Review on main sources and impacts of urban ultrafine particles: Traffic emissions, nucleation, and climate modulation

Identifying and quantifying the sources and clarifying the impacts of ultrafine particles (UFP) in the complicated urban environments are important for particle pollution control and UFP-climate interaction understanding. The previous studies have made notable contributions to these aspects and it is necessary to review the achievements. Here, the characteristics of traffic emissions and new particle formation (NPF) events/processes and their effects on urban UFP are summarized mainly based on the latest progresses. The constantly improved techniques of measuring UFP have played a vital role for knowing the sources and impacts of UFP. Meanwhile, the emissions inventories, dispersion models, and receptor models generally perform better when working together and using high resolution input and corrected algorithms. Besides, the interaction between UFP and climate is discussed mainly by linking radiation, cloud condensation nuclei, particle deposition, and the environmental conditions required for nucleation processes. Although for urban UFP, there are consensuses that traffic emissions and nucleation processes are two main sources and UFP and climate interact mainly via radiation and cloud condensation nuclei (CCN), there are many other crucial tasks for future and this work lists seven of them. They involve, scientifically, how much other sources such as industrial and regional sources mix with traffic emissions and nucleation processes in source contributions and how primary pollutants collaborate with UFP (aerosols) in aerosol-climate interactions; and engineeringly, how to improve the integration of the instruments and the instrument customization services according to actual situations. These progresses and future perspectives would help in more accurately quantifying the contributions of emissions and nucleation processes to UFP and better evaluating the impacts of UFP. Despite our efforts, knowledge on the main sources and impacts of urban UFP is limited and detailed solutions for the future tasks are missing here, which need joint efforts from UFP and related fields.