Magnetic and geochemical characterization of iron pollution in subway dusts in Shanghai, China

Dust samples collected from subway platforms in Shanghai, China, have been examined using magnetic measurements and geochemical analysis. Our results indicate that the Shanghai subway platform dusts have extremely strong magnetic signatures. These results, combined with X-ray diffraction analysis and scanning and transmission electron microscope examinations, indicate that the magnetic mineralogy of the dust is dominated by iron scraps due to wheel-rail mechanical abrasion and spherules rich in magnetite from fossil fuel combustion. Although the magnetic particles are primarily micrometer sized, fine submicron magnetic grains are also evident in the dust. The underground platform dusts have a much higher iron flake abundance and magnetic susceptibility than those from aboveground platforms because the latter ones are diluted by inputs of magnetically weaker ambient aerosols with a higher proportion of magnetite spherules. Geochemical analysis indicates that underground platform dusts have elevated Fe and Mn, but lower Al and Ti contents relative to aboveground subway dust. This is consistent with the closed nature of underground platforms, which therefore reduces exposure to soil-derived dust. Since the adverse environmental effects of subway particles may be linked to higher contents of iron and other metals, our results demonstrate that magnetic measurements provide a novel and effective approach for characterizing iron mineralogy and grain size in subway dusts.