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.