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Most particles breathed on rail subway platforms are highly ferruginous (FePM) and extremely small (nanometric to a few microns in size). High magnification observations of particle texture and chemistry on airborne PM10 samples collected from the Barcelona Metro, combined with published experimental work on particle generation by frictional sliding, allow us to propose a general model to explain the origin of most subway FePM. Particle generation occurs by mechanical wear at the brake-wheel and wheel-rail interfaces, where magnetic metallic flakes 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 (10-20nm) that become pseudomorphed by maghemite. Continued oxidation results in extensive alteration of the magnetic nanostructure to more rounded aggregates of non-magnetic hematite nanocrystals, with magnetic precursors (including iron metal) still preserved in some particle cores. Particles derived from steel wheel and rails contain a characteristic trace element chemistry, typically with Mn/Fe=0.01. Flakes released from brakes are chemically very distinctive, depending on the pad composition, being always carbonaceous, commonly barium-rich, and texturally inhomogeneous, with trace elements present in nanominerals incorporated within the crystalline structure. In the studied subway lines of Barcelona at least there appears to be only a minimal aerosol 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. ; This work was supported by the + D Project CGL2012-33066 (METRO), the Spanish Ministry of Economy and Competitiveness and FEDER funds within the I European Union Seventh Framework Programme (FP7/2007–2013) under grant agreement no. 315760 HEXACOMM and the IMPROVE LIFE project ( LIFE13 ENV/ES/000263 ). F. Amato is beneficiary of an AXA Research Fund postdoctoral grant. We acknowledge useful discussion with our colleague Greg Power. ; Peer reviewed

The present work assesses indoor air quality in stations of the Barcelona subway system. PM2.5 concentrations on the platforms of 4 subway stations were measured during two different seasons and the chemical composition was determined. A Positive Matrix Factorization analysis was performed to identify and quantify the contributions of major PM2.5 sources in the subway stations. Mean PM2.5 concentrations varied according to the stations design and seasonal periods. PM2.5 was composed of haematite, carbonaceous aerosol, crustal matter, secondary inorganic compounds, trace elements, insoluble sulphate and halite. Organic compounds such as PAHs, nicotine, levoglucosan and aromatic musk compounds were also identified. Subway PM2.5 source comprised emissions from rails, wheels, catenaries, brake pads and pantographs. The subway source showed different chemical profiles for each station, but was always dominated by Fe. Control actions on the source are important for the achievement of better air quality in the subway environment. © 2015 The Authors. ; The present study was supported by the European Union Seventh Framework Programme (FP7/2007-2013) under grant agreement no. 315760 HEXACOMM, the Spanish Ministry of Economy and Competitiveness and FEDER funds ( METRO CGL2012-33066 ), and the IMPROVE LIFE project ( LIFE13 ENV/ES/000263 ). Fulvio Amato is beneficiary of an AXA Research Fund postdoctoral grant. The authors would like to thank the Transports Metropolitans de Barcelona METRO staff who arranged the sampling campaign and contributed actively to this work. Appendix A ; Peer reviewed

Air quality in subway systems is of interest not only because particulate matter (PM) concentrations can be high, but also because of the peculiarly metalliferous chemical character of the particles, most of which differ radically from those of outdoor ambient air. We report on the oxidative potential (OP) of PM2.5 samples collected in the Barcelona subway system in different types of stations. The PM chemical composition of these samples showed typically high concentrations of Fe, Total Carbon, Ba, Cu, Mn, Zn and Cr sourced from rail tracks, wheels, catenaries, brake pads and pantographs. Two toxicological indicators of oxidative activity, ascorbic acid (AA) oxidation (expressed as OPAA μg−1 or OPAA m−3) and glutathione (GSH) oxidation (expressed as OPGSH μg−1 or OPGSH m−3), showed low OP for all samples (compared with outdoor air) but considerable variation between stations (0.9–2.4 OPAA μg−1; 0.4–1.9 OPGSH μg−1). Results indicate that subway PM toxicity is not related to variations in PM2.5 concentrations produced by ventilation changes, tunnel works, or station design, but may be affected more by the presence of metallic trace elements such as Cu and Sb sourced from brakes and pantographs. The OP assays employed do not reveal toxic effects from the highly ferruginous component present in subway dust. © 2016 The Authors ; This study was supported by the Spanish Ministry of Economy and Competitiveness and FEDER funds (METRO CGL2012-33066 ), the I MPROVE LIFE Project ( LIFE13 ENV/ES/000263 ) and the European Union Seventh Framework Programme ( FP7/2007-2013 ) under grant agreement no. 315760 HEXACOMM. Support is also acknowledged to Generalitat de Catalunya 2014 SGR33 . Fulvio Amato is beneficiary of an AXA Research Fund postdoctoral grant . In the UK the research was funded by the National Institute for Health Research Health Protection Research Unit (NIHR HPRU) in Health Impact of Environmental Hazards at King's College London in partnership with Public Health England (PHE). The views expressed are those of the authors and not necessarily those of the NHS, the NIHR, the Department of Health or Public Health England. Special thanks to Wes Gibbons for improving an early draft of the manuscript. Appendix A ; Peer reviewed

Access to detailed comparisons in air quality variations encountered when commuting through a city offers the urban traveller more informed choice on how to minimise personal exposure to inhalable pollutants. In this study we report on an experiment designed to compare atmospheric contaminants inhaled during bus, subway train, tram and walking journeys through the city of Barcelona. Average number concentrations of particles 10-300 nm in size, N, are lowest in the commute using subway trains (N5.0×104cm-3), with extreme transient peaks at busy traffic crossings commonly exceeding 1.0×105cm-3 and accompanied by peaks in Black Carbon and CO. Subway particles are coarser (mode 90nm) than in buses, trams or outdoors (1200ppm in crowded buses and trains. There are also striking differences in inhalable particle chemistry depending on the route chosen, ranging from aluminosiliceous at roadsides and near pavement works, ferruginous with enhanced Mn, Co, Zn, Sr and Ba in the subway environment, and higher levels of Sb and Cu inside the bus. We graphically display such chemical variations using a ternary diagram to emphasise how "air quality" in the city involves a consideration of both physical and chemical parameters, and is not simply a question of measuring particle number or mass. © 2015 The Authors. ; This work was supported by the ACS Foundation contributing to the dissemination of good environmental practices and environmental protection activities, the Spanish Ministry of Economy and Competitiveness and FEDER funds within the I+D Project CGL2012-33066 (METRO), and the IMPROVE LIFE project ( LIFE13 ENV/ES/000263 ). VM and ASF acknowledge funding from the European Union Seventh Framework Programme ( FP7/2007-2013 ) for a Marie Curie ITN (Grant agreement no. 315760 HEXACOMM ). Additional funding from AXA Research Fund is acknowledged. Fulvio Amato is beneficiary of the Juan de la Cierva postdoctoral Grant ( JCI-2012-13473 ) from the Spanish Ministry of Economy and Competitiveness. Appendix A ; Peer reviewed