Airborne particulate matter emissions from vehicle brakes in micro- and nano-scales: Morphology and chemistry by electron microscopy
2019
Brake wear particles constitute a significant part of the non-exhaust related particulate matter (PM) associated to traffic. These particles derive from various components of the vehicles’ brake system and were recognized as an important pollution source only recently. In the present study, electron microscopy was applied to elucidate brake wear particle morphology and associated chemical composition. Sampling was carried out on a brake test bench. Particles were collected on 13 successive polycarbonate foil bags and a backup filter by means of a Dekati Low Pressure Impactor (DLPI), based on the equivalent aerodynamic particle diameter. The results revealed a broad size range from the micro-to the nano-scale, as measured on electron microscope images, including coarse (2.5–10 μm), fine (0.1–2.5 μm) and ultrafine ones (<0.1 μm), down to a few nanometers. It cannot be implied with certainty to what extent the particle size spectrum identified under the experimental conditions of the present study strictly applies to real-world conditions. The particles occur typically in form of aggregates, also in the ultrafine scale; single, i.e. non-agglomerated particles are more common in the ultrafine fraction than at larger sizes. Imaged particles have commonly rounded outlines probably due to friction and associated abrasion during braking; melting/evaporation at high temperature braking phases, associated with the rate of the generated frictional energy of braking, and subsequent rapid solidification can be invoked to interpret the nearly spherical shape of nanometer size particles. The individual constituents of the aggregates have various sizes and chemical composition. Energy dispersive X-ray analysis of the imaged aggregates and of their constituents showed that Fe is prevalent in all size fractions but is less pronounced in the smaller sizes, where Ca, occasionally in combination with S and/or P becomes more frequent. Other frequently analysed elements are Al, Sn, Mg, Si, Cr, Ti, K and W; less frequent are Ni, Zn, Zr, Ba, S, C, P, F, Mn and rarely Bi and Sb.
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