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

• Published in: The Science of the total environment 2015-02, Vol.505, p.367-375
• Main Authors: Moreno, Teresa, Martins, Vânia, Querol, Xavier, Jones, Tim, BéruBé, Kelly, Minguillón, Maria Cruz, Amato, Fulvio, Capdevila, Marta, de Miguel, Eladio, Centelles, Sonia, Gibbons, Wes
• Format: Article
• Language: English
• Subjects: Aerosols; Air Pollutants - analysis; Air Pollution, Indoor - analysis; Air Pollution, Indoor - statistics & numerical data; Brakes; Environmental Monitoring; Flakes; Iron oxides; Metals - analysis; Nanoparticles; Nanostructure; Particle Size; Particulate Matter - analysis; Platform air quality; Platforms; Railroads; Rails; SEM; Subway PM; Subways; Surface layer; TEM; Texture
• ISSN: 0048-9697; 1879-1026
• DOI: 10.1016/j.scitotenv.2014.10.013

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. •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.