New particle formation (NPF) events in China urban clusters given by sever composite pollution background.

New Particle Formation (NPF) refers to transformation of gaseous precursors in the atmosphere due to nucleation and subsequent growth process through physicochemical interaction. It has generated a lot of interest due to its profound impact on global and regional environment, climate and human health. We reviewed the studies on NPF in three city clusters of China: the North China Plain, the Yangtze River Delta and the Pearl River Delta obtained through experiment simulations (e.g., chamber simulation, flow-tube simulation, etc.), field observations, and numerical simulations. Due to its atmospheric background pollution and strong oxidation capacities resulting in high source rate of precursors, China's atmosphere possesses challenges different from those evaluated in previous studies on cleaning sites and other developing countries. Hence, NPF events can simultaneously exhibit high condensable sink, formation rate and growth rate. In addition, the high intensity of anthropogenic emissions in urban China has led to greater diversity of pollutant species involved in NPF nucleation and subsequent growth, compared to the dominant role of biogenic precursors at cleaning sites. Differences in geographical location and industrial structure also lead to significant distinctions in NPF characteristics of the three city clusters. Consequently, the lack of understanding of nucleation mechanism of complexly polluted background sites makes the global and regional climate models with submodels based on clean background have enormous uncertainty when applied to urban China. The establishment of a mature research ecosystem including field observations, laboratory simulations and numerical simulations is the key to the breakthrough of NPF research in China.