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T. Moreno, V. Martins, X. Querol, T. Jones, K. BéruBé, MC. Minguillón, F. Amato, M. Capdevila, E. de Miguel, S. Centelles, W. Gibbons. 2015. A new look at inhalable metalliferous airborne particles on rail subway platforms. Science of The Total Environment 505: 367-375, ISSN 0048-9697, http://dx.doi.org/10.1016/j.scitotenv.2014.10.013. Open access.

Inception Report 30th of June 2015 ANNEX III – Deliverable 13 4 Science of the Total Environment 505 (2015) 367–375

Contents lists available at ScienceDirect

Science of the Total Environment

journal homepage: www.elsevier.com/locate/scitotenv

A new look at inhalable metalliferous airborne particles on rail subway platforms

Teresa Moreno a,⁎, Vânia Martins a,b, Xavier Querol a,TimJonesc, Kelly BéruBé d, Maria Cruz Minguillón a, Fulvio Amato a, Marta Capdevila e, Eladio de Miguel e, Sonia Centelles e, Wes Gibbons f a Institute of Environmental Assessment and Water Research (IDÆA-CSIC), C/Jordi Girona 18-24, 08034 Barcelona, Spain b Dept. of Analytical Chemistry, Faculty of Chemistry, University of Barcelona, Av. Diagonal 647, 08028 Barcelona, Spain c School of Earth and Ocean Sciences, Cardiff University, CF10 3YE Cardiff, Wales, UK d School of Biosciences, Cardiff University, Museum Avenue, Cardiff CF10 3AX, Wales, UK e Transports Metropolitans de Barcelona (TMB), Santa Eulalia 08902, Av. del Metro s/n L'Hospitalet de Llobregat, Spain f WPS, C/Major 13, 08870 Sitges, Spain

HIGHLIGHTS

• Most particles breathed on subway platforms are ferruginous and nanometric in size. • Particles derive dominantly from mechanical processes at brake–wheel–rail interface. • Fe-particles have a flake shape with inhomogeneous chemistry (Ba/Zn/Cu). • Fe-PM undergoes progressive atmospheric oxidation from metal Fe to magnetite/hematite. • It is still unclear whether subway air is more or less toxic than outdoor air.

article info abstract

Article history: Most particles breathed on rail subway platforms are highly ferruginous (FePM) and extremely small Received 5 August 2014 (nanometric to a few microns in size). High magnification observations of particle texture and chemistry on air- Received in revised form 2 October 2014 borne PM10 samples collected from the Barcelona Metro, combined with published experimental work on parti- Accepted 4 October 2014 cle generation by frictional sliding, allow us to propose a general model to explain the origin of most subway Available online xxxx FePM. Particle generation occurs by mechanical wear at the brake–wheel and wheel–rail interfaces, where mag- fl Editor: P. Kassomenos netic metallic akes and splinters are released and undergo progressive atmospheric oxidation from metallic iron to magnetite and maghemite. Flakes of magnetite typically comprise mottled mosaics of octahedral nanocrystals Keywords: (10–20 nm) that become pseudomorphed by maghemite. Continued oxidation results in extensive alteration of Subway PM the magnetic nanostructure to more rounded aggregates of non-magnetic hematite nanocrystals, with magnetic Platform air quality precursors (including iron metal) still preserved in some particle cores. Particles derived from steel wheel and SEM rails contain a characteristic trace element chemistry, typically with Mn/Fe = 0.01. Flakes released from brakes TEM are chemically very distinctive, depending on the pad composition, being always carbonaceous, commonly Iron oxides barium-rich, and texturally inhomogeneous, with trace elements present in nanominerals incorporated within Nanoparticles the crystalline structure. In the studied subway lines of Barcelona at least there appears to be only a minimal aero- sol contribution from high temperature processes such as sparking. To date there is no strong evidence that these chemically and texturally complex inhalable metallic materials are any more or less toxic than street-level urban particles, and as with outdoor air, the priority in subway air quality should be to reduce high mass concentrations of aerosol present in some stations. © 2014 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/).

1. Introduction

With nearly 200 rail subway systems in operation worldwide, transporting more than 100 million people daily, air quality both on sta- ⁎ Corresponding author. Tel.: +34 934006123. tion platforms and inside trains is an important urban air pollution issue E-mail address: [email protected] (T. Moreno). (Nieuwenhuijsen et al., 2007). Average return journey times last around

http://dx.doi.org/10.1016/j.scitotenv.2014.10.013 0048-9697/© 2014 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/). 368 T. Moreno et al. / Science of the Total Environment 505 (2015) 367–375 one hour so that many underground rail commuters are routinely ex- context of subway particle physicochemical characterisation there is in posed to transient doses of inhalable particulate matter (PM) levels particular a striking lack of high quality imaging of subway PM at high 3 higher than the 50 μg/m mean PM10 (PM b10 μm in size) limit legally magnification. Several papers have published secondary electron images, imposed for outdoor European city air. In fact, PM levels are commonly but these usually provide only limited information about the range of par- much higher than those above ground, as demonstrated by an array of re- ticle types that exist in the samples and are not the main focus of the cently published studies in subway systems from cities as varied as Los study (e.g. Abbasi et al., 2012, 2013; HJ. Jung et al., 2012; Loxham et al., Angeles, Barcelona, Milan, Paris, Prague, Rome, Stockholm, Seoul, Shang- 2013; Midander et al., 2012; Múgica-Alvarez et al., 2012; Murruni et al., hai and Taipei (Fromme et al., 1998; Johansson and Johansson, 2003; 2009). Seaton et al., 2005; Braniš, 2006; Ripanucci et al., 2006; Salma et al., This paper aims to contribute to and clarify the debate about the char- 2007; Kim et al., 2008; Park and Ha, 2008; Raut et al., 2009; Ye et al., acter of subway air by presenting and interpreting high magnification im- 2010; Cheng and Yan, 2011; Kam et al., 2011; Querol et al., 2012; ages and chemical analyses obtained during an ongoing study conducted Colombi et al., 2013; Moreno et al., 2014). More interesting than the sim- on samples collected from the Barcelona Metro. This subway system, op- ple elemental mass concentration of inhalable PM is the fact that these erated by Transports Metropolitans de Barcelona (TMB), carries a large particles have a peculiar physico-chemical character specifictothesub- proportion of the city daily commute, around 1.25 million people way environment, being loaded with inhalable ferruginous particles (Moreno et al., 2014). Here we focus in particular on the ferruginous par- (FePM) commonly accompanied by other elements such as C, Si, Ca, ticles that dominate subway platform air, and on the significant contribu- Mn, Cr and Ba. tion from inhalable-sized carbonaceous polymetallic particles interpreted It has been argued that the metalliferous character of the particles pro- to be derived from the application of asbestos-free pneumatic brakes. duced by underground rail transport has a considerable potential capacity to stimulate reactive oxygen species (ROS) generation and cause DNA 2. Methodology damage (M. Jung et al., 2012), especially since the work of Karlsson et al. (2005), who presented toxicological evidence apparently in favour The subway aerosol PM examined for this study was collected during of enhanced genotoxicity for subway PM. Not only is the particle chemis- an intensive monitoring campaign carried out over four months (April– try implicated in this concept of enhanced toxicity, but also the range of July 2013) at different stations within the Barcelona Metro system. sizes, morphologies and surface bioreactivities are likely to differ from Barcelona's Metro includes 11 metro lines built since 1929 to 2010, the those more typical of outdoor urban PM (Knibbs et al., 2011). Of key inter- latest (L9–L11) with platform screen door systems and advanced ventila- est here is FePM morphology and the speciation of the inhalable ferrugi- tion systems. All trains are operated electrically (80% of them being motor nous material, as some iron species are reported to be more toxic than carriages) and run from 5 a.m. until midnight every day, with additional others (Park et al., 2014). services on Friday nights (finishing at 2 a.m. of Saturday) and Saturday Our understanding of the origin and potential toxicity of subway par- nights (running all night long), with a frequency between 2 and 15 min, ticles is limited by the fact that there have been relatively few studies elu- depending on the day (weekend or weekday) and time of day. Night cidating the detailed morphology and chemistry of individual inhalable maintenance works involving diesel vehicles or yielding operations are aerosols. Indeed, reviewing the available literature there is still uncertain- occasional but can have an impact on the platform air quality. ty over some issues, such as the exact speciation of ferruginous particles The braking system is electric when approaching the platform, available for inhalation on station platforms, the importance of mechani- changing to non-asbestos pneumatic braking when slowing down cal versus thermal processes in subway particle generation, and the rela- below a 5 km/h velocity for all lines independently of the platform tive importance and origins of other elements also present in FePM. In the design, using either frontal or lateral brake pads.

Tetuán

Joanic

LLefià Santa Coloma

Fig. 1. Stations selected for the study. Santa Coloma (Line 1), Tetuan (Line 2), Joanic (Line 4), and Llefià (in the new sliding door system Line 10). T. Moreno et al. / Science of the Total Environment 505 (2015) 367–375 369

At all sampling sites subway platform PM10 was collected on polyure- using a JEOL5900LV Scanning Electron Microscope (SEM) via an energy thane foam substrates (PUF) during 8 consecutive days (from 9:00 a.m. dispersive X-ray microanalysis system (EDX). PUF substrates were on Monday to 9:00 a.m. of Tuesday of the following week) using an Air- directly ‘flat-mounted’ onto aluminium SEM stubs using epoxy-resin borne Sample Analysis Platform system (ASAP; Model 2800 Thermo, (Araldite) as an adherent between the PUF and the stub. The samples USA) with a high sample flow-rate of 200 l/min. The equipment was lo- were then gold/palladium-coated using a 208HR Sputter Coater cated at the end of the platform corresponding to the entrance point of (Cressington, UK) and an MTM20 Thickness Controller (Cressington, the train into the station, with no exit access to minimise the complexity UK). The microscope working distance was 10 mm, with an accelerating of air turbulence and annoyance to passengers. Four stations were select- voltage of 20 kV. ed (Fig. 1), each with different designs chosen to obtain a wide range of In addition, sampled particles were visualised using a high resolution PM sample characteristics, including a station with open double rail transmission electron microscope (HR-TEM). For this, particles were track (Santa Coloma), double track separated by a wall in the station suspended in molecular biology (MB) grade water (Sigma-Aldrich, UK;

(Joanic), single rail track (Tetuan) and a new station with platform screen 2 μgPM/1μlH2O) and 40 μl of this suspension was pipetted onto the sur- doors (PSD's) separating the single rail track from the platform (Llefià). face of a 200 mesh Au grid with carbon film (Agar Scientific). Samples Individual particles were characterised using scanning electron mi- were imaged using a Philips CM12HR-TEM at 80 kV accelerating voltage, croscopy. The size and shape of individual particles were observed and images were obtained with a SIS MegaView III digital camera.

a) b) c)

d) e) f)

Fe

Fe Si

g) h) i)

k)j) l)

Fe+soot FeC

Fig. 2. SEM images demonstrating the typical morphological aspect of FePM in subway samples: a) General aspect of a PUF sample with subway PM10 dominantly ferruginous and flake- like; b–f) FePM with flake and splintery morphologies; g) Lineated FePM flake; h) Botryoidal morphology interpreted as hematite; i) Large FePM flake with rough, coarsely cleaved surface and attached nanoparticles; j) FeCPM with nanometric crystals growing on the flake surface; k) FePM flake adjacent to amorphous gypsum (calcium sulphate) particle on which has grown nanometric crystalline needles of new gypsum; l) FePM flakes and carbonaceous nanometric soot aggregates. 370 T. Moreno et al. / Science of the Total Environment 505 (2015) 367–375

3. Results euhedral cubic crystals that have grown within the PUF substrate after sample collection (Fig. 3e). Metals other than Fe, although detected at Particle concentrations (8 day means) were highest in the double trace levels in the chemical analyses of PM filter bulk samples, are 3 rail track station (Santa Coloma 72 μgPM2.5/m ), followed by the station only rarely evident under the SEM. Barium sulphates were observed in 3 with single rail track (Tetuan 44 μgPM2.5/m ), whereas the stations both old and new stations (Santa Coloma and Llefià), these being attrib- with a wall separating both rail tracks (Joanic) and the sliding door sys- uted to the use of barite in the fabrication of brakes in both trains and 3 tem (Llefià) both had values of 29 μgPM2.5/m . These variations in PM road vehicles (Aarnio et al., 2005). Other trace metals identified under mass concentration were mostly related to the differences in platform the SEM include Zn, Ti, and Sb, the latter again most probably related design and ventilation rather than particle sources, because particle to brake abrasion and always attached to or integrated within larger composition and morphologies were very similar in all stations (as ob- particles (Fig. 3f–i). Occasional bright spots under SEM backscatter re- served after studying the samples under SEM), indicating a common veal concentrations of Sb, either in irregular, sub-angular grains associ- mechanism of formation. Compositionally the most abundant particles ated with Ca (also present in brakes as calcite), or as spheres with were Fe-rich (Fig. 2a), in agreement with the previous subway studies associated traces of S and V. Copper PM was only identified in the sam- elsewhere (e.g. Aarnio et al., 2005; Kang et al., 2008; Salma, 2009; HJ. ples from Joanic and Santa Coloma, the two stations where the panto- Jung et al., 2012; Midander et al., 2012; Minguillón et al., 2012). Indeed, graph connecting to the electric supply catenary (which is made of it is difficult to analyse a subway particle that does not register at least 95% Cu wire) is made of copper or graphite (50% of trains each), where- some iron, an observation that we shall discuss further below. as it is only graphite in the other two stations. These trace metallic par- The inhalable-sized fraction of ferruginous particles (FePM) in the ticles in the subway samples are always in the nanosized scale and occur Barcelona subway samples displays a spectrum of morphologies under either embedded obscurely within ferruginous masses or adhered to the SEM, but by far the most common occurs in the form of irregular, the surface of larger particles, with morphologies ranging from amor- rough-surfaced flakes measuring at most just a few microns in size phous to spherical. In general micrometric metallic spheres are not (Fig. 2a), with the larger examples commonly displaying cracked and common in the Barcelona Metro samples which are overwhelmingly corroded textures (Fig. 2b, c). Sharply angular shard-like and splintery dominated by ferruginous flakes and splinters. forms often with relatively smooth surfaces (Fig. 2d, e, f) are also com- In order to elucidate further the chemistry of subway platform FePM mon in the FePM, and a range of other textures include lineated at high magnifications, Table 1 provides a selection of analyses of (Fig. 2g), botryoidal (Fig. 2h), or roughly cleaved (Fig. 2i). It is normal micrometric sized areas viewed under the transmission electron micro- to observe nanometric PM attached to the host particle (Fig. 2c, i, j, l), in- scope (similar to the data shown in the chemical maps of Fig. 4)com- deed, in terms of particle number, submicron FePM is the most abun- paring PM from new and old lines. All areas analysed in this way were dant type. highly ferruginous (Fe 40–65 wt.%), and when Mn was registered as Many FePM flakes are carbonaceous, with C being present either in- an accompanying minor element this was almost always in the ratio tegrated within the grain structure or as nanometric crystals growing on of Mn/Fe = 0.01. In addition, the TEM analyses confirm the carbona- the surface (Fig. 2j). In addition, more familiar carbonaceous particles ceous character of many FePM, and our data suggest the presence of within the subway PM10 samples include diesel soot attached to the at least three groups based on carbon contents registered as low C host FePM (Fig. 2l), and biological aerosols (Fig. 3a, b, c, d) introduced (b5 wt.%), medium C (10–20 wt.%) or high C (N40 wt.%) (Table 1). probably from outside city air. Coarser PM10 (N5 μm) is much rarer in Both Si and Ca are commonly present, as are traces of other elements these samples, and is more commonly associated with “geological” par- Al, Mg, Na, K, Cr, Co, Sb, S, and Cl (Table 1). These analyses provide ticles such as quartz, calcium carbonate, calcium sulphates (from soil some clear indicators as to the likely source materials responsible for and building materials) and both aluminium and iron–magnesium sili- subway FePM generation. A Mn/Fe ratio of 0.01 is highly consistent cates. Halite particles are also present, commonly in the shape of with an origin from steel used in wheels, rails and brakes (Abbasi

Table 1 Chemical composition (elemental weight %) obtained by TEM microanalysis from typical FePM images of the Barcelona subway samples comparing particles from the new and old lines. See text for discussion. Cu was not analysed as it is the major component of the sample holder in the TEM.

Element (wt.%) New line Old line

C 1.96 3.46 12.35 12.83 18.46 19.52 13.72 49.85 11.06 O 22.23 32.47 9.08 22.01 18.65 16.42 21.01 4.73 15.12 Na 0.86 1.87 0.62 0.25 0.89 0.32 0.39 Mg 0.92 0.23 0.89 0.38 1.46 0.28 0.27 1.41 A1 0.93 0.32 0.24 2.06 2.12 0.56 0.53 Si 2.54 1.05 8.47 7.07 6.06 3.58 2.34 39.38 2.31 S 0.56 0.85 1.07 0.75 1.8 0.57 2.70 1.98 4.34 Cl 0.59 0.71 1.1 0.18 0.53 0.4 0.74 0.36 0.76 K 0.86 0.84 0.19 0.3 1.47 0.31 0.18 0.39 Ca 2.35 1.75 1.52 0.91 3.59 1.28 2.04 0.96 2.14 Ti 0.05 0.05 V 0.13 Cr 0.35 0.31 0.36 0.33 0.15 Mn 0.65 0.58 0.58 0.53 0.48 0.6 Fe 64.69 55.88 60.4 49.02 40.16 51.23 47.93 0.42 42.11 Co 0.21 0.52 0.31 0.42 0.06 Zn 0.05 0.02 0.23 0.68 0.99 As 0.08 0.05 0.05 0.03 Sn 0.64 Sb 0.39 0.11 0.13 0.89 0.09 0.42 0.28 Ba 2.49 4.1 4.1 1.69 10.41 1.27 18.56 Total: 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00 Mn/Fe 0.010 0.010 0.010 0.011 0.012 0.012 ––– T. Moreno et al. / Science of the Total Environment 505 (2015) 367–375 371

a) b) c)

Fe-Ti-Zn

d) e) f) NaCl

Ca-S

g) i)h)

Fe-Sb

Fig. 3. SEM images of particles other than typical FePM also present in subway samples: a) Angular FePM splinter with attached calcium carbonate and (below left) cluster of nanometric spheres, brochosomes, secreted by leafhoppers; b) Close-up of brochosomes alongside nanometric cluster of metallic PM; c) Pollen grain; d) Insect fragment; e) Halite cube (with surface crystals of gypsum) growing on PUF substrate; f) FePM flakes alongside polycrystalline particle rich in Ca and Sb; g) Close up of CaSb particle; h) FePM rich in Sb and interpreted as derived from brake pad abrasion; i) Brake flakes containing Sb and Cu. et al., 2012 and references therein). Carbonaceous particles in the sub- produced by frictional processes at the wheel–rail interface. The parti- way environment, excluding those imported from above ground and cles appear relatively unoxidised and may therefore have been freshly those created by oxidation reactions of volatile organic compounds, generated just prior to collection. They are accompanied by Al-silicate can source from the high resistance commutator brushes in the electric and MgAl-silicate (lower and left part of image), and calcium carbonate motor and train pantographs connecting to the catenary. As discussed (lower centre) particles. Fig. 4b offers a close-up of a single micrometric below, however, we consider that most of the subway-generated carbo- carbonaceous FePM in a more advanced state of oxidation. This particle naceous particles on station platforms are derived from interaction be- contains traces of Si, Ca, Mn, and Co, and there is a markedly inhomoge- tween the brake pad and train wheel. Train brakes in the Barcelona neous distribution of Ba and S. The presence of these minor elements, subway can be frontal or lateral dependent on the train model, and especially Ba, along with the highly ferruginous nature of the particles, are chemically either carbonaceous-Fe poor (frontal pads: CNNFe) or leads us to interpret this FePM as derived from a metallic lateral ferruginous–carbonaceous (lateral pads: Fe N C) (own data). As is the brake–wheel interface. Fig. 4c images two FePM flakes, both of which case with road traffic brake pads (Amato et al., 2012) the main chemical are again highly oxidised, carbonaceous, contain Si, Ca, and Al and are components of the train brake pads are heterogeneously distributed rich in barium and sulphur. These are also interpreted as metallic lateral within the pad and accompanied by a wide spectrum of other cations brake particles and show a typically inhomogeneous distribution of zinc such as Mg, Ca, Ba, Cr, Co, Sr, and Mo. Iron-poor pads (frontal types) ad- which is clustered into nuggets in the lower particle. Finally Fig. 4dpro- ditionally contain enhancements in a range of typical “crustal” elements vides a close-up of the lower brake flake imaged in Fig. 4c. At these mag- (Al, Na, Ti, Li, V, Rb, Y, Nb, Th, U, and lanthanoid group, especially La, Ce, nifications the nugget-like texture displayed by Zn distribution is even Pr and Nd) whereas metallic pads contain higher Mn, Ni, and especially more obvious, and it becomes increasingly clear that the secondary Zn, Cu, As, Sb, and in some cases Pb (own data). Barium is a particularly electron image of Fe itself has a mottled appearance implying internal characteristic component within the brakes utilised in the Barcelona inhomogeneity. metro, and in some pads can be abundant enough to comprise the dom- We zoom further into the detailed microstructure of these FePM inant cation, combined with S in the form of barium sulphate (Table 1). flakes in Fig. 5, initially with an imaging sequence from Fig. 5atoc. Examples of the distribution of trace elements within subway FePM The image in Fig. 5a may be compared to the featheredge of the flake are provided by the TEM data in Fig. 4 with images of low C FePM shown in Fig. 4d (top of the image), and itself has ultrathin featheredges (C b2 wt.%, Fig. 4a) and medium C FePM (C b20 wt.%, Fig. 4b–d). The which again display the same mottled appearance. This texture be- first example (Fig. 4a) shows a cluster of micrometric-sized angular comes clearer in Fig. 5b and c, which reveals the mottling to comprise MnCr steel fragments (centre and upper part of image) interpreted as clusters of mostly rounded ferruginous nanocrystals typically 372 T. Moreno et al. / Science of the Total Environment 505 (2015) 367–375

a) b)

Element Wt% Element Wt% C 19.52 C 1.96 O 16.42 O 22.23 Na 0.32 Na 0.86 Mg 0.28 Mg 0.92 Al 2.12 Al 0.93 Si 3.58 Si 2.54 S 0.57 S 0.56 Cl 0.40 Cl 0.59 K 1.47 K 0.86 Ca 1.28 Ca 2.35 Mn 0.60 Cr 0.35 Fe 51.23 Mn 0.65 Co 0.42 Fe 64.69 Sb 0.09 Zn 0.05 Ba 1.69 As 0.08 Total: 100.00 Sb 0.39 Total: 100.00 c) d)

Element Wt% Element Wt% C 11.06 C 13.72 O 15.12 O 21.01 Mg 1.41 Al 0.56 Al 0.53 Si 2.34 Si 2.31 S 2.70 S 4.34 Cl 0.74 Cl 0.76 K 0.31 K 0.39 Ca 2.04 Ca 2.14 Fe 47.93 Fe 42.11 Zn 0.68 Zn 0.99 Sb 0.42 Sb 0.28 Ba 10.41 Ba 18.56 Total: 100.00 Total: 100.00

Fig. 4. TEM elemental mapping of Barcelona subway PM10 samples showing, a) Agglomerate of steel wheel/rail splinters and mineral dust particles; b) Oxidised steel FePM containing Si, C, Co, Ca, S, Mn and Ba, and interpreted as derived from metallic lateral brake pad: note inhomogeneous distribution of Ba and S; c–d) Brake flake particles showing inhomogeneous distri- bution of trace components and mottled nanocrystalline structure of iron oxide (continue to Fig. 5a–c).

measuring from 10 nm or less to a few 10s of nm. The highly crystalline submicron-scale oxidation are attributed to the stresses induced by nature of these samples is very apparent under the TEM, as demonstrat- a smaller lattice parameter of the oxidised shell as compared to the ed by an abundance of diffraction patterns available to determine the less oxidised core (Feitknecht and Gallagher, 1970; Özdemir et al., particle structure and lattice spacing. Thus most TEM images reveal 1993). The enduring presence of iron metal within platform FePM, the presence of nanocrystalline magnetite (Fig. 5d, e) and hematite however, is the exception: most inhalable-sized platform FePM (Fig. 5f), as well as onion-like concentric layers (Fig. 5g) indicative of car- flakes are composed of iron oxide nanocrystals within which are bon nanoparticle structure integrated within the FePM microstructure. intimately incorporated nanocrystals of other compounds originally The nanometric aggregates are not entirely iron oxide and carbon, how- also present within the parent materials. ever, as other tiny particles of compounds such as barium sulphate, bar- ium carbonate, and halite can be identified (Fig. 5f, h). 4. Discussion Although the previous X-ray diffraction data from platform dust (Querol et al., 2012) have indicated that the bulk of Fe-particles in the Our observations on subway platform particles confirm that they are Barcelona subway are present as hematite (Fe2O3), it is clear under pervasively ferruginous and both physically and chemically different the TEM that magnetite also exists within these nanometric aggregates, from the typical outdoor urban cocktail of inhalable PM breathed and in some places it is possible to identify the presence of Fe metal. above ground in the city. This in turn indicates that the dominant mix One such FePM preserved native Fe is imaged in Fig. 5i as a slit-like of source materials and processes of PM generation are unique to the core revealed inside a cracked submicron Fe particle with an subway environment. This conclusion is nothing new: a considerable oxidised coating. The stretching and cracking of this particle during number of papers published on subway PM over the last ten years T. Moreno et al. / Science of the Total Environment 505 (2015) 367–375 373

a) b) c)

d) e) f)

g) h) i)

C Fe Fe-Ox

Fig. 5. High magnification images of FePM composed of iron oxide nanocrystals (a–c) containing magnetite (d, e), hematite (f), concentric carbon nanocrystals (g), barium salts (f, h) and halite (f). Fig. 5i images a cracked nanoparticle revealing an iron metal core and oxidised shell: see text for discussion.

have highlighted the ferruginous nature of such aerosols and concluded with platform sliding doors, ambient FePM has been reported to be that the obvious sources distinctive to the subway are related to train less ferruginous and less magnetic (Kim et al., 2010; HJ. Jung et al., movement, with wheels, rails, brakes, and electrical supply system all 2012), this presumably reflecting reduced amounts of fresh tunnel- implicated in FePM generation. However, reading the available litera- generated Fe-metal-bearing particles gaining access to the platform. ture can reveal a lack of precision over the relative importance of One of the commonest oxide mineral reactions in nature is the oxi- these different sources, the specific processes that generate different dation of magnetite to maghemite (Özdemir et al., 1993), which is in FePM, and whether subway aerosols are more toxic than those breathed turn metastable relative to hematite (Guo and Barnard, 2013; Machala at street level. In this context, the following discussion section will focus et al., 2011). The presence of a mixture of iron metal and various oxides on FePM chemical composition, oxidation history, microstructure, gen- in the same subway particle, with the oxides sometimes seen as an eration processes, and conclude with a brief comment on concerns over external carapace (e.g. Fig. 5i), presumably records stages in the pro- potential toxicity. gressive oxidation of freshly exposed highly active metal surfaces of Our current understanding of the chemical speciation of subway iron splinters and flakes to magnetite, maghemite and hematite, with FePM owes much to a series of publications on air quality in the Seoul smaller particles having greater surface areas available for reaction underground rail system of South Korea (e.g. Kang et al., 2008; Kim (Kang et al., 2008; Knibbs et al., 2011; HJ. Jung et al., 2012; Eom et al., et al., 2010; HJ. Jung et al., 2012; Eom et al., 2013). Ambient FePM10 in 2013). One of the key, and perhaps surprising, characteristics of the tunnel and platform has been recorded as comprising a mixture of inhalable subway particles is that most of them are extremely small, iron metal (α-Fe), magnetite (Fe3O4), maghemite (γ-Fe2O3) and hema- as shown in our study and as emphasised in several other recent publi- tite (α-Fe2O3), with unoxidised Fe-metal being less common than cations (e.g. Jansson et al., 2010; HJ. Jung et al., 2012; Midander et al., oxide, and a majority of FePM being carbonaceous. Samples of subway 2012; Loxham et al., 2013). Indeed there appears to be an increasing re- dusts appear to be more magnetic if collected closer to the rail, indicat- alisation that ferruginous nanoparticulate material (nanoFePM) is the ing the presence of more iron metal, magnetite and maghemite relative dominant inhalable aerosol in the subway environment and that it is to non-magnetic hematite (HJ. Jung et al., 2012), and revealing spatial not necessary to invoke the traditional view that such materials are pro- variation in the oxidation state of FePM. Similarly, in those stations duced by high temperature processes such as vapourisation and 374 T. Moreno et al. / Science of the Total Environment 505 (2015) 367–375 condensation. This new understanding stems from the combination of of Eom et al. (2013) indicate the more obvious presence of micrometric direct observation of subway PM (as in our study) with the results Fe-spheres which they attribute to iron metal sparking vapourisation from experimental studies (e.g. Olofsson, 2011; Sundh et al., 2009; followed by condensation, analogous to particle generation during arc- Abbasi et al., 2012). Wear rates and particle generation at the train welding. Also the passing of diesel trains during nightwork can help the rail–wheel contact can be shown both to increase with increasing load formation of particles by condensation and thermal fragmentation of and produce a large amount of ultrafine PM in the size range 0.25– nanoparticle aggregates (Burchill et al., 2011). 1 μm(Sundh et al., 2009). Similarly, disc brake PM released from region- Finally, we return to the subject of FePM toxicity and the possibility al trains contains abundant amounts of nanometric particles which can that subway air is more capable of damaging human health than city air be as small as 50 nm (Abbasi et al., 2012). In a particularly revealing outdoors. The main evidence in favour of subway FePM being more study, Olofsson (2011) produced frictionally-generated nanoparticles genotoxic and capable of inducing oxidative stress has been presented by simulating the sliding of brake materials against railway wheels, by Karlsson et al. (2005) using platform PM10 collected on glass fibre and demonstrated that under their chosen experimental conditions filters (see also Karlsson et al., 2006, when part of the subway sample metalliferous brake pads generate many more nanoFePM than compos- bioreactivity was found to be due to the lingering presence of glass filter ite pads. fibres in the extracted samples). They considered their FePM samples to Much of the finer inhalable FePM on subway platforms in the Barce- be mostly magnetite, although in a subsequent study (Karlsson et al., lona metro comprises iron oxide nanocrystal aggregates like those im- 2008) they were unable to prove their original speculation that high aged in Fig. 5. These nanoFePM have been observed by some of the Fe content was behind an increased ability to induce oxidative stress. few previous subway publications to utilise TEM, with Salma (2009) de- Comparisons with various samples of magnetite, hematite, Cu and Cu– scribing structured aggregates of rounded crystals 5–15 nm in size, and Zn failed to produce the same toxic response (Karlsson et al., 2008). Zhang et al. (2011) recording “clumped” submicron-sized magnetic Papers subsequently published on rail-generated particles usually FePM. These nanoFePM materials are commonly flake-like in form quote at least one of these papers (e.g. Kang et al., 2008; Salma, 2009; which, rather than suggesting condensation of Fe vapour (Kang et al., Murruni et al., 2009; Olofsson, 2011; Zhang et al., 2011; Múgica- 2008; Salma, 2009; Salma et al., 2009; HJ. Jung et al., 2012; Loxham Alvarez et al., 2012; HJ. Jung et al., 2012; Midander et al., 2012; Abbasi et al., 2013) we interpret as generated mechanically by frictional wear, et al., 2013; Eom et al., 2013; Loxham et al., 2013; Querol et al., 2012; especially by the sliding of two metallic surfaces such as between lateral Moreno et al., 2014). In a study of PM subway doses Seaton et al. brake pads and wheel, followed by oxidation. Their parent material is (2005) reached the conclusion that “those principally at risk from dust steel or cast iron which has become pervasively, but not always inhalation by working or travelling in the London Underground should completely, oxidised to magnetic oxide species (magnetite and not be seriously concerned, although efforts to reduce dust concentra- maghemite) and non-magnetic hematite. The morphologies of natural tions should continue, since the dust is not without toxicity”.Withre- magnetite nanoparticles are octahedral and rhombo-dodecahedral gard to epidemiological evidence, Gustavsson et al. (2008) concluded (Guo and Barnard, 2013), a microtexture preserved even when further that no increased lung cancer risk was found amongst subway train oxidation produces maghemite as this mineral simply pseudomorphs drivers, and their study thus failed to support any hypothesis that sub- the pre-existing magnetite without creating a new crystal form. Sugges- way particles are more potent in inducing lung cancer than particles tions of formerly euhedral nanocrystals with octahedral form are dis- in ambient air. Given the uncertain and limited nature of the evidence cernable in some of the ferruginous flakes (e.g. Fig. 5c). However, for subway metalliferous PM toxicity, there appears to us to be no obvi- further conversion to hematite alters the magnetic microstructure ous cause for alarmist concern, although given the peculiar metal chem- texture to produce more rounded crystal forms. We invoke the partial istry of these particles, with enhanced amounts of toxic trace elements or total replacement of precursor magnetic minerals by non-magnetic such as Mn and Cr (e.g. see Chillrud et al., 2004), the presence of hematite to explain the typically less well-defined crystal faces in more nanocrystals of magnetite and maghemite (Park et al., 2014), and the amorphous aggregates such as those depicted in our subway PM images possibility that some brake pad materials may produce more toxic (Fig. 5). Our images of nanometric FePM magnetite–maghemite clusters flakes than others, adoption of the precautionary principle would are similar to those of naturally occurring iron magnetite–maghemite– seem wise. Thus, as with outside city air in traffic hot spots (where hematite nanocrystals (Guo and Barnard, 2013). toxicological effects have been demonstrated, Janssen et al., 2012; Our chemical data on individual platform FePM indicates that many of Sysalová et al., 2012), at this stage the priority in subway air quality them are brake flakes, namely slices of oxidised ferruginous and carbona- should be to reduce the high mass concentrations of aerosol present in ceous material dissociated from the brake pad (Fig. 4b, c) during brake some stations rather than worry about attempting to remove some application immediately prior to and during station entry. Brake dust is chemical component of unspecified toxicity. not, of course, exclusive to subway air, being abundantly generated by road traf fic in outdoor city air, although diluted by the products of hydro- 5. Conclusions carbon combustion and particle resuspension. Sagnotti et al. (2009) for example identified outdoor brake flakes forming rough-surfaced particles 1. Ambient subway platform aerosol particles mostly source from train of non-stoichiometric nanocrystalline magnetite with an inhomogeneous movement and are pervasively ferruginous (FePM), commonly microchemistry and a morphology “remarkably different from the typical carbonaceous, and both physically and chemically distinctive from spherical particles of industrial flyashes”.Inthecaseoftherailsubway, PM breathed in outdoor urban air. brake flakes will be swept through and settle in the station under the diminishing influence of each train arrival piston effect. These subway 2. The most common form of subway FePM is very small (nanometric brake flakes, undiluted by road vehicle emissions, are therefore expected up to a few microns in size) and derives dominantly from mechanical to be much more common on the platform than in the tunnel or outdoors. processes of sliding and wear at the brake–wheel and wheel–rail in- In addition to the brake flakes, many other platform FePM will be derived terfaces, with, in Barcelona at least, only minor contributions from from the chemically more homogeneous Mn steel of the wheel and/or high temperature processes such as sparking. rails, as demonstrated by their distinctive trace element chemistry 3. Metallic flakes and splinters released from the wheel area undergo (such as a Mn/Fe ratio of 0.01: Fig. 4a, b). Although we have emphasised progressive atmospheric oxidation from native iron through magne- the overwhelmingly dominant role of mechanical processes in the gener- tite, maghemite and hematite organised within a nanocrystalline ation of the Barcelona metro atmospheric environment, where few parti- oxide matrix which images as a distinctive mottled nanostructure cles are spheroidal, this will not be necessarily the case for all other under TEM. Individual nanocrystals, now mostly hematite, are systems worldwide. In the Seoul subway, for example, the descriptions typically 10–20 nm and rounded in appearance, although they can T. Moreno et al. / Science of the Total Environment 505 (2015) 367–375 375

preserve remnants of octahedral forms indicative of their origin as Kam W, Cheung K, Daher N, Sioutas C. Particulate matter concentrations in underground and ground-level rail systems of the Los Angeles Metro. Atmos Environ 2011;45: magnetite derived from the oxidation of iron metal. 1506–16. 4. The chemistry of individual FePM under TEM allows differentiation Kang S, Hwang H, Park Y, Kim H, Ro CU. Chemical compositions of subway particles in between different anthropogenic sources, such as an origin from Seoul, Korea determined by a quantitative single particle analysis. Environ Sci Technol 2008;42:9051–7. Mn steel (wheels, rail) or brake pads. Particles released during Karlsson HL, Nilsson L, Möller L. Subway particles are more genotoxic than street particles frictional brake wear in particular exhibit a highly distinctive and induce oxidative stress in cultured human lung cells. 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V. Martins, T. Moreno, MC. Minguillón, F. Amato, E. de Miguel, M. Capdevila, X. Querol. 2015. Exposure to airborne particulate matter in the subway system. Science of The Total Environment, 511, 711–722. http://dx.doi.org/10.1016/j.scitotenv.2014.12.013. Open access.

Inception Report 30th of June 2015 ANNEX III – Deliverable 13 5 Science of the Total Environment 511 (2015) 711–722

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Science of the Total Environment

journal homepage: www.elsevier.com/locate/scitotenv

Exposure to airborne particulate matter in the subway system

Vânia Martins a,b,⁎, Teresa Moreno a, María Cruz Minguillón a, Fulvio Amato a, Eladio de Miguel c, Marta Capdevila c,XavierQuerola a Institute of Environmental Assessment and Water Research (IDAEA), CSIC, C/Jordi Girona 18-26, 08034 Barcelona, Spain b Department of Analytical Chemistry, Faculty of Chemistry, University of Barcelona, Av. Diagonal 647, 08028 Barcelona, Spain c Transports Metropolitans de Barcelona, TMB Santa Eulàlia, Av. Del Metro s/n L'Hospitalet de Llobregat, 08902, Spain

HIGHLIGHTS

• Higher PM concentrations were found on platforms compared to outdoor. • Air quality was better in the new lines with PSDs. • PM concentrations were higher in the colder than in the warmer period. • Ventilation and air conditioning systems improve air quality in the subway system. • Time commuting in the subway contributes substantially to the personal exposure.

article info abstract

Article history: The Barcelona subway system comprises eight subway lines, at different depths, with different tunnel dimen- Received 18 September 2014 sions, station designs and train frequencies. An extensive measurement campaign was performed in this subway Received in revised form 1 December 2014 system in order to characterise the airborne particulate matter (PM) measuring its concentration and investigat- Accepted 5 December 2014 ing its variability, both inside trains and on platforms, in two different seasonal periods (warmer and colder), to Available online 21 January 2015 better understand the main factors controlling it, and therefore the way to improve air quality. The majority of Editor: Lidia Morawska PM in the underground stations is generated within the subway system, due to abrasion and wear of rail tracks, wheels and braking pads caused during the motion of the trains. Substantial variation in average PM concentra- Keywords: tions between underground stations was observed, which might be associated to different ventilation and air Barcelona conditioning systems, characteristics/design of each station and variations in the train frequency. Average

Platform stations PM2.5 concentrations on the platforms in the subway operating hours ranged from 20 to 51 and from 41 to Trains 91 μgm−3 in the warmer and colder period, respectively, mainly related to the seasonal changes in the subway Ventilation ventilation systems. The new subway lines with platform screen doors showed PM2.5 concentrations lower than Indoor those in the conventional system, which is probably attributable not only to the more advanced ventilation setup, Metro but also to the lower train frequency and the design of the stations. PM concentrations inside the trains were gen- erally lower than those on the platforms, which is attributable to the air conditioning systems operating inside the trains, which are equipped with air filters. This study allows the analysis and quantification of the impact of different ventilation settings on air quality, which provides an improvement on the knowledge for the general understanding and good management of air quality in the subway system. © 2014 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/).

1. Introduction private vehicles. The subway, being an electrical system and one of the cleanest public transport systems in large urban agglomerations, is con- Citizens usually spend a considerable amount of their daily time sidered to be the most appropriate public transport since it diverts the commuting. Considering that in urban areas road trafficisamajoremis- burdens of superficial traffic congestion. Its high capacity in terms of sion source of air particles (Viana et al., 2008), travelling by public trans- number of daily commuters makes it an environmentally friendly alter- portation saves energy and produces less pollution than travelling in native. The energy efficiency and reduced urban atmospheric emissions make this kind of public transport a powerful tool to reduce energy de- mands and improve air quality in urban environments. ⁎ Corresponding author at: Institute of Environmental Assessment and Water Research (IDAEA), CSIC, C/Jordi Girona 18-26, 08034 Barcelona, Spain. However, prior studies in subway systems of several cities world- E-mail address: [email protected] (V. Martins). wide indicate, with few exceptions, that particulate matter (PM)

http://dx.doi.org/10.1016/j.scitotenv.2014.12.013 0048-9697/© 2014 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/). 712 V. Martins et al. / Science of the Total Environment 511 (2015) 711–722 concentrations are generally higher in these environments than those In order to gather information on the relationship between pollutant measured in ambient air (Nieuwenhuijsen et al., 2007). The underground levels and the characteristics of the sampling sites, the measurements subway system is a confined space that promotes the concentration of were obtained in several subway lines, including stations with different contaminants entering from the outside atmosphere in addition to characteristics (design and ventilation of the station and tunnels, num- those generated inside. The subway aerosol particles are mainly generat- ber and location of connections with the outdoor level, and train fre- ed by the abrasion of rail tracks, wheels, catenary and brake pads pro- quency). This monitoring scheme was designed to characterise the duced by the motion of the trains, and the movement of passengers temporal and spatial variation of particles at each site and to identify which promotes the mixing and suspension of PM (Querol et al., 2012). their possible sources. Therefore, the four subway stations studied on PM levels have been reported in many subway systems, such as in daily basis have different characteristics; in particular, one of the sta- Milan (Colombi et al., 2013), Barcelona (Querol et al., 2012; Moreno tions is equipped with platform screen doors (PSDs) for commuters' et al., 2014), Taipei (Cheng et al., 2008, 2012; Cheng and Lin, 2010), safety but also resulting in less mixing of air between the platform Seoul (Kim et al., 2008, 2012; Park and Ha, 2008; Jung et al., 2010), and tunnels. The influence of the installed PSDs on aerosol characteris- Mexico City (Mugica-Álvarez et al., 2012; Gómez-Perales et al., 2004), tics is also investigated in this work. Los Angeles (Kam et al., 2011a,b), New York (Wang and Gao, 2011; Chillrud et al., 2004, 2005), Shanghai (Ye et al., 2010), Sydney (Knibbs 2. Methodology and de Dear, 2010), Buenos Aires (Murruni et al., 2009), Paris (Raut et al., 2009), Budapest (Salma et al., 2007), Beijing (Li et al., 2006, 2007), 2.1. Field study Prague (Braniš,2006), Rome (Ripanucci et al., 2006), Helsinki (Aarnio et al., 2005), London (Seatonetal.,2005;Adamsetal., The subway system in the city of Barcelona (managed by Transports 2001), Stockholm (Johansson and Johansson, 2003), Hong Kong Metropolitans de Barcelona, TMB) is one of the oldest underground (Chan et al., 2002a), Guangzhou (Chan et al., 2002b), Tokyo (Furuya transport systems in Europe, with its first line beginning operation in et al., 2001), Boston (Levy et al., 2000), and Berlin (Fromme et al., 1924. By the present decade, the Barcelona subway system comprises 1998). However, results are not always directly comparable because of 8 lines (3 of them in operation over the last five years) with a total differences in sampling and measurement methods, data analysis, length of 102.6 km and including 140 train stations. The new stations duration of the measurements and the type of environment studied have platforms separated from the tunnel by a wall with mechanical (Nieuwenhuijsen et al., 2007). There are important factors influenc- doors (PSDs) that are opened simultaneously with the train doors. ing PM concentrations in underground railway systems around the Trains run from 5 a.m. until midnight every day, with additional services world, which include differences in the length and design of the sta- on Friday nights (finishing at 2 a.m. of Saturday) and Saturday nights tions and tunnels, system age, wheel and rail-track materials and (running all night long), with a frequency between 2 and 15 min, de- braking mechanisms, train speed and frequency, passenger densi- pending on the day (weekend or weekday), subway line and time of ties, ventilation and air conditioning systems, cleaning frequencies, day. The Barcelona subway absorbs around 50% of the urban commuting among other factors (Moreno et al., 2014 and references therein). load, transporting 1.25 million commuters on weekdays, with the most Despite the number of studies on PM in underground subway sys- frequent average journey time being 35 min (Querol et al., 2012). tems, the main focus of most of them has been to monitor variations In all subway systems, two main types of environments are in mass concentration of PM on platforms and in a reduced number of connected: the platform station and the inside of the train. Both types of stations. Therefore, there is a need for extensive studies of entire environments were investigated in this study. Four underground stations subway systems, covering the vast diversity of lines, trains and stations with distinct designs belonging to different lines were selected for contin- and providing an overview of the overall exposure to PM in this uous monitoring in two one-month periods: Joanic on the yellow line environment. (L4), Santa Coloma on the red line (L1), Tetuan on the purple line (L2), With this in mind, this work is the first study that presents a large and Llefià on the new light blue line (L10). The architecture of the stations dataset from an extensive campaign, able to characterise 24 stations in and tunnels is different for each station: one wide tunnel with two rail the Barcelona subway system and providing valuable information for tracks separated by a middle wall in Joanic station and without middle human PM exposure studies in such environment, considering its possi- wall in Santa Coloma, a single narrow tunnel with one rail track in Tetuan, ble adverse health effects (Pope et al., 2004; Seaton et al., 2005; Karlsson and a single tunnel with one rail track separated from the platform by a et al., 2006, 2008; Gustavsson et al., 2008). For this, continuous PM wall with PSDs in Llefià(Table 1). measurements were carried out in 4 underground subway stations in The study was conducted in the warmer (2 April–30 July 2013) and Barcelona, on a daily basis during two months and supplementary sam- colder (28 October 2013–10 March 2014) periods (Table 1), according plings were also performed in a total of 20 additional stations. Measure- to TMB ventilation protocols to ascertain seasonal differences. In total, ments inside the trains were also carried out in 6 subway lines. the air quality at each station was measured continuously for 30 days

Table 1 Features of the subway stations and measurement periods.

Subway station (line) Measurement period Station

Warmer Colder Depth Design

Joanic (L4) 2 Apr–2 May 2013 28 Oct–25 Nov 2013 −7.6 m

Santa Coloma (L1) 1 Jul–30 Jul 2013 10 Feb–10 Mar 2014 −12.3 m

Tetuan (L2) 2 May–31 May 2013 25 Nov–20 Dec 2013 −14.8 m

Llefià (L10) 31 May–1 Jul 2013 13 Jan–10 Feb 2014 −43.6 m V. Martins et al. / Science of the Total Environment 511 (2015) 711–722 713 per season to obtain statistically representative data. Both seasons were The high volume sampler, which permits the sequential sampling of analysed using the same analytical methodology and monitoring instru- 15 filters, was equipped with quartz microfiber filters and programmed ments. For comparison purposes, outdoor ambient PM concentrations to sample PM2.5 over 19 h (from 5 a.m. to 12 p.m., subway operating were measured concurrently at the urban background station of Palau hours) at a sampling flow rate of 30 m3 h−1.Afield blank was taken

Reial (41°23′14″ N, 02°06′56″ E) which was used during both cam- at each station. PM2.5 concentrations were determined gravimetrically paigns as a reference site. using a microbalance (Model XP105DR, Mettler Toledo) with a sensitivity The platform ventilation conditions in the stations are regulat- of ±10 μg. The sampled filters were pre-equilibrated before weighing for ed by introducing outdoor air into the tunnel and/or platform at least 48 h in a conditioned room (20 °C and 50% relative humidity). The (impulsion) and removing indoor air towards the outdoor envi- quartz filters were used only for gravimetric purpose in this study, how- ronment (extraction). The mechanical ventilation settings ever, a detailed chemical analysis will be performed in subsequent (Table 2), with strong or low impulsion and/or extraction of air be- studies. tween the platform stations and tunnels, were adjusted for this Continuous measurements (24 h day−1) with a 5-minute time reso- study according to different TMB protocols during the sampling lution were performed using the DustTrak monitor for PM1,PM2.5 and periods in order to evaluate the influence in PM concentrations PM10 concentrations and the IAQ-Calc for CO2 and CO concentrations, and to determine the best operating conditions for air quality on T and RH. CO concentrations were always below the detection limit the platform. Each selected ventilation setting was maintained at (3 ppm) and hence they will not be further mentioned in this study. least during one week, in order to better evidencing their effects PM2.5 concentrations provided by DustTrak monitor were corrected on PM levels. against the in-situ and simultaneous gravimetric PM2.5 for each station. Levels of PM1 and PM10 were corrected using the same correction fac- 2.2. Measurements and sampling equipment tors obtained for PM2.5. However, previous HVS-DustTrak intercompar- isons for PM1 and PM10 concentrations were done for the ambient Air monitoring equipment included a light-scattering laser photom- outdoor air and weak correlations were obtained. The PM10 and PM1 eter (DustTrak, Model 8533, TSI) for PM1,PM2.5 and PM10 (particulate concentrations provided for the DustTrak monitor were undervalued matter with aerodynamic diameter less than 1 μm, 2.5 μm and 10 μm, and overvalued, respectively. Since the aerosol properties in the subway respectively) concentrations, a high volume sampler (HVS, Model are different from the outdoor aerosol, the previously determined corre-

CAV-A/MSb, MCV) with a PM2.5 head, and an indoor air quality meter lations are not suitable to correct the measurements. Therefore, in this (IAQ-Calc, Model 7545, TSI) for CO, CO2, T and RH values. The instru- study only the PM2.5 concentrations are used in absolute terms. ments were placed at the end of the platform corresponding to the In the urban background station of Palau Reial, continuous measure- train entry point, far from the passengers' access-to-platform point, ments were performed by a Laser Aerosol Spectrometer (Environmen- and behind a light fence for safety protection. This location was chosen tal Dust Monitor, Model EDM180, Grimm), with a 30-minute time as a compromise between meeting conditions for undisturbed mea- resolution, corrected with in-situ and simultaneous measurements ob- surement and obstructing commuter's path as little as possible. The tained with a high volume sampler (HVS, Model CAV-A/MSb, MCV), aerosol inlets were placed at roughly 1.5 m above the ground level. working for 24 h every third day. Two protocols were undertaken concurrently during the study for continuous PMX measurements: 1) additional measurements of PMX 2.3. Additional platform measurements concentrations on platforms to characterise spatial variations along the platform, and 2) monitoring of air quality inside the trains (see Measurements at the 4 selected platforms and at 20 additional plat- Sections 2.3 and 2.4). forms with wide variety of designs, from 6 subway lines, were carried

Table 2 Operating conditions for tunnel and platform ventilations.

Operating conditions Experimental period Mode Stations Day Night (week of each month) Platform Tunnel Platform Tunnel

I.W J, SC, T 1st and 3rd Low Imp. St. Imp. St. Ext. No ventilation II.W J, SC, T 2nd and 4th Low Ext.

III.W L 1st and 3rd St. Imp. + Ext. St. Imp. + Ext. St. Imp. + Ext. St. Imp. + Ext. IV.W L 2nd Low Imp. + Ext. Low Imp. + Ext. V.W L 4th St. Imp. + Ext. Low Imp. + Ext. Low Imp. + Ext.

I.C J, SC, T 1st and 4th Low Extr. Low Imp. II.C J, SC, T 2nd Low Imp. No ventilation No ventilation

III.C J, SC, T 3rd Low Extr. Low Ext.

IV.C L 1st and 2nd St. Imp. + Ext. St. Imp. + Ext. (*) St. Imp. + Ext. St. Imp. + Ext. V.C L 3rd Low Imp. + Ext. Low Imp. + Ext. (*)

VI.C L 4th St. Imp. + Ext. Low Imp. + Ext. St. Imp. + Ext. (*) Low Imp. + Ext. W: warmer period; C: colder period; J: Joanic station; SC: Santa Coloma station; T: Tetuan station; L: Llefià station; St: strong; Imp: impulsion; Ext: extraction; and (*): some fans switched off. Normal ventilation conditions marked in shadow. 714 V. Martins et al. / Science of the Total Environment 511 (2015) 711–722

out using a DustTrak monitor to obtain PM1,PM2.5 and PM10 concentra- DustTrak monitor. During the colder period, CO2 concentrations were tions with 5-second time resolution, enabling us to see the effect of also monitored using an Indoor Air Quality meter (IAQ-Calc). The log- trains and commuters movements. Out of the 24 stations, 4 were new ging interval for all measurements was set at 5 s. stations (line 10) and the remaining were old stations (lines 1–5). The The measurements were performed from 10 a.m. on weekdays in sampling protocol was as follows: the measurements at each station duplicate at each route and were carried out in the middle of the central lasted for 1 h, divided into periods of 15 min in 4 positions approximate- car of the train, with instrumentation being transported in a bag with ly equidistant along the platform; the first measurement point was lo- the air uptake inlet placed at shoulder height when sitting. A manual cated at the sampling site (placed at the far end of the platform record of the time when train doors open and close was performed. corresponding to the train entry point) for comparison with the average During the colder period of the campaign, measurements were carried

PMX concentrations measured across the platform; additionally, in the out along the whole length of the line with and without air conditioning colder period, the sampling in the first point was repeated during (not possible during warmer period due to passengers comfort 5 min after the 4 positions as a control; a manual record of the exact ar- requirements). rival and departure times of the trains was kept; sampling was per- formed during weekdays between 9 a.m. and 2 p.m. Sampling was 3. Results and discussion conducted twice in each season campaign (colder and warmer periods), resulting in 96 total sampling studies. In addition, in 12 old stations, in 3.1. PMX concentrations on platforms the colder period, measurements were performed once more to study the influence of the piston effect (with the ventilation mode II.C, 3.1.1. Influence of outdoor environment

Table 2) on the air quality of the platforms. The PMX mass concentrations discussed in this section are those de- The PMX concentrations reported in this study for the 4 selected sta- termined by the DustTrak instrument and corrected against gravimetric tions are those corrected for spatial variation determined at each plat- measurements. Fig. 1 shows a comparison between the average PM2.5 form based on the aforementioned measurements (Table S1). The concentrations on the subway platforms and outdoor, considering all

PM2.5 correction factors for spatial variation obtained for the light- day data and only operating hours data. Some outliers in the DustTrak scattering laser photometer (DustTrak) data at Joanic, Santa Coloma, time series were identified and associated with occasional, mostly Tetuan and Llefià were 1.05, 0.71, 0.92 and 0.71 in the warmer period night-time, maintenance or cleaning operations, and were included in and 0.96, 1.06, 1.02 and 0.95 in the colder period, respectively. These the analysis of daily concentrations. In any case the most relevant data values were obtained by dividing the average PM2.5 concentrations in are those measured during subway operating hours, due to the com- all station by the average PM2.5 concentrations in the first point, located muters' exposure to PM. at the sampling site. In general, the factors indicate that levels measured PM2.5 concentrations on the platforms were significantly higher than at the sampling sites were very similar to the exposure levels of com- those in the outdoor environment. The average concentrations were muters waiting elsewhere along the platform, as factors were very 1.3–6.1 and 1.3–6.7 times higher on the platforms than outdoors for close to 1, with the exceptions of Santa Coloma and Llefià, in the warmer all day period and in the operating hours period, respectively. The out- period, in which exposure levels were around 40% higher in the far end door PM concentrations do not seem to influence significantly the air of each platform. quality in the subway stations, since most of the PM load in the under- ground stations is generated within the subway system, due to the abra- 2.4. Measurements inside the trains sion and wear of rail tracks and wheels caused by the motion of the trains as well as to the braking systems (Querol et al., 2012). In

Measurements inside the trains from 6 subway lines (L1, L2, L3, L4, Stockholm the exposure levels for PM2.5 were 5–10 times higher than L5 and L10) were carried out during a return trip along the whole sub- the corresponding values measured on the busiest streets in that city way line. PM1,PM2.5 and PM10 concentrations were measured using a (Johansson and Johansson, 2003).

subway platform (all day) subway platform (subway operating hours) outdoor (all day) outdoor (subway operating hours)

120

100 )

-3 80

(µg m 60 2.5 40 PM

20

0 Joanic W Joanic C Sta Coloma W Sta Coloma C Tetuan W Tetuan C Llefià W Llefià C

2.5 6.1 2.9 4.7 2.8 3.8 1.3 3.6

2.2 6.7 3.1 4.9 2.8 3.9 1.3 3.6

Station design:

one wide tunnel with two rail tracks separated by a middle wall single narrow tunnel with one rail track

one wide tunnel with two rail tracks without middle wall single tunnel with one rail track separated from the platform by a wall with PSDs

Fig. 1. Average PM2.5 concentrations and standard deviations on the subway platforms and outdoor in both periods, and the ratios of PM2.5 concentrations with respect to the outdoor levels (W — warmer period; C — colder period). V. Martins et al. / Science of the Total Environment 511 (2015) 711–722 715

3.1.2. Comparison at different periods the year. During night-time however the pattern was very similar to

Average PM2.5 concentrations during operating hours were general- the one described above for the warmer period. ly higher than those corresponding to the all day, which indicates the The results in Llefià station (equipped with PSDs) for the warmer pe- importance of PM2.5 sources related to the subway operation activities. riod showed that its stronger ventilation systems can achieve much The opposite trend was only observed during the warmer period in Joanic lower and stable PMX concentrations on the platforms, with only a when night-time maintenance or cleaning operations were more intense, slighter increase of PM levels between 6 and 9 a.m. especially during generating larger amounts of PM2.5 during non-operating hours (Fig. 1). weekdays. Again, in the colder period, the daily pattern of PMX concen- Average PM2.5 concentrations for operating hours on Joanic, Santa trations presents higher and less stable values during the whole week Coloma, Tetuan and Llefià subway platforms were 32, 51, 40 and due to the lower ventilation rates. −3 −3 20 μgm in the warmer period, and 70, 65, 91 and 41 μgm in the Regarding the three PMX size fractions, the PM1/PM10 and PM2.5/ colder period, respectively. Highest concentrations occurred thus during PM10 ratios were lower in the warmer period (Fig. 2), indicating that the colder period, mainly due to platform ventilation differences between the ventilation of the subway system was more efficient removing seasons. The new station (Llefià) showed on average PM2.5 concentra- coarser particles. Thus, the PM1 were the principal size fraction compos- tions lower (around 50%) than the old stations (Joanic, Santa Coloma ing the PM in the subway system, especially during the warmer period. and Tetuan), which is probably attributable to the design of the stations, On the platforms, the PMX concentrations were lower during week- but also due to the less train frequency and more advanced ventilation ends, probably due to the lower frequency of trains, as Aarnio et al. setup. Nieuwenhuijsen et al. (2007) mentioned that the high levels of (2005) and Johansson and Johansson (2003) observed in the Helsinki PM could be observed in underground environments resulting from the and Stockholm subway systems, respectively. The average weekday generation or accumulation of PM in a confined space, particularly in values were between 1.2 and 1.5 times higher than those measured old subway systems. on weekends. Averages, maximum and minimum PM2.5 concentrations for operating hours and standard deviations for the four stations are 3.1.3. Daily patterns summarized in Table S2 for weekdays and weekends.

Average intra-day variations of PMX and CO2 concentrations are plotted versus the train traffic frequency separately for weekdays, Sat- 3.1.4. Influence of different ventilation settings urdays and Sundays on the Joanic and Llefià platforms in Fig. 2,for Regarding ventilation settings, several protocols (Table 2) were test- both warmer and colder periods. Similar daily trends of PMX and CO2 ed during weekly periods to detect PMX concentration differences and concentrations were observed among the conventional stations (Joanic, determine the best operating conditions for optimizing the air quality Santa Coloma and Tetuan), only Joanic is shown as example in Fig. 2. on the platforms. The ventilation modes varying during day/night and

The PMX daily pattern during weekdays of the warmer period pre- platform/tunnel, were the same for the three old stations monitored, sents a concentration increase in the morning with the arrival of the being Joanic (old) shown as example in Fig. 3, together with Llefià first trains until the maximum concentration at around 6 a.m., when (new, with PSDs system). the ventilation rate increased. From then the PMX concentration de- Generally, on the old platforms, when comparing the I.W and II.W creased towards a rather stable concentration throughout the day. modes (Table 2, with different ventilation in tunnel at night) higher

With the reduction of the ventilation rate at around 9 p.m. the PMX PMX concentrations were recorded in the situation II during night- levels rise again until midnight (when the trains operation stops) and time hours (see shadow area in Fig. 3a, b), evidencing that the impulsion tends to decrease during the night. In the conventional stations, in- of outdoor air was more efficient than the extraction of indoor air for air creases in PMX concentration up to a factor of 2 were observed around quality purposes. The same effect of ventilation result was obtained dur- 3 a.m., and they were associated with occasional night-time mainte- ing the I.C and III.C modes, also with different ventilation in tunnel at nance or cleaning operations. However, for Joanic W (Fig. 2)there night (only I.C mode shown in Fig. 3c). Note that the general concentra- were higher average concentrations during the night than during sub- tions during the colder period were higher, attributed to the lower day- way operating hours, mainly due to the intense maintenance works or time ventilation than in the warmer period, as previously discussed. The cleaning operations, as previously discussed. The CO2 concentrations ventilation mode II.C (Fig. 3d) was tested to observe if the piston effect can also have a slight peak caused by the workers' exhalation and by (with no additional mechanical ventilation in the tunnel) produced by the use of machinery. The highest peak of CO2 concentrations on week- the movement of the trains was enough to reach a good air quality in- days was found in the morning rush hour between 7 and 9 a.m., due to side the subway system. On average the PM2.5 concentrations were the higher influx of commuters. The commuters generate CO2 through around 29% higher during this week compared to the levels observed exhalation and at the same time they lead to the re-suspension of the with the normal ventilation in the colder period (I.C mode).

PMX created by walking. On weekends it is possible to observe the On Llefià platform, the ventilation III.W and IV.W modes resulted in same pattern in relation to the ventilation rate. On Saturday, the PMX similar diurnal patterns (only III.W mode shown in Fig. 3e) and the V.W levels only decrease after 2 a.m., when the trains stop operating, and mode resulted in higher PMX concentrations during all day (Fig. 3f). on the night of Saturday to Sunday the PMX concentration decreases These results revealed that changes in the ventilation settings on the gradually as the train frequency also decreases, which shows the train platform did not influence the air quality in the station, while the oppo- frequency influence in the absence of strong ventilation. Hence, the site was observed for the tunnel ventilation, demonstrating that only daily pattern of PMX and CO2 concentrations was primarily influenced the changes in the tunnel ventilation were relevant in the air quality by the ventilation settings and secondarily by the train frequency. The within the new system.

PMX concentrations on the platforms are the result of a dynamic system There were no differences among the ventilation modes tested in the controlled by the train frequency (source) and ventilation settings (re- colder period (IV.C, V.C and VI.C), but the use of a lower number of fans moval), however, it is evident that the impact of train frequency on on the platforms resulted in higher PMX concentrations. PMX levels only becomes relevant when lower ventilation rates occur (Fig. 2). 3.1.5. Spatial and temporal variations along platforms In the colder period it is possible to observe that the stable and rela- Some clear spatial and temporal trends were obtained among all tively low concentration registered in the warmer period (with stronger measurements, although in some platforms there were day-to-day ventilation) is replaced by higher concentrations that tend to increase fluctuations in PMX concentrations. Representative cases are discussed during the day, especially during weekend, reflecting the increasing below (Fig. 4), whereas all the results for PM2.5 are displayed in number of trains, and probably enhanced by the accumulation of parti- Table S1. As mentioned before, the PMX concentration on the platforms cles in the station caused by the weaker ventilation during this time of was generally lower in the warmer period, when compared with the 716

120 15 Joanic W Joanic W Joanic W 450 14 (Weekdays) (Saturdays) (Sundays) 100 400 13

80 350 12 )

-3 300 11 60 10

250 (ppm) 2

X 40 9

200 CO 8 PM 150 7

20 100 Train frequency (trains/hour) 6

0 5

120 15 450 Joanic C C Joanic Joanic C 14 711 (2015) 511 Environment Total the of Science / al. et Martins V. 100 (Weekdays) (Saturdays) (Sundays) 400 13

350 80 12 )

-3 300 11 60 10

250 (ppm) 2 9 X 40

200 CO 8 PM 150 7

20 100 Train frequency (trains/hour) 6 5 0

15 120 450 Llefià W Llefià W Llefià W 14 (Weekdays) (Saturdays) (Sundays) 100 400 13

350 80 12 )

-3 300 11 60 10

250 (ppm) 2

X 40 9

200 CO

PM 8 150 7

20 100 Train frequency (trains/hour) 6

0 5

120 15 – Llefià C Llefià C Llefià C 450 722 (Weekdays) (Saturdays) (Sundays) 14 100 400 13

350 80 12 )

-3 300 11 60 250 10 (ppm) 2

X 40 9

200 CO

PM 8 150 7

20 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 100 Train frequency (trains/hour) 6

0 5 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 019 120 221 322 423 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23

Time Time Time

Fig. 2. Relation between the train frequency per hour and the hourly average PM10 ,PM2.5 ,PM1 and CO2 concentrations on the subway platforms of Joanic (old) and Llefià (new) stations, during weekdays, Saturdays and Sundays in both periods (W — warmer period; C — colder period). The (lower) night ventilation is highlighted in grey. See text for details. V. Martins et al. / Science of the Total Environment 511 (2015) 711–722 717

Fig. 3. Hourly average PM10,PM2.5,PM1 and CO2 concentrations and train frequency on the subway platforms of Joanic and Llefià stations with different ventilation settings. The night ventilation is highlighted in grey (W — warmer period; C — colder period).

colder period, due to the increased use of ventilation throughout the Gorg station, which is located in the end of one of the new lines and station diluting PMX (Figs. 4a, b, 5). In addition in colder period, the has an uncommon design (directly connected with the street level in PMX concentrations on the platforms were generally more variable in the P4 location), also shows high PMX concentrations at the point of shorter time scales (five second periods) ranging for example from 33 entry and exit of the train (P1, Fig. 4g) caused by the trains' motion. −3 to 133 μgPM2.5 m in Joanic station. Smaller concentration peaks were observed along the rest of the plat- High time resolution PMX measurements evidenced that PMX con- form related to the open PSDs while the train was stopped, allowing centrations on the platform increased when the train entered the plat- air exchanges between the tunnel and the platform. In any case the form pushing in polluted air from the tunnel (by piston effect) and PMX concentrations in the rest of the platform were lower than those decreased when it departed. While the train was stopped in the station measured in other stations, which can be strongly influenced by out- the PMX concentration on the platform was kept stable, due to polluted door air that may enter the station, influencing the dilution of PMX. air introduced by piston effect and PMX generated by resuspension. The In the areas closer to the passengers' access to the platforms there is decrease of PMX concentrations when the train left the station can also also a high probability of air turbulences, created by the commuters be explained by the reverse piston effect as the train moves polluted air walking and the air flowing in and out of the station. This turbulence from the station, renewing the air of the platform. The same PMX time can cause PMX resuspension, which explains the higher mass concen- patterns were described by Salma et al. (2007) for the Budapest subway, trations measured in these points at Llucmajor and Encants stations although different patterns were found in other study (Ma et al., 2014). (Fig. 4e, f), as it has already been described by Moreno et al. (2014).

The passage of trains was a very important factor in the PMX concentra- However, due to the design of both stations (one wide tunnel with tions on some platforms, being especially strong in the new stations two rail tracks without middle wall) it is impossible to assure if the (Fig. 4c) and with single rail track (Fig. 4d). In some stations with two nearest point of entry of the train had also influence in these results. rail tracks without middle wall (Fig. 4e, f) this pattern was also observed More specific measurements will be required in these cases. but in general less frequently. Measurements carried out with normal ventilation used in the

In some stations, higher PMX mass concentrations, especially the colder period (C1) and without ventilation in the tunnel (C2, as the coarse particles, were recorded at one end of the platform, coinciding II.C ventilation mode in Table 2), allowed evidencing different spatial with the train entry edge, and a clear decreasing trend for PMX concen- variation of PMX concentrations in some stations (Fig. 4d, h). When trations was observed along the platform (Fig. 4c). This variation can be the ventilation of the tunnel was turned off (i.e. only piston effect ven- attributed to the turbulence generated by the trains entry, due to the tilation, Table S1), average PMX concentrations on the platform were wind blasts caused by the trains when they pull into the stations. 26% higher than those registered on a fully operational ventilation sys-

The results obtained in the new lines equipped with PSDs showed tem, indicating an accumulation of PMX in the tunnel. This percentage that this system, despite being an effective security barrier, does not was very similar to the result obtained for the extensive campaigns on prevent completely air exchange between the railway and the platform. the four platforms studied on daily basis (29%).

Therefore, the PMX values were also influenced by the arrival and Overall, a substantial variation in PMX concentrations between departure of trains similarly to older platforms. distinct subway stations was observed (averages ranging from 13 to 718 V. Martins et al. / Science of the Total Environment 511 (2015) 711–722

PM10 PM2.5 PM1 Train arrival Train departure Train direction

a) Joanic W b) Joanic C 250 250

200 200 ) -3

) P1 P2 P3 P4 P1 P2 P3 P4 -3 150 150 X

X 100 100 PM PM 50 50

0 access 0 access 12:03 13:05 12:04 13:10 Time (hh:mm) Sampling date: 4 Apr 2013 Time (hh:mm) Sampling date: 31 Oct 2013

c) Onze de Setembre C d) Tetuan C1 250 250

200 200 ) ) 3 - P1 P2 P3 P4 -3 150 150 X X 100 100 PM PM

50 50 P1 P2 P3 P4

0 access 0 access 13:22 14:32 9:00 10:06 Time (hh:mm) Sampling date: 30 Jan 2014 Time (hh:mm) Sampling date: 19 Dec 2013

e) Llucmajor C f) Encants C 250 250

200 200 ) ) 3 -3 P1 P2 P3 P4 - P1 P2 P3 P4 150 150 X X 100 100 PM PM

50 50

0 access access 0 access 10:42 11:46 13:17 14:26 Time (hh:mm) Sampling date: 21 Nov 2013 Time (hh:mm) Sampling date: 28 Nov 2013

g) Gorg W h) Tetuan C2 250 250

200 200 ) 3 -

) P1 P2 P3 P4 3 - 150 150 X

X 100 100 PM PM

50 50 P1 P2 P3 P4

0 access access 0 access 10:21 11:30 9:05 10:11 Time (hh:mm) Sampling date: 21 Jun 2013 Time (hh:mm) Sampling date: 5 Dec 2013

Station design:

one wide tunnel with two rail tracks separated by a middle wall single narrow tunnel with one rail track

one wide tunnel with two rail tracks without middle wall single tunnel with one rail track separated from the platform by a wall with PSDs

Fig. 4. Concentrations profiles of PM10,PM2.5 and PM1 on selected platforms at 4 different sites (P1, P2, P3 and P4). Locations of accesses to platforms, train arrival/departure and direction are indicated (W — warmer period; C — colder period).

−3 154 μgm of PM2.5, Table S1. — excluding the piston effect measure- lower (around 50%) than the conventional system (Fig. 5), as previously ments) and this might be related to the differences in the length and mentioned (Section 3.1.2). Among the conventional system, the stations design of the stations and tunnels, variations in the train frequency, pas- with single narrow tunnel and one rail track showed on average PM2.5 senger densities and ventilation systems, among other factors (Moreno concentrations higher than those observed in stations with one wide et al., 2014 and references therein). In general, the stations composed by tunnel and two rail tracks separated by a middle wall. The stations a single tunnel with one rail track separated from the platform by a wall with one wide tunnel and two rail tracks without middle wall presented with PSDs (new system) showed on average PM2.5 concentrations average PMX concentrations much more variable (Fig. 5). V. Martins et al. / Science of the Total Environment 511 (2015) 711–722 719

160 Colder Warmer

120 ) -3

80 2.5 PM 40

0

single tunnel with one one wide tunnel with two single narrow one wide tunnel with two rail tracks without middle wall rail track separated from rail tracks separated by a tunnel with one the platform by a wall middle wall rail track with PSDs

NEW SYSTEM CONVENTIONAL SYSTEM

Fig. 5. Average PM2.5 concentrations for colder and warmer periods of measurements sorted by subway station design categories.

3.2. PMX concentrations inside the trains concentrations monitored along the lines presented temporary in- creases after the train doors close in a number of cases, possibly due All measurements carried out inside trains from 6 different lines are to turbulence and consequent PM re-suspension produced by the shown in Table 3.Lines1–5 are the oldest ones whereas line 10 is one of movement of passengers inside the train. The CO2 concentration profile the newest, most technologically advanced lines with more efficient was most probably proportional to the number of passengers inside the mechanical ventilation system. PM2.5 concentrations inside the trains carriages of the trains. Hence the CO2 concentrations presented always in line 10 were on average 2.2 times lower than in the rest of the lines the maximum peak in the central part of each line, coinciding with the (Table 3). Repetition of measurements showed important variations in maximum influx of people. some cases (Table 3) indicating a possible dependency on variables such as the number of passengers (not counted in this campaign) al- 3.3. Comparison with other studies though measurements were done at the same hours of the day. Regard- ing seasonal variations, there was not a regular variability among all Table 4 shows a comparison of the average PM2.5 concentrations on fl fi results, perhaps in uenced by changes of the air lters in the trains subway platforms worldwide with the results of this study. PM2.5 levels (the trains are fitted with air filters coupled to the air conditioning sys- measured in the conventional system were in the range of those mea- tem that are changed monthly). A future study will be performed taking sured in Budapest (Salma et al., 2007), Helsinki (Aarnio et al., 2005), into account the changes of the filters and analysing their influence in Los Angeles (Kam et al., 2011a), New York (Wang and Gao, 2011), the PMX measurements to obtain a conclusive result. Mexico (Mugica-Álvarez et al., 2012) and Paris (Raut et al., 2009), and From the measurements carried out with and without air condition- were lower than those from London (Seaton et al., 2005), Buenos ing, it is possible to conclude that the air conditioning had a clear effect Aires (Murruni et al., 2009) and Shanghai (Ye et al., 2010). The average on both concentration and variability of PMX inside the trains. The re- PM2.5 value referred by Kim et al. (2012) to the PSDs system present sults indicate that the ventilation system provides a clear abatement also in Seoul was relatively higher than the result obtained in the cur- of PM concentrations inside the trains (Fig. 6), resulting in lower PMX rent study for a similar system (L10). The PM concentrations found in concentrations (by around 47% for PM2.5) and finer particles (around the present study were lower than those found in a previous study per- 15% finer). Similarly, a study in Hong Kong also reported that the filter formed in July 2011 in 2 stations of Barcelona subway (Querol et al., in the air-conditioning system was supposed to be capable of removing 2012) for both conventional and new systems. the larger portion of coarse particles (Chan et al., 2002a). Given that the lowest PMX concentrations were found in the new The PMX and CO2 concentration profiles during trips inside line both on the platforms and inside the trains, it is possible to conclude trains showed dissimilar behaviours (Fig. 6). Generally, the PMX that PMX levels inside the trains were affected by the surrounding

Table 3

Average PM2.5 concentrations inside the trains in both periods of measurements.

Warmer period Colder period

−3 −3 Sampling date PM2.5 (μgm ) Sampling date PM2.5 (μgm ) With air conditioning With air conditioning Without air conditioning

Line 1 05 Jul 2013 74.8 11 Feb 2014 42.1 58.9 Line 1 repetition 19 Jul 2013 59.5 04 Mar 2014 38.9 56.6 Line 2 09 May 2013 34.4 26 Nov 2013 37.5 77.1 Line 2 repetition 16 May 2013 30.2 17 Dec 2013 46.4 98.8 Line 3 24 May 2013 43.8 11 Nov 2013 62.9 75.5 Line 3 repetition 29 May 2013 49.4 09 Dec 2013 71.6 90.9 Line 4 08 Apr 2013 29.3 29 Oct 2013 63.2 87.1 Line 4 repetition 19 Apr 2013 51.1 18 Nov 2013 43.9 72.9 Line 5 12 Jun 2013 43.3 20 Jan 2014 19.2 27.7 Line 5 repetition 28 Jun 2013 41.2 24 Feb 2014 39.1 47.1 Line 10 05 Jun 2013 30.7 14 Jan 2014 23.6 30.1 Line 10 repetition 20 Jun 2013 20.3 27 Jan 2014 18.6 21.2 720 V. Martins et al. / Science of the Total Environment 511 (2015) 711–722

PM10 PM2.5 PM1 Train doors open Train doors close CO2 T 800 23.5

Line 2 (Winter) 23 600 22.5 C)

22 O 400 (ppm)

2 21.5 Temp. ( Temp. CO 21 200 20.5 1 0 20 200

160 ) 3 - 120

X 80 PM 40

0 10:33 11:38 Time (hh:mm)

Fig. 6. PMX,CO2 concentrations and temperature measured inside the train of line 2, with and without (grey area) air conditioning. conditions, such as those on the platforms. Hence, air exchange between measurements inside the trains, the data used were obtained in the platforms and the inside of the trains occurred when doors were open commuters normal conditions during the warmer period (with air con- with the consequent exchange of air pollutants. Table 4 presents the ditioning) and without air conditioning in the colder period for the ex-

PM2.5 levels inside the trains for different subway systems worldwide. posure calculations. PM2.5 levels inside the trains in the conventional Barcelona subway sys- For a subway commuting travel of 30 min in the train and 5 min on the −3 tem are lower than those measured in Seoul (Kim et al., 2008; Park and platform, the average PM2.5 exposure would reach 53 μgm .Thisvalue Ha, 2008), Beijing (Li et al., 2007) and London (Seaton et al., 2005), and was reduced to 27 μgm−3 inthecaseofline10,whereasforL1,L2,L3,L4 similar to those measured in Mexico (Gómez-Perales et al., 2004) and and L5 lines the exposures were 66, 62, 67, 59 and 40 μgm−3,respective-

New York (Chillrud et al., 2004). The average PM2.5 levels of the remain- ly. The average commuter exposure levels for the warmer and colder pe- −3 ing subway systems referred in Table 4 are more similar to the value ob- riods among all lines were 43 and 63 μgm of PM2.5, respectively, tained for the new system. Both in the conventional and new systems emphasizing that in the colder period the commuters are exposed to the average concentrations inside the trains found in the present worse air quality when commuting. When air conditioning was switched study were higher than those from the previous study in Barcelona on, a decrease of 32% of PM2.5 exposure levels was reached, being an effec- (Querol et al., 2012). tive approach to reduce exposure levels. Comparing the results with previous worldwide studies measuring It has been recognized in several studies that concentrations inside the concentrations of PMX on subway platforms and inside the trains, the trains are lower than in subway stations (Chillrud et al., 2004; there is a remarkable variation among respective results. This could be Aarnio et al., 2005; Seaton et al., 2005; Braniš, 2006), suggesting that explained by differences in the monitoring conditions such as the time spent in stations may be a better predictor of personal exposure time, place, or season of the measurements, the differences in the length than total time spent underground. The exposure is repeated almost and design of the stations and tunnels, the system age, the wheels and every day for most commuters, which may cause cumulative or chronic rail-track materials, the type of brake mechanism, the train speed and health effects over time. Nevertheless, higher health risks for sensitive frequency, the measurement equipment used, the ventilation systems, groups, such as children, the elderly, and individuals with pre-existing the passenger density, among other factors (Moreno et al., 2014 and health conditions exacerbated by air pollution (many respiratory and references therein). Therefore, the results are not always directly cardiovascular diseases) may be significant, even for short periods comparable because of differences in sampling methods, data analysis, spent in underground environment (Salma et al., 2007). Train drivers duration of the measurements and the type of environment studied and other workers, who spend several hours a day within the under-

(Nieuwenhuijsen et al., 2007). ground subway are subject to higher exposure to PMX levels than the The PM2.5 concentrations inside the trains were lower (around 15%) commuting public and thus possibly greater health risks. In a study of than those on station platforms (Table 4). These measurements results PM exposure of pregnant women in Barcelona, a train/subway source can be explained by PM that was re-suspended on platforms due to contribution was identified, and its contribution was found not related train or commuter movement. Moreover, PM concentrations can also to the time spent during commuting but only to the fact of using the be diluted rapidly via the air conditioners inside the trains as the subway, pointing to a maximum exposure on the platform, as opposed space is confined during operation. Nieuwenhuijsen et al. (2007) impli- to inside the train (Minguillón et al., 2012). cated the air conditioning in trains as a possible factor favouring low PM The average PM2.5 exposure during subway commuting in Barcelona levels inside the trains. obtained in the current study is higher than that reported (26 μgm−3) by Querol et al. (2012), which might be related to the higher amount of

3.4. PM2.5 exposure during subway commuting measurements carried out in this study. Comparing to subway systems worldwide, the PM2.5 exposure obtained in the current study was also −3 The PM2.5 exposure was calculated taking into account all data ob- higher than that reported in Mexico (33 μgm ; Gómez-Perales et al., tained during both the intensive campaign on the 4 selected stations 2007), Taipei (35 μgm−3; Tsai et al., 2008), Hong Kong (33 μgm−3; and the additional 20 platform measurements. Regarding the Chan et al., 2002a) and Guanzhou (44 μgm−3; Chan et al., 2002b), V. Martins et al. / Science of the Total Environment 511 (2015) 711–722 721

Table 4

Comparison of PM2.5 concentrations measured on platforms and inside the trains at different subway systems worldwide.

−3 Measurement City PM2.5 (μgm ) Reference environment Range Average (min–max)

On the platform Budapest – 51 Salma et al. (2007) Helsinki 23–103 60 Aarnio et al. (2005) London – 270–480 Seaton et al. (2005) Los Angeles 9–130 57 Kam et al. (2011a) New York 60–77 68 Wang and Gao (2011) New York – 62 Chillrud et al. (2004) Buenos Aires – 152–270 Murruni et al. (2009) Mexico 41–67 48 Mugica-Álvarez et al. (2012) Paris – 61–93 Raut et al. (2009) Seoul 82–176 129 Kim et al. (2008) Seoul – 105 Park and Ha (2008) Seoul 39–129 66 Kim et al. (2012) Seoul PSDsa 20–166 58 Shanghai – 287 Ye et al. (2010) Stockholm WDb 105–388 258 Johansson and Johansson (2003) Stockholm WEc 24–334 185 Taipei 7–100 35 Cheng et al. (2008) Barcelona L3d 110–186 125 Querol et al. (2012) Barcelona L9e 12–99 46 Barcelona L1, L2, L3, L4 and L5d 18–154 66 Current study Barcelona L10e 13–61 32

Inside the train Beijing – 113 Li et al. (2007) Beijing 13–111 37 Li et al. (2006) Guangzhou – 44 Chan et al. (2002b) Helsinki 17–26 21 Aarnio et al. (2005) Hong Kong 21–48 33 Chan et al. (2002a) London – 130–200 Seaton et al. (2005) Los Angeles 11–62 24 Kam et al. (2011b) Mexico 31–99 57 Gómez-Perales et al. (2004) Mexico 8–68 – Gómez-Perales et al. (2007) New York 34–44 40 Wang and Gao (2011) New York – 62 Chillrud et al. (2004) Seoul 115–136 126 Kim et al. (2008) Seoul – 117 Park and Ha (2008) Sydney – 36 Knibbs and de Dear (2010) Taipei 8–68 32 Cheng et al. (2008) Taipei 3–48 24 Cheng et al. (2012) Barcelona L3 and L5d 17–32 25 Querol et al. (2012) Barcelona L9e 11–18 15 Barcelona L1, L2, L3, L4 and L5d 28–99 57 Current study Barcelona L10e 20–31 26

a PSDs—platform screen doors. b WD—weekdays. c WE—weekends. d Conventional system. e New system.

and lower than that referred for London (202 μgm−3; Adams et al., matter mass concentration. The following main conclusions were 2001)andNewYork(62μgm−3; Chillrud et al., 2004). drawn:

In addition, an assessment on PM2.5 exposure for different com- muting modes reported in several studies worldwide (Querol et al., • PMX concentrations on the platforms were higher than those in out- 2012 and references therein) was done to compare the data obtain- door environment approximately 1.3–6.7 times, revealing the preva- ed in the present study. The PM2.5 exposure while commuting by lence of PM sources on the platform and tunnel level. bus and passenger car reached values in the range of • The new system (L10) with PSDs showed on average PMX concentra- 33–75 μgm− 3 and 22–83 μgm− 3, respectively, comparable to tions lower (around 50%) than the conventional system (L1–L5). those reported for subway commuting in Barcelona during • The measured PM2.5 concentrations on all types of platforms were this study (27–67 μgm− 3). Cycling/motorbike and pedestrian lower or in the range of other reported subway systems worldwide. commuting were reported with PM2.5 exposure levels of 68–88 • The measurements in the warmer period (strong ventilation) showed and 63 μgm− 3, respectively, being markedly higher than in the lower concentrations than in the colder period (weak ventilation).

Barcelona subway. Variations in PMX levels in different seasons were thus clearly influ- enced by the ventilation system. This suggests that an appropriate ventilation mode should be applied to the subway system to obtain 4. Conclusions both PM reduction and energy saving. • The piston effect alone (with no additional mechanical ventilation in Subway aerosol particles have been monitored in Barcelona on di- the tunnel) produced by the movement of the trains was not an effec- verse platform stations and inside the trains, focusing on particulate tive approach to obtain a good air quality in the subway system. 722 V. Martins et al. / Science of the Total Environment 511 (2015) 711–722

Gómez-Perales, J.E., Colvile, R.N., Nieuwenhuijsen, M.J., Fernández-Bremauntz, A., • PMX concentrations displayed a typical diurnal cycle during the week- Gutiérrez-Avedoy, V.J., Páramo-Figueroa, V.H., et al., 2004. Commuters' exposure to days, driven by the ventilation settings and secondarily by the train PM2.5, CO, and benzene in public transport in the metropolitan area of Mexico City. frequency. Atmos. Environ. 38 (8), 1219–1229. Gómez-Perales, J.E., Colvile, R.N., Fernández-Bremauntz, A.A., Gutiérrez-Avedoy, V., Páramo- • Both lower PMX concentrations and less marked cycles were observed Figueroa, V.H., Blanco-Jiménez, S., et al., 2007. Bus, minibus, metro inter-comparison of on Saturdays and Sundays. commuters' exposure to air pollution in Mexico City. Atmos. Environ. 41 (4), 890–901. • Real-time measurements of PMX showed temporal and spatial varia- Gustavsson, P., Bigert, C., Pollán, M., 2008. Incidence of lung cancer among subway drivers tions along the platforms, related to the differences in the time, in Stockholm. Am. J. Ind. Med. 547 (51), 545–547. place, or season of the measurements, design of the stations and tun- Johansson, C., Johansson, P.-A., 2003. Particulate matter in the underground of Stockholm. Atmos. Environ. 37, 3–9. nels, variations in the train frequency, passenger densities and venti- Jung, H.-J., Kim, B., Ryu, J., Maskey, S., Kim, J.-C., Sohn, J., et al., 2010. Source identification lation systems, among other factors. of particulate matter collected at underground subway stations in Seoul, Korea using – • The use of air conditioning inside the trains was an effective approach quantitative single-particle analysis. Atmos. Environ. 44 (19), 2287 2293. Kam, W., Cheung, K., Daher, N., Sioutas, C., 2011a. Particulate matter (PM) concentrations to reduce exposure levels. The PMX concentrations inside the trains in underground and ground-level rail systems of the Los Angeles Metro. Atmos. Envi- were lower (around 15%) than those on station platforms. ron. 45 (8), 1506–1516. • The ventilation and air conditioning systems were more efficient re- Kam, W., Ning, Z., Shafer, M.M., Schauer, J.J., Sioutas, C., 2011b. Chemical characterization and fi fi redox potential of coarse and ne particulate matter (PM) in underground and ground- moving coarse particles, resulting in a relatively ne-dominated PM levelrailsystemsoftheLosAngelesMetro.Environ.Sci.Technol.45(16),6769–6776. in the subway system. Karlsson, H.L., Ljungman, A.G., Lindbom, J., Möller, L., 2006. Comparison of genotoxic and • This study shows that the time spent commuting in the subway sys- inflammatory effects of particles generated by wood combustion, a road simulator and collected from street and subway. Toxicol. Lett. 165 (3), 203–211. tem can contribute substantially to total daily exposure to PM2.5 and Karlsson, H.L., Holgersson, A., Möller, L., 2008. Mechanisms related to the genotoxicity of be associated with adverse health effects. particles in the subway and from other sources. Chem. Res. Toxicol. 21 (3), 726–731. Kim, K.Y., Kim, Y.S., Roh, Y.M., Lee, C.M., Kim, C.N., 2008. Spatial distribution of particulate

matter (PM10 and PM2.5) in Seoul Metropolitan Subway stations. J. Hazard. Mater. 154 (1–3), 440–443. Kim, K.-H., Ho, D.X., Jeon, J.-S., Kim, J.-C., 2012. A noticeable shift in particulate matter levels Acknowledgements after platform screen door installation in a Korean subway station. Atmos. Environ. 49, 219–223. fi This study was supported by the Spanish Ministry of Economy and Knibbs, L.D., de Dear, R.J., 2010. Exposure to ultra ne particles and PM2.5 in four Sydney transport modes. Atmos. Environ. 44 (26), 3224–3227. Competitiveness and FEDER funds (METRO CGL2012-33066), the Levy, J.I., Houseman, E. a, Ryan, L., Richardson, D., Spengler, J.D., 2000. Particle concentra- European Union Seventh Framework Programme (FP7/2007-2013) tions in urban microenvironments. Environ. Health Perspect. 108 (11), 1051–1057. under grant agreement no. 315760 HEXACOMM and the IMPROVE LIFE Li, T.-T., Bai, Y.-H., Liu, Z.-R., Liu, J.-F., Zhang, G.-S., Li, J.-L., 2006. Air quality in passenger cars of – Project (LIFE13 ENV/ES/000263). Fulvio Amato is a beneficiary of an thegroundrailwaytransitsysteminBeijing,China.Sci.TotalEnviron.367(1),89 95. Li, T.-T., Bai, Y.-H., Liu, Z.-R., Li, J.-L., 2007. In-train air quality assessment of the railway AXA Research Fund postdoctoral grant. The authors would like to thank transit system in Beijing: a note. Transp. Res. Part D: Transp. Environ. 12 (1), 64–67. the Transports Metropolitans de Barcelona METRO staff who Ma, H., Shen, H., Liang, Z., Zhang, L., Xia, C., 2014. Passengers' exposure to PM2.5,PM10,andCO2 – arranged the sampling campaign and contributed actively to this work. in typical underground subway platforms in Shanghai. Lect. Notes Electr. Eng. 237 245. Minguillón, M.C., Schembari, A., Triguero-Mas, M., de Nazelle, A., Dadvand, P., Figueras, F.,

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V. Martins, T. Moreno, M.C. Minguillón, X. Querol. Air quality in stations and trains in the Barcelona subway system . POSTER. 2015 SETAC Europe, 25th Annual Meeting. Barcelona, Spain. May 2015

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