Research progress on the influencing factors of polycyclic aromatic hydrocarbons and derivatives from vehicle exhaust and non-exhaust emissions

Polycyclic aromatic hydrocarbons (PAHs) are a group of toxic organic compounds from vehicle emissions. Many PAHs are carcinogenic, teratogenic, mutagenic, and immunotoxic, causing a negative impact on human health and severe damage to the environment and ecosystems. Although PAH derivatives, including nitro-polycyclic aromatic hydrocarbons (NPAHs) and oxygenated polycyclic aromatic hydrocarbons (OPAHs), are one to three orders of magnitude lower in concentration than PAH parents, some components are far more mutagenic and carcinogenic than PAH parents. The PAHs and their derivatives in motor vehicle exhaust emission are mainly caused by the incomplete combustion of fossil fuels, and their emission characteristics vary with the combustion conditions and fuel compositions. With the increasingly strict control of exhaust emission standards and the gradual popularization of electric vehicles, non-exhaust emissions have become the main contributors to traffic air pollution. Therefore, as the main source of PAHs in an urban environment, non-exhaust emissions, including brake wear, tire wear, road dust resuspension, and road wear emissions, cannot be ignored in terms of their contribution proportion. The emission characteristics of PAHs and their derivatives from vehicles are mainly affected by many factors, such as combustion conditions, road conditions, and the types of motor vehicle parts and materials. This paper collates and summarizes the existing data on vehicle exhaust and non-exhaust emissions of PAHs and their derivatives at home and abroad. In general, for exhaust emission, stricter emission standards lead to lower emission of PAHs and their derivatives; under operating modes, including cold start and acceleration, the engine combustion efficiency is reduced, leading to an increase in emission; the emission of diesel vehicles is much higher than that of gasoline vehicles; gasoline direct injection (GDI) exhibits higher PAH emissions than port fuel injection (PFI); and emissions increase with increasing vehicle mileage. At present, studies on the non-exhaust emission of PAHs and their derivatives are lacking. Existing studies find that the chemical composition of brake pads, braking conditions, tire materials, and pavement conditions affect non-exhaust emissions, but these findings have a high degree of uncertainty and need further research. This paper is intended to analyze the emission characteristics of PAHs and their derivatives from motor vehicles under different influence factors to provide a scientific basis for developing emission control technology and formulating policy standards.