Insights on non-exhaust emissions: An approach for the chemical characterization of debris generated during braking

Up to 50 % of total PM_2.5 emissions are due to particles derived from the automotive sector, and both exhaust and non-exhaust emissions contribute to the pollution of urban areas. Fuel incomplete combustion, or lubricant degradation due to high temperatures during the combustion process, are responsible for exhaust emissions. The non-exhaust ones concern brakes, tires and road surface-wear emissions and road resuspension contribution. The present study aims to provide a methodological approach for a detailed chemical characterization of wear friction products by means of a large array of techniques including spectroscopic tools, thermogravimetric analysis (TGA), chromatography, morphological and elemental analysis. The dust sample derived from the wear of a brake pad material was collected after a Noise & Vibration Harshness (NVH) test under loads similar to a Worldwide Light vehicle Test Procedure (WLTP) braking cycle. The TGA shows that only a small fraction is burned during the test in an oxidizing environment, testifying that the sample consists mostly of metals (more than 90 %). Fe exhibits the highest concentrations (50-80 %, even in the form of oxides). Also other kinds of metals, such as Zn, Al, Mg, Si, S, Sn, Mn, occur in small quantities (about 1-2% each). This finding is confirmed by X-ray diffraction (XRD) analysis. The organic fraction of the debris, investigated by means of Raman spectroscopy, has an evident aromatic character, probably due to oxidative phenomena occurring during the braking cycle test. Noteworthy, the extraction of the dust sample with organic solvents, revealed for the first time the presence of ultrafine particles (UFPs), even in the range of few nanometers (nanoparticles), and polycyclic aromatic hydrocarbons (PAHs), recognized as highly toxic compounds. The simultaneous presence of toxic organic carbon and metals makes of concern the non-exhaust emissions and mandatory a deep insight on their structure and detailed composition.