Impacts of Northward Typhoons on Autumn Haze Pollution Over North China Plain

Although air quality in China has improved substantially over recent years, haze pollution events still occur frequently, especially over the North China Plain (NCP). Previous studies showed that typhoons are conducive to regional pollution events in eastern China; however, the underlying mechanism and quantitative understanding of the typhoons' impact on haze pollution remain unclear there. Here, based on ground-based and satellite observations, reanalysis data, and model simulations, we show that northward typhoons approaching China are essential for autumn haze pollution over NCP. Elevated relative humidity levels and enhanced pollution accumulation, caused by northward typhoons and the corresponding high-pressure systems, are responsible for the pollution enhancements over NCP. Compared with episodes without typhoon influence, cities near Taihang and Yan Mountain suffer from heavier haze pollution when typhoons approach, with PM2.5 concentrations increasing from 87.1 to 106.4 mu g m-3. More water vapor from the Yellow and Bohai Seas and pollutants from eastern China are transported to these cities by typhoon-induced southeasterly wind anomalies, facilitating the chemical formation of aerosols there. In addition, by the block of mountains, these southeasterly wind anomalies also lead to stronger local accumulation over cities and an elevation of pollutants along the mountains. What is more, with the implementation of emission reduction, the relative changes of PM2.5 concentrations between typhoon-induced episodes and no-typhoon episodes increase. This work highlights the importance of understanding the impact of synoptical weather on PM2.5 transport, accumulation, and formation processes in haze pollution mitigation in eastern China. Air pollution is harmful to human health and the ecosystem. Although great efforts have been made in emission reduction, haze pollution events still frequently occur in China. Previous studies have shown that typhoons (also named tropical cyclones or hurricanes) are conducive to regional pollution events in eastern China. In this study, we find that by influencing pollutants transport, accumulation, and formation processes, Northwest Pacific northward typhoons accompanied with the corresponding high-pressure systems can lead to severe PM2.5 pollution events over the North China Plain (NCP), especially for cities near mountains west and north of NCP. Despite emission control implementation, cities near mountains still experience relatively higher PM2.5 pollution when typhoons approach. This study highlights the importance of typhoons on PM2.5 pollution in NCP and will be helpful for further pollution prevention and control. Northward typhoons with corresponding high-pressure systems can trigger heavy PM2.5 pollution events over the North China Plain (NCP) Due to enhanced transport, accumulation, and chemical formation of aerosols, cities near mountains are more polluted when typhoons approach The contribution of typhoon-induced episodes to haze in NCP has amplified in recent years as anthropogenic emissions reduced