Dinitrogen pentoxide (N₂O₅) and nitryl chloride (ClNO₂) are key species in nocturnal tropospheric chemistry, and have significant effects on particulate nitrate formation and the following day's photochemistry. To better understand the roles of N₂O₅ and ClNO₂ in the high aerosol loading environment of northern China, an intensive field study was carried out at a high-altitude site (Mt. Tai, 1465 m a.s.l.) in the North China Plain

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... (NCP) during the summer of 2014. Elevated ClNO₂ plumes were frequently observed in the nocturnal residual layer with a maximum mixing ratio of 2.1 ppbv (1-min), whilst N₂O₅ was typically present at very low levels (<30 pptv), indicating fast heterogeneous N₂O₅ hydrolysis. Combined analyses of chemical characteristics and backward trajectories indicated that the ClNO₂-laden air was caused by the transport of NO_x-rich plumes from the coal-fired power plants in the NCP. The heterogeneous N₂O₅ uptake coefficient (γ) and ClNO₂ yield (ϕ) during the campaign exhibited high variability, with means of 0.061 ± 0.025 and 0.27 ± 0.24, respectively. These derived values are higher than those derived from previous laboratory and field studies in other regions, and cannot be well characterized by model parameterizations. Fast heterogeneous N₂O₅ reactions dominated the nocturnal NO_x loss in the residual layer over this region, and contributed to substantial nitrate formation of up to 17 μg m^−3. The determined nocturnal nitrate formation rates ranged from 0.2 to 4.8 μg m^−3 hr^−1 in various plumes, with a mean of 2.2 ± 1.4 μg m^−3 h^−1. The results demonstrate the significance of heterogeneous N₂O₅ reactivity and chlorine activation in the NCP, and their unique and universal roles in fine aerosol formation and NO_x transformation, and thus potential impacts on regional haze pollution in northern China.