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Title

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

AuthorsMoreno, Teresa CSIC ORCID ; Martins, Vânia ORCID; Querol, Xavier CSIC ORCID ; Jones, Tim Peter; Jones, Tim Peter; Minguillón, María Cruz CSIC ORCID ; Amato, Fulvio CSIC ORCID ; Capdevila, Marta; de Miguel, Eladio; Centelles, Sonia; Gibbons, Wes
KeywordsNanoparticles
Subways
Platform air quality
Iron oxides
Subway PM
Air quality
TEM
SEM
Issue Date1-Feb-2015
PublisherElsevier
CitationScience of the Total Environment 505: 367-375 (2015)
AbstractMost 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.
Publisher version (URL)https://doi.org/10.1016/j.scitotenv.2014.10.013
URIhttp://hdl.handle.net/10261/185855
DOIhttp://dx.doi.org/10.1016/j.scitotenv.2014.10.013
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