Fast Secondary Aerosol Formation in Residual Layer and Its Impact on Air Pollution Over Eastern China.

Though China's air quality has been improved in the past decade, haze still engulfs megacities in winter with fast increasing secondary aerosol like nitrate. Given that aerosol and its precursors exhibit great vertical heterogeneity, we conducted airship measurements at Shanghai in December 2017 to understand the critical drivers for the rapidly increased secondary pollution. By integrating in‐situ observations and model simulations, we found that cold front favors the long‐range transport of pollutants, during which chemical formation of nitrate in residual layer was faster than near the surface due to concentrated precursors and sufficient oxidation. The nitrate aloft would be entrained down once the daytime convective boundary layer develops, thereby deteriorating near‐surface air pollution. It is shown that the local surface‐level chemical production played a minor role, whereas the synoptic weather and boundary layer evolution largely contributed to enhanced secondary aerosol pollution (>60%). This study demonstrates that weather systems, boundary layer evolution, and chemical processes could jointly shape secondary aerosol production, highlighting the importance of a vertical understanding of haze pollution. Plain Language Summary: Severe haze events occur frequently in eastern China during winter, mainly caused by secondary aerosol like nitrate. Given that aerosol and its precursors feature great vertical heterogeneity, it is of great significance to understand the critical drivers for secondary pollution from a vertical perspective. By conducting airship measurements in Shanghai, high loading of secondary aerosols was observed in the upper air. It is revealed that cold fronts promoted the transport of pollution, resulting in the accumulation and rapid oxidation of precursors in residual layer. Nitrate aloft mixed downward with the development of boundary layer, which leads to enhanced secondary pollution near thesurface. This work sheds light on the importance of a vertical understanding of haze pollution. Key Points: Cold‐front driven regional transport of pollutants aloft was observed by a tethered airshipConcentrated precursors and rapid oxidation promote secondary aerosol formation in residual layerSecondary aerosols in the upper air make a significant impact on surface air quality through vertical mixing process [ABSTRACT FROM AUTHOR]