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With the implementation of stringent emission regulations and the growth of electric vehicles, more attention should be shifted to road traffic-derived non-exhaust emissions. Tyre wear particle (TWP) emissions have outweighed those from the exhaust and are potential for microplastic pollution. However, there are no regulations that specifically limit TWP emissions and no feasible technologies to curb the emissions. Due to the varied measurements and discrepant data, the related results are incomparable and uncertain. This paper gives a comprehensive overview of TWP from four aspects: formation mechanisms, experimental methods, emission characteristics, and influencing factors. Real-road TWP are generally incorporated with existing road particles, so the sampling methods need to be carefully designed to eliminate the influence of undesired particles. In this case, the TWP scattering paths, the effects of sampling flow rate and sampling inlet location, the design principles of sampling geometry based on sampling flow rate and vehicle speed are discussed. The micro-vibration and the stick-slip oscillation between the tyre and road, coupled with the evaporation of tread volatile compounds, lead to the TWP formation. Thus, TWP covers a broader size distribution, diverse shapes, and multiple chemical components. Further studies should be focused on the physicochemical properties of TWP by standardizing experimental procedures and narrow controlling the main influencing factors of vehicle driving behaviours, road types, tyre specifications, and the ambient environment. • TWP sampling, analysis, and parameters affecting the efficiency are summarized. • TWP formation, scattering path, emission, and physicochemical property are compared. • Effects of major influencing factors of driving and environment are discussed. [ABSTRACT FROM AUTHOR]

Particles emitted during braking account for a significant proportion of transport-related particles. The brake particles are still beyond supervision because no mandatory regulations were introduced to control the brake particle emissions. Many studies have been conducted about the brake particles, but the materials & methods used by these studies vary a lot, thus reducing the result comparability among these studies. This paper reviewed brake particle emissions from four aspects: experimental methods, emission characteristics, influencing factors, and future works. Six experimental methods corresponding to different application scenarios are summarized, and these methods require different modifications, skills, and expenses. Whatever the experimental methods, the sampling method needs careful design to reduce the particle transport loss and to ensure particle sampling efficiency. The reported brake particle emissions could exceed the sixth stage exhaust emission regulation (both EU and China), depending on the test procedures, friction materials, and data processing. The brake particle covers a broad size distribution (from a few nanometers to tens of microns), and more studies are needed about its physicochemical characteristics. The friction materials, system running-in, and friction temperature will significantly affect the brake particle emission characteristics, and these factors should be considered when designing the study procedure. The patterns about gaseous emission and non-brake particle emission need more attention, as these patterns might be helpful to understand the brake particle emission characteristics. Some filter&collection-based brake particle after-treatments have also been invented. Future studies should focus on the homogeneous test procedure, mechanism analysis, after-treatment, and non-airborne wear et al. • Different brake particle research methods were assessed. • Brake particle emission characteristics are summarized and compared. • Major factors affecting the brake particle emission are analyzed. • Future concerns for brake particle emission are proposed. [ABSTRACT FROM AUTHOR]