Comprehensive elemental and physical characterization of vehicle brake wear emissions from two different brake pads following the Global Technical Regulation methodology

Abstract

Non-exhaust emissions have gained increasing attention during the last years, with the upcoming EURO 7 regulation defining maximum PM₁₀ emission factors for tire and brake emissions for the first time. This study, therefore, focusses on broadening the knowledge on chemical composition and physical characteristics of brake dust to define emission factors for heavy metal and organic pollutants. Particles from two pads were analyzed utilizing the Worldwide Harmonised Light Vehicle Test Procedure (WLTP) brake cycle. Geometric mean diameters for both pads were found with a bimodal distribution in the ultrafine range. PM₁₀ emission factors of 15.1 ± 0.1 mg/km and 16.3 ± 0.4 mg/km were measured, which is 2.15 and 2.32 times higher than upcoming maximum permitted emission factor of 7 mg/km. On average 54.9% and 58.1% of PM10 was emitted as iron, with a wide variety of Fe concentrations between 43 – 75% by mass found in individual particles. Other heavy metals, such as Cu, Cr, Mn and Zn, were also found and a high contribution of wear from the brake disc was noticeable, based on the elemental composition. Fe emission factors calculated from the WLTP brake cycle were 8-9 times higher than previously reported values in literature, while Cu levels were significantly lower based on recent trends in brake pad formulations. Four different PAH were detected even at the relatively low temperatures that are common for the WLTP brake test cycle.