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

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 PM_2.5 mass concentrations during the observation periods. The long-distance transport of sandstorms, characterized by decreasing PM_2.5/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 NO_x emission reduction measures must be made to better achieve air quality standards in Beijing and the NCP region.