Overview of Persistent Haze Events in China

As the world’s second largest economy, China has experienced severe haze pollution, with fine particulate matter (PM) reaching unprecedentedly high levels across many cities. In addition to the profound impacts on human health and ecosystems, fine PM interacts directly and indirectly with the Earth’s radiation budget, influencing weather and climate. An understanding of the PM formation mechanism is critical in the development of efficient mitigation policies to minimize the local, regional, and global impacts. The formation mechanisms leading to severe haze episodes with exceedingly high PM2.5 levels in China remain uncertain, and the abundance and chemical constituents of PM2.5 depend on the complex interplay between meteorology, emission sources, and atmospheric chemical processes. This paper discusses the various aspects relevant to persistent haze in China, including the fundamental chemistry and meteorological conditions. The similarity and difference in PM formation between Beijing and other world regions are illustrated. Future research needs relevant to persistent haze formation in China are discussed.