Fast heterogeneous N₂O₅ uptake and ClNO₂ production in power plant and industrial plumes observed in the nocturnal residual layer over the North China Plain

Abstract. 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 through chlorine radical production and NO_x recycling upon photolysis of ClNO₂. 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 (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 industry and power plants in the NCP. The heterogeneous N₂O₅ uptake coefficient (γ) and ClNO₂ yield (ϕ) were estimated from steady-state analysis and observed growth rate of ClNO₂. The derived γ and ϕ exhibited high variability, with means of 0.061 ± 0.025 and 0.28 ± 0.24, respectively. These 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 estimated nocturnal nitrate formation rates ranged from 0.2 to 4.8 µg m⁻³ h⁻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 their potential impacts on regional haze pollution in northern China.