Mixed and intensive haze pollution during the transition period between autumn and winter in Beijing, China

Abstract

In the Northern China Plain (NCP), extreme haze events with high concentrations of fine particles occur frequently during the winter but rarely occur in autumn. In this study, we present a synthetic analysis of particulate constituents during the historically polluted transition period of autumn–winter in 2018, revealing that mixed-type haze episodes are the result of regional transport, homogeneous/heterogeneous conversion, and sandstorm influences. The hydrolysis process of N2O5 at higher relative humidity levels (>70%), which feature an enhanced nitrate oxidation ratio (0.30–0.70) and NO3− concentration (>60 μg m−3), was the driving factor for high PM2.5 mass concentrations during the observation periods. The long-distance transport of sandstorms, characterized by decreasing PM2.5/PM10 ratios (<30%) from the north/northwest, is the most important factor for the explosive growth of PM10 concentration. These results can help us gain a comprehensive understanding of haze formation and highlight the importance of nitrate chemistry in the aqueous phase. The results suggest that persistent NOx emission reduction measures must be made to better achieve air quality standards in Beijing and the NCP region.