg/m3 [29], which is slightly lower than the finding in this study. The mechanical ventilation near the M station was improved in January 2017, and it could be considered to be the difference in cleanliness in the subway tunnels. Figures 5 and 6 show the number, volume, and size distribution of PM in the subway tunnel at the M station between 7:00 and 8:00 p.m. on 15 May 2017. In the M station, a total of 36 trains stopped during that time. The number and volume of PM varied in a constant pattern. It is thought that the particles were generated from the subway and resuspended by the piston effect in the subway tunnels [19]. The volume in Figure 5A shows a similar pattern, as does the volume and number of PM in Figure 5B. However, the pattern of Figure 5A compared to Figure 5B is different. This is due to the

Figures 5 and 6 show the number, volume, and size distribution of PM in the subway tunnel at the M station between 7:00 and 8:00 p.m. on 15 May 2017. In the M station, a total of 36 trains stopped during that time. The number and volume of PM varied in a constant pattern. It is thought that the particles were generated from the subway and resuspended by the piston effect in the subway tunnels [19]. The volume in Figure 5A shows a similar pattern, as does the volume and Int. J. Environ. Res. Public Health 2018, 15, 2534 7 of 17 number of PM in Figure 5B. However, the pattern of Figure 5A compared to Figure 5B is different. This is due to the size distribution. The size distribution of the volume shows a bimodal shape with peaks at around 2–3 m and 7–8 µ m, while the number distribution shows a unimodal shape with a size distribution. Theµ size distribution of the volume shows a bimodal shape with peaks at around peakµm at and around µ m. 2–3 7–80.02–0.03 µm, while the number distribution shows a unimodal shape with a peak at around

8000

Volume of PM10

Volume of PM2.5

A 6000

4000

2000

0 1.8e+5 1.6e+5 1.4e+5

50 Number of PM10 Number of PM2.5

Number ofTime(s) PM0.1 Volume of PM0.1

B

40

1.2e+5 30

1.0e+5 8.0e+4

20

6.0e+4 4.0e+4

10

2.0e+4 0.0 19:00

19:10

19:20

19:30

19:40

0 20:00

19:50

Volume concentration(um3/cm3)

Number concentration(#/cm3)

Volume concentration(um3/cm3)

0.02–0.03 µm.

Time(s)

Figure 5. The number and volume of PM in the subway tunnel at the M station between 7:00 and 8:00 8 of 18 p.m. p.m. on 15 May 2017.

Int. J. Environ. Res. Public Health 2018, 15, x FOR PEER REVIEW 2.4e+5

14000 Number Volume

2.0e+5

12000 10000

6000 8.0e+4 4000

dVdlogdP/cm

dNdlogdP/cm

3

8000 1.2e+5

3

1.6e+5

4.0e+4 2000 0.0

0

-4.0e+4

-2000 0.01

0.1

1

10

Particle diameter(um) Figure 6. 6. Size Size distribution distribution of of PM PM in in the the subway subway tunnel tunnel at at the the M M station station between 7:00 and 8:00 p.m. on on Figure 15 May 2017.

3.2. Chemical Composition of PM 3.2. Chemical Composition of PM 3.2.1. Carbonaceous Compounds 3.1.1. Carbonaceous Compounds Figure 7 shows the mass concentrations of carbonaceous compounds in PM. The total carbon Figure 7 shows mass concentrations of carbonaceous compounds in 3PM. The total carbon (TC) contents of PM10the and PM2.5 were 35.8 ± 7.0 (16.7%) and 18.2 ± 2.6 µg/m (23.2%), respectively. (TC) contents of PM10 and PM2.5 were 35.8 ± 7.0 (16.7%) and 18.2 ± 2.6 µ g/m3 (23.2%), respectively. Meanwhile, TC minus CC represented 16.1% of PM10 and 22.3% of PM2.5 (Table 3). These concentrations were higher than those of PM in outdoor air. The OC and EC concentrations of PM 10 were 23.4 ± 3.7 and 11.1 ± 3.8 µ g/m3, and those of PM2.5 were 13.8 ± 2.1 and 3.8 ± 0.8 µ g/m3, respectively. CC comprised 1.3 ± 0.6 µ g/m3 of PM10 and 0.7 ± 0.3 µ g/m3 of PM2.5. However, it should


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Meanwhile, TC minus CC represented 16.1% of PM10 and 22.3% of PM2.5 (Table 3). These concentrations were higher than those of PM in outdoor air. The OC and EC concentrations of PM10 were 23.4 ± 3.7 and 11.1 ± 3.8 µg/m3 , and those of PM2.5 were 13.8 ± 2.1 and 3.8 ± 0.8 µg/m3 , respectively. CC comprised 1.3 ± 0.6 µg/m3 of PM10 and 0.7 ± 0.3 µg/m3 of PM2.5 . However, it should be noted that the TOT method overestimates the OC content due to the mineral oxides (e.g., Fe2 O3 and SiO2 ) present in the PM in subway tunnels [41]. Carbonate estimated using the ion balance comprised 3.1% of PM10 and 2.9% of PM2.5 [29,32]. In addition, CaCO3 and SiO2 were detected by XRD (Figure 8). The carbonaceous components of PM in subway tunnels originate from diesel PM emitted by the diesel motor car operated to maintain the tunnel [16,20], as well as organic material transported from the outdoor air Res. through the ventilation system [16,20], and the pantograph carbon strip–catenary 9wire Int. J. Environ. Public Health 2018, 15, x FOR PEER REVIEW of 18 interface [4,42].

Figure 7. Mass concentrations of the carbonaceous components of PM in the subway tunnel at the M Figure 7. Mass concentrations of the carbonaceous components of PM in the subway tunnel at the M station (n = 13). OC—organic carbon; CC—carbonate carbon; EC—elemental carbon. station (n = 13). OC—organic carbon; CC—carbonate carbon; EC—elemental carbon. Table 3. Physical and chemical characteristics of PM at the M station (n = 13). Table 3. Physical and chemical characteristics of PM at the M station (n = 13). PM10

PM2.5

PM10

This Study This Study

(%)(%) Mass Mass (g/m3(g/m ) 3Ratio ) Ratio 213.7 Total Total 213.7 ± 50.4± 50.4 TC * TC * 34.5 ±34.5 6.8 ± 6.8 Anion ** 11.0 ± 5.1 CationAnion ** 4.0 ±11.0 1.8 ± 5.1 Inorganic *** 1.5± 1.8 Cation 11.1 ±4.0 Fe 86.1 ± 30.5 Inorganic *** 11.1 ± 1.5 Fe2O3 **** 123.1 ± 37.7 Unknown Fe 30.0 ±86.1 22.1± 30.5

100100 16.1 16.1 5.2 1.95.2 5.21.9 40.3 5.2 57.6 1440.3

In In Seoul In[16] Seoul InMexico Mexico City [43] [16]

City [43]

3 Mass Mass(g/m (g/m3))

200.75 200.75 -18.16 + 18.16 +++ 9.82 +++ 8.68 9.82 ++ 72.51+++ 8.68 72.51 -

PM2.5

This Study This Study

In

In Seoul

In

Barcelona Mexico In Seoul [29]In Barcelona In Mexico

Mass Ratio (%) 3 ) 3) Ratio Mass (g/m (%) (g/m

[29]

[44]

[44]

City [43]

City [43]

3) 3) MassMass (g/m(g/m

89.55 89.55

78.4 78.4±±8.8 8.8

100 100

55.1 55.1 20.7–93.2 20.7–93.2 48.34 48.34

---5.57 5.57 -

17.5±±2.6 2.6 17.5 6.7 ± 3.4 6.7 2.5±±3.4 1.3 2.9±±1.3 0.6 2.5 26.3 ± 6.5 2.9 ± 0.6 37.7 ± 9.3 26.3 11.1±±6.5 9.7

22.3 22.3 8.6 8.63.2 3.23.7 33.6 3.7 48 33.6 14.1

- 3.2–17.1 3.2–17.1 6.4 ** 6.4 ** 3 3 6.9–52.4 3.1 -

3.1 -

Fe2O3 ****carbonate 123.1 ±C; 37.7 57.6 - excluding 37.7 ± 9.3 - 4 2− ; * Excluding ** including carbonate C; *** Fe; **** 48 Fe as Fe2 O- 3 ; + Cl−6.9–52.4 , NO3 − , and SO + , K+ , Mg2+ , and Ca2+ ; +++ Al, Ba, Cr, Cu, Fe, Mn, Ni, Pb, Si, Ti, and Zn. TC—total carbon. NaUnknown 30.0 ± 22.1 14 11.1 ± 9.7 14.1 -

++

* Excluding carbonate C; ** including carbonate C; *** excluding Fe; **** Fe as Fe2O3; + Cl−, NO3−, and SO42−; ++ Na+, K+, Mg2+, and Ca2+; +++ Al, Ba, Cr, Cu, Fe, Mn, Ni, Pb, Si, Ti, and Zn. TC—total carbon.

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Figure 8. X-ray diffraction (XRD) patterns of PM in the subway tunnel at the S station.

3.2.2. Ionic Compounds The mean mass concentrations of ionic compounds in PM10 and PM2.5 were 11.0 ± 5.1 and 6.7 ± Figure 8. X-ray diffraction (XRD) patterns of PM in the subwayof tunnel at the S station. CO32–) and 3.4 µ g/m3, respectively (Figure 9). The mean normal concentrations anions (excluding Figure 8. X-ray diffraction (XRD) patterns of PM in the subway tunnel at the S station. 3 cations in PM10 were 0.09 ± 0.05 and 0.20 ± 0.09 eq/m , respectively, whereas those in PM2.5 were 0.05 3.2.2. Ionic Compounds ± 3.2.2. 0.04 and ± 0.06 eq/m3, respectively. These results indicated that PM in the subway tunnels Ionic0.12 Compounds The mean concentrations of ionic compounds in PM and PM2.5 were 11.0 ± was 5.1 and contained highermass cation than anion (excluding CO32–) contents. In10this study, CO 32– analysis not The mean mass concentrations of ionic compounds in PM 10 and PM2.5 were 11.0 ± 5.1 and 6.7 3 6.7 ± 3.4 µg/m respectively 9). The mean normal of anions (excluding CO3 2− ) ± conducted, and ,anions were (Figure lost from the filters becauseconcentrations of the high temperature and humidity 3, respectively (Figure 9). The mean normal concentrations of anions (excluding CO32–) and 3.4cations µ g/m 3 ,3respectively, 2– content based and in PM[45]. 0.05 and ± 0.09 the eq/m whereas those in PM were during sampling For0.09 this± reason, we0.20 estimated CO on the cation and2.5anion 10 were cations in PM 10 were 0.09 ± 0.053 and 0.20 ± 0.09 eq/m3, respectively, whereas those in PM2.5 were 0.05 0.05 ± 0.04 and equivalence [34].0.12 ± 0.06 eq/m , respectively. These results indicated that PM in the subway tunnels 3, respectively. These results indicated that PM in the subway tunnels ± 0.04 0.12 ±cation 0.06 eq/m 2− ) contents. 2− analysis The 3, respectively. Theand CO 32– concentrations of PM10 and PM2.5CO were ± 3.2 andIn 4.5this ± 1.5 µ g/mCO contained higher than anion (excluding study, was 3 6.6 3 2–) contents. In this study, CO32– analysis was not contained higher cation than anion (excluding CO 3 2– 2– + – CO , SO4 , Na and , and NO3 were concentrations in the Seoul subway system the summer of 2010 not3 conducted, anions lost from the filters because of the highduring temperature and humidity conducted, and anions lost from the filtersthe because theconcrete, high temperature and humidity 2−of were reported previously [29]. Subway tunnels are made of the of which during sampling [45]. For were this reason, we estimated CO content based on thedebris cation and anion 3 during sampling Forfloor. this reason, we estimated the CO32– content based on the cation and anion accumulates on the[45]. tunnel equivalence [34]. equivalence [34]. The CO32– concentrations of PM10 and PM2.5 were 6.6 ± 3.2 and 4.5 ± 1.5 µ g/m3, respectively. The CO32–, SO42–, Na+, and NO3– concentrations in the Seoul subway system during the summer of 2010 were reported previously [29]. Subway tunnels are made of concrete, the debris of which accumulates on the tunnel floor.

Figure 9. 9. Normal Normal and and mass mass concentrations concentrations of of ionic ionic components components in in PM PM at at the the M M station station (n (n == 13). 13). Figure

The CO3 2− concentrations of PM10 and PM2.5 were 6.6 ± 3.2 and 4.5 ± 1.5 µg/m3 , respectively. The CO3 2− , SO4 2− , Na+ , and NO3 − concentrations in the Seoul subway system during the summer of 2010 were reported previously [29]. Subway tunnels are made of concrete, the debris of which accumulates on9.the tunneland floor. Figure Normal mass concentrations of ionic components in PM at the M station (n = 13).

oxides) comprised 40.3% of PM10 and 33.6% of PM2.5, which was higher than previously reported (36.1% of PM10) [16]. The mass concentrations of Fe in PM10 and PM2.5 were 86.1 ± 30.5 and 26.3 ± 6.5 µ g/m3, respectively, whereas the mass concentrations of other inorganic compounds (Ni, Mn, Ba, Pd, V, Cr, Int. J. Environ. Res. Public Health 2018, 15, 2534 10 of 17 Cu, Zn, and Al ranged from 0.63 to 1.65 µ g/m3 in PM10 and 0.027 to 0.684 µ g/m3 in PM2.5 (Figure 10). Indeed, nano-sized PM were reported to be emitted from the wheel–rail–brake interface [2,5,46]. additionCompounds to iron oxides in PM in the subway tunnel, we detected two iron oxides as hematite 3.2.3.InInorganic (α-Fe2O3) and maghemite-C (γ-Fe2O3) using XRD (Figure 8). Fe2O3 represented 57.6% of the PM10 and compounds represented 47.0% of PM10 and 38.2%isofpresent PM2.5 . as Feiron (principally ironmineral oxides) 48.0%Inorganic of the PM 2.5 in the subway tunnel. Fe in subway tunnels oxides and comprised 40.3% of PM and 33.6% of PM , which was higher than previously reported (36.1% 3 elements; e.g., FeOx/SiO210[23]. The estimated2.5mass of Fe2O3 was 123.1 ± 37.7 µ g/m in PM10 and 37.7of ± PMµ ) [16]. 3 in PM2.5 (Table 3). In the case of European subway systems, Fe2O3 is the most abundant 10g/m 9.3 The on mass concentrations Fe in PM were 86.1 andPM 26.3 ± 6.5 µg/m3 , 10 and PM element platforms withoutof PSDs, accounting for2.5 30–66% of ±the30.5 total 2.5, followed by respectively, whereas the mass concentrations other inorganic compoundsthe (Ni, Mn, Ba,systems Pd, V, Cr, carbonaceous components (18–37%) [32,47]. Fe of reportedly also predominates subway of 3 3 Cu, Zn, and Al ranged from 0.63 to 1.65 µg/m in PM and 0.027 to 0.684 µg/m in PM (Figure 10). 10 2.5 other cities [43,48–50]. Indeed, nano-sized PM were reported to be emitted from the wheel–rail–brake interface [2,5,46].

Figure 10. Mass concentrations of inorganic components in PM at the M station. Figure 10. Mass concentrations of inorganic components in PM at the M station.

In addition to iron oxides in PM in the subway tunnel, we detected two iron oxides as hematite Meanwhile, Ni, Mn, Ba, Pd, V, Cr, Cu, Zn, and Al together comprised 5.2% of PM10 and 3.2% of (α-Fe2 O3 ) and maghemite-C (γ-Fe2 O3 ) using XRD (Figure 8). Fe2 O3 represented 57.6% of the PM10 PM2.5. Fe, Ba, and Mn are used in the wheel–rail–brake system, and Ni, V, and Cr are included in and 48.0% of the PM2.5 in the subway tunnel. Fe in subway tunnels is present as iron oxides and lubricant and are generated by oil combustion [28]. The Fe/Mn ratio in PM10 was 99.7, which was mineral elements; e.g., FeOx /SiO2 [23]. The estimated mass of Fe2 O3 was 123.1 ± 37.7 µg/m3 in PM10 similar to previous reports regarding rail, wheel, and electric sliding collectors (Fe:Mn ratio, 92–121) 3 and 37.7 ± 9.3 µg/m in PM2.5 (Table 3). In the case of European subway systems, Fe2 O3 is the most [16,48,51–53]. Cu, the major component of catenary wire, comprised 0.98 ± 1.09 µ g/m3 of PM10 and abundant element on platforms without PSDs, accounting for 30–66% of the total PM2.5 , followed by 0.53 ± 0.97 µ g/m3 of PM2.5. The carbon strip–catenary wire interface undergoes wear, and a short carbonaceous components (18–37%) [32,47]. Fe reportedly also predominates the subway systems of circuit between the wire and the strip generates an arc discharge [54,55]; according to a previous other cities [43,48–50]. study [16], electrical cable wear is estimated to contribute about 8.1% of PM10. Meanwhile, Ni, Mn, Ba, Pd, V, Cr, Cu, Zn, and Al together comprised 5.2% of PM10 and 3.2% of PM2.5 . Fe, Ba, and Mn are used in the wheel–rail–brake system, and Ni, V, and Cr are included 3.3. Morphology and Energy-Dispersive Spectroscopy (EDX) in lubricant and are generated by oil combustion [28]. The Fe/Mn ratio in PM10 was 99.7, which analyzed the morphology of PM inrail, the subway tunnel (Figure 11). PM of 1-µ m(Fe:Mn diameter is was We similar to previous reports regarding wheel, and electric sliding collectors ratio, 3 of considered to be abrasive PMmajor generated by mechanical wear the wheel–rail–brake interface 92–121) [16,48,51–53]. Cu, the component of catenary wire,atcomprised 0.98 ± 1.09 µg/m [24,56]. The majority dust3 particles diameters of below 1 µwire m PM displayed irregular spheres. PM10 and 0.53 ± 0.97of µg/m of PM2.5 .with The carbon strip–catenary interface undergoes wear, and a SEM/EDX mapthe ofstrip PM2.5 is shown 12. [54,55]; Fe accounted for to44.14 wt.%, shortAn circuit betweenelemental the wire and generates an in arcFigure discharge according a previous followed C (28.07cable wt.%) andisOestimated (20.43 wt.%). The Fe component was evenly in the study [16],byelectrical wear to contribute about 8.1% of PMdistributed 10 . elemental map, suggesting that nano-sized Fe PM generated by friction was condensed with or 3.3. Morphology Spectroscopy (EDX) attached to otherand PMEnergy-Dispersive [18]. Through EDXtheanalysis, Fe was detected in PM ≥tunnel 94 nm (Table Studies showed that We analyzed morphology of PM in the subway (Figure 11).4).PM of 1-µm diameter is 100–200-nm aregenerated generatedby from the wheel–rail–brake interface [2,18].interface The ratio[24,56]. of Fe considered tonanoparticles be abrasive PM mechanical wear at the wheel–rail–brake The majority of dust particles with diameters of below 1 µm PM displayed irregular spheres. An SEM/EDX elemental map of PM2.5 is shown in Figure 12. Fe accounted for 44.14 wt.%, followed by C (28.07 wt.%) and O (20.43 wt.%). The Fe component was distributed evenly in the elemental map, suggesting that nano-sized Fe PM generated by friction was condensed with or attached to other PM [18]. Through EDX analysis, Fe was detected in PM ≥ 94 nm (Table 4). Studies showed that 100–200-nm nanoparticles are generated from the wheel–rail–brake interface [2,18]. The ratio of Fe increased with

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increased with increasing particle size, reaching 47.2% for 1.6-µ m-diameter PM. Ca and Si were detected inparticle 380-nmsize, PM.reaching Meanwhile, S was detected in PM ≥PM. 30 nm, withSithe highest proportion in increasing 47.2% for 1.6-µm-diameter Ca and were detected in 380-nm 250-nm PM. S entered subway tunnel from Cu was detectedinin250-nm PM between 94 and PM. Meanwhile, S was the detected in PM ≥ 30 nm,outside with the[29]. highest proportion PM. S entered 150 To supply a catenary attachedin toPM the between pantograph; however, short the nm. subway tunnelelectricity, from outside [29]. Cuwire wasisdetected 94 and 150 nm. To circuits supply generate discharge, in to thethe emission of Cuhowever, nanoparticles. was generate also present as electricity,ana arc catenary wire resulting is attached pantograph; short Cu circuits an arc copper oxides, e.g., CuO. nanoparticles have toxicity than microparticles discharge, resulting in theNotably, emissionCuO of Cu nanoparticles. Cugreater was also present asCuO copper oxides, e.g., [57]. PMCuO (Ca and Si) ≥ 380 nm could havetoxicity been generated due to the deterioration of subway CuO.Crustal Notably, nanoparticles have greater than CuO microparticles [57]. Crustal PM facilities and≥the of soil dust from outdoors. Bato (which is present inofbrake components) PMthe of (Ca and Si) 380inflow nm could have been generated due the deterioration subway facilities and 0.940–2.500 µ m was also detected. inflow of soil dust from outdoors. Ba (which is present in brake components) PM of 0.940–2.500 µm was also detected. Table 4. Chemical composition of PM according to size at the M station (unit, %). Table 4. Chemical composition of PM according to size at the M station (unit, %).

Particle Size Particle Size (µm) (µm) 0.016 0.016 0.030 0.030 0.054 0.054 0.094 0.094 0.150 0.150 0.250 0.250 0.380 0.380 0.600 0.600 0.940 0.940 1.600 1.600 2.500 3.600 2.500

3.600

CK CK

OK OK

SK SK

Fe K Fe K

Cu L Ca K Si K Ba L Br L Cu L

Ca K

Si K

Ba L

Br L

KK KK

Mo L Mo L

Mg K Mg K

91.58 7.21 1.21 91.58 7.21 1.21 84.81 12.25 2.94 84.81 12.25 2.94 95.65 4.35 95.65 4.35 80.95 11.49 3.39 1.73 1.73 1.89 1.89 0.56 80.95 11.49 3.39 0.56 83.17 8.62 8.62 1.48 1.48 1.45 1.45 1.87 1.87 3.42 83.17 3.42 70.67 70.67 8.26 8.26 8.86 8.86 12.21 12.21 64.03 17.95 1.55 14.80 0.7 0.96 0.96 64.03 17.95 1.55 14.80 0.7 60.89 14.31 0.87 20.54 1.32 1.27 0.80 60.89 14.31 0.87 20.54 1.32 1.27 0.80 40.37 18.65 0.78 34.19 2.06 1.83 2.11 40.37 22.97 18.65 0.79 0.78 47.20 34.19 2.06 21.69 2.38 1.83 3.09 2.11 1.87 21.69 20.43 22.97 0.60 0.79 44.14 47.20 2.38 28.07 1.95 3.09 2.73 1.87 1.64 0.44 38.62 1.63 2.73 2.74 1.64 28.07 18.28 20.43 0.76 0.60 37.96 44.14 1.95 0.44 elemental. 38.62 18.28 K , 0.76 37.96quantum number 1.63 of 2.74 L is principal , is principal quantum number of elemental.

K L

Figure Figure 11. 11. Morphology Morphology of of PM PM in in subway subway tunnels tunnels according according to size at the M station.

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Figure 12. 12. Elemental map of PM PM2.5 2.5 at Figure at the the M M station. station.

According to prior studies [2,18], nanoparticles are generated at sites of wheel–rail contact that surpass a given temperature threshold as the the train train brakes. brakes. It is assumed that nanoparticles were emitted at the wheel–rail contact rather than the brake pad–disc interface due to the difference in Ba and Fe particle size. According totothe patterns (Figure 8), the subway tunnels comprised calcium carbonate According theXRD XRD patterns (Figure 8),PM thein PM in subway tunnels comprised calcium (CaCO ; JCPDS card number, 00-047-1743), quartz (SiO ; 01-075-8322), hematite (α-Fe O ; 00-033-0664), carbonate (CaCO3; JCPDS card number, 00-047-1743),2 quartz (SiO2; 01-075-8322), hematite (-Fe2O3; 3 2 3 and maghemite-C (γ-Fe2 O3 ; 00-039-1346). Calcium carbonate is a major component cement, and 00-033-0664), and maghemite-C (γ-Fe2O3; 00-039-1346). Calcium carbonate is a majorofcomponent of quartz the quartz second is most mineral in continental crust. Upon exposure to air, Fe is oxidized cement,isand the abundant second most abundant mineral in continental crust. Upon exposure to air, intoisiron oxide into or iron hydroxide; the latter is reportedly present on the surface of rails in surface the Tokyo Fe oxidized iron oxide or iron hydroxide; the latter is reportedly present on the of metroin[58]. Previous studies reported studies the presence of calcium carbonate (CaCO (SiO23),), rails the Tokyo metro [58]. Previous reported the presence of calcium carbonate (CaCO 3 ), quartz hematite (α-Fe O3 ), maghemite (Fe and(α-FeOOH), akaganeite quartz (SiO 2), 2 hematite (-Fe2O(γ,ε-Fe 3), maghemite (γ,ε-Fe2O 3),3 O magnetite (Fe3O(α-FeOOH), 4), Fe, goethite 2 O3 ), magnetite 4 ), Fe, goethite (β-FeOOH) in PM [24,49,58–60]. However, in this study, we did and akaganeite (-FeOOH) in PM [24,49,58–60]. However, in not this detect study,Fe, wegoethite, did notakaganeite, detect Fe, or magnetite using XRD spectroscopy. goethite, akaganeite, or magnetite using XRD spectroscopy. TEM images of PM in the subway tunnels tunnels showed showed flakes flakes of of iron iron oxide oxide nanocrystals nanocrystals (Figure (Figure 13). 13). According to previous reports and the XRD patterns, iron oxide PM contains various chemical e.g.,hematite hematite(α-Fe (α-Fe ), magnetite and goethite (α-FeOOH) PM species, e.g., 2O23O ), 3magnetite (Fe3(Fe O4),3 O and (α-FeOOH) [23,58].[23,58]. The PMThe showed 4 ), goethite showed mottling of clusters of rounded ferruginous nanocrystals a few nanometers in width, mottling of clusters of rounded ferruginous nanocrystals a few nanometers in width, along with along with highly crystalline and C nanocrystals. According to Moreno highly crystalline magnetite,magnetite, hematite, hematite, and C nanocrystals. According to Moreno [23],[23], Fe Fe nanoparticles are generated mechanically by frictional wear, especially through the sliding of two nanoparticles are generated mechanically by frictional wear, especially through the sliding of metallic surfaces, followed by oxidation, while others suggested their generation via condensation of Magnetite crystals have octahedral and rhombic dodecahedral Fe vapor vapor[18,24,30,61,62]. [18,24,30,61,62]. Magnetite crystals have octahedral and rhombic morphologies, dodecahedral whereas maghemite nanoparticles show an elongated cuboctahedral morphology [59]. Table 5 shows morphologies, whereas maghemite nanoparticles show an elongated cuboctahedral morphology the chemical of PM incompositions Figure 13. The (Table 5). The elemental [59]. Table 5 compositions shows the chemical of PM PMcontained in Figure Fe 13.and TheOPM contained Fe and O ratios of5).particles A and B were while particle comprised 16.33% (Table The elemental ratiossimilar, of particles A and C B presented were similar, while 71.32% particleFeCand presented O. C was excluded due to the use of a TEM grid composed of Cu and C. However, concentric C comprised 71.32% Fe and 16.33% O. C was excluded due to the use of a TEM grid composed of Cu nanocrystals wereconcentric detected in A (Figure and C. However, C particle nanocrystals were13). detected in particle A (Figure 13).


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Figure 13. Morphology Morphology of PM in subway tunnels according to size at the S station. Table 5. Chemical compositions * of three samples of PM in Figure 13 (unit, %). Table 5. Chemical compositions * of three samples of PM in Figure 13 (unit, %). A B C Element Element A B C Iron (Fe) 43.98 43.94 71.32 Iron (Fe) 43.98 43.94 71.32 Oxygen (O) 40.40 40.58 16.33 Oxygen (O)Silicon (Si)40.40 6.59 40.58 3.71 16.33 6.59 Silicon (Si) 6.59 0.34 6.59 3.71 Magnesium (Mg) 0.34 1.42 (Al) 1.17 1.76 MagnesiumAluminum (Mg) 0.34 1.18 0.34 1.42 Calcium (Ca) 1.77 1.76 Aluminum (Al) 1.18 2.80 1.17 1.76 Sodium (Na) 2.79 Calcium (Ca) 1.77 1.76 Phosphorus (P) 0.68 0.68 0.47 Sodium (Na)Sulfur (S) 2.80 0.47 2.79 Chlorine (Cl) 0.54 0.54 Phosphorus (P) 0.68 0.68 Potassium (K) 1.14 1.14 Sulfur (S)Titanium (Ti)0.47 0.12 0.47 Zirconium (Zr) Chlorine (Cl) 0.54 0.54 Potassium (K) Sum 1.14 100.00 100.00 1.14 100.00 * Cu and C were excluded Titanium (Ti) 0.12due to the use of a Cu–C TEM grid. Zirconium (Zr) 4. Conclusions Sum 100.00 100.00 100.00 In summary, Fe was abundant theofsubway tunnels, * Cuthe andmost C were excluded element due to theinuse a Cu–C TEM grid. accounting for higher proportions of PM; Fe was detected in PM with diameters >94 nm, and was observed as mottling of 4. Conclusions clusters in rounded ferruginous nanocrystals with a few nanometers. Fe was present mostly as iron oxides, which were emitted from the wheel–rail–brake interface. Copper particles were 96–150 nm in In summary, Fe was the most abundant element in the subway tunnels, accounting for higher diameter and were likely emitted via catenary wire arc discharges. The carbonaceous components of proportions of PM; Fe was detected in PM with diameters >94 nm, and was observed as mottling of PM in subway tunnels originate from diesel PM emitted by the diesel motor car operated to maintain clusters in rounded ferruginous nanocrystals with a few nanometers. Fe was present mostly as iron the tunnel. Organic material was transported from the outdoor air through the ventilation system and oxides, which were emitted from the wheel–rail–brake interface. Copper particles were 96–150 nm also originated from the pantograph carbon strip–catenary wire interface. Ca and Si in the subway in diameter and were likely emitted via catenary wire arc discharges. The carbonaceous tunnel, present as calcium carbonate (CaCO3 ) and quartz (SiO2 ), were detected in PM with diameters components of PM in subway tunnels originate from diesel PM emitted by the diesel motor car >380 nm. Calcium carbonate (CaCO3 ) is a major component of cement, and quartz is the second most operated to maintain the tunnel. Organic material was transported from the outdoor air through the abundant mineral in continental crust. ventilation system and also originated from the pantograph carbon strip–catenary wire interface. Ca and Si in the subway tunnel, present as calcium carbonate (CaCO3) and quartz (SiO2), were


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In the near future, our research team will conduct measurements at three locations (outdoors, platform, and tunnel) to identify PM exposure to passengers for each location. These findings suggest that the majority of nano- and micro-PM in the subway tunnel in Seoul, South Korea is generated due to the operation of the subway. Our results can be used to improve the indoor air quality (IAQ) in subway tunnels and to prevent the generation of PM due to subway operation. Author Contributions: D.P. and T.K. planned the study and contributed the main ideas; Y.L. and J.S.C. obtained and analyzed the data. Y.L. and Y.-C.L. were principally responsible for the writing of the manuscript; Y.-C.L. and D.P. commented on and revised the manuscript. Funding: This work was supported by a research grant for the Railway Technology Research Project from the Ministry of Land, Infrastructure, and Transport, Republic of Korea (18RTRP-B082486-05). This research received no external funding. Conflicts of Interest: The authors declare that they have no competing interests.

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10. 11. 12.

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