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Secondary air pollution, especially ozone (O3) and secondary aerosols, are emerging air quality challenges confronting China. Nitrous acid (HONO), as the predominant source of hydroxyl radicals (OH), are acknowledged to be essential for secondary pollution. However, HONO concentrations are usually underestimated by current air quality models due to the inadequate representations of its sources. In the present study, we revised the Weather Research and Forecasting & Chemistry (WRF‐Chem) model by incorporating additional HONO sources, including primary emissions, photo‐/dark oxidation of NOx, heterogeneous uptake of NO2 on surfaces, and nitrate photolysis. By combining in‐situ measurements in the Yangtze River Delta (YRD) region, we found the improved model show much better performance on HONO simulation and is capable of reproducing observed high concentrations. The source‐oriented method is employed to quantitatively understand the relative importance of various processes, which showed that heterogeneous NO2 uptake on the ground surface was the major contributor to HONO formation in urban areas. Comparatively, photo‐oxidation of NOx is a main contributor in rural areas. The introduction of multiple sources of HONO led to an apparent increase in OH and hydroperoxyl (HO2) radicals. The promoted HO2 levels further increased diurnal O3 concentration by 4.5–12.9 ppb, while secondary inorganic and organic concentrations were also increased by 14%–32% during a typical secondary pollution event. The improved description of HONO emission and formation in the model substantially narrowed the gaps between simulations and observations, highlighting the great importance in understanding and numerical representations of HONO in secondary pollution study. Plain Language Summary: Nitrous acid (HONO) has a vigorous potential to produce hydroxyl radicals, contributing to tropospheric oxidation reactions. However, current air quality models lack detailed representation of HONO‐related processes. We revised a widely‐used air quality model based on recently reported mechanisms. The improved model can reasonably simulate actual HONO concentrations measured in the Yangtze River Delta region, one of the most urbanized city clusters in China. It is identified that the heterogeneous conversion of nitrogen dioxide on the ground surface and photo‐oxidation of nitrogen oxide may play an important role in HONO formation. The modeling result is indicative of the pronounced enhancement of HONO on secondary air pollutants, including ozone and secondary aerosols. This study highlights the critical impact of HONO on deteriorating secondary air pollution in city clusters. Key Points: Incorporating missing sources of HONO in the air quality model helps to better represent the observed magnitude and temporal variations of HONOHeterogeneous uptake of NO2 on the ground surface and photo‐oxidation of NOx are major contributors to HONO formationHONO can significantly deteriorate ozone and secondary aerosol pollution by enhancing atmospheric oxidizing capacity [ABSTRACT FROM AUTHOR]

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]