Fast heterogeneous N₂O₅uptake and ClNO₂production in power plant and industrial plumes observed in the nocturnal residual layer over the North China Plain

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 NOxrecycling upon photolysis of ClNO2. To better understand the roles of N2O5and ClNO2in the high-aerosol-loading environment of northern China, an intensive field study was carried out at a high-altitude site (Mt. Tai, 1465ma.s.l.) in the North China Plain (NCP) during the summer of 2014. Elevated ClNO2plumes were frequently observed in the nocturnal residual layer with a maximum mixing ratio of 2.1 ppbv (1 min), whilst N2O5was typically present at very low levels (<30 pptv), indicating fast heterogeneous N2O5hydrolysis. Combined analyses of chemical characteristics and backward trajectories indicated that the ClNO2-laden air was caused by the transport of NOx- rich plumes from the coal-fired industry and power plants in the NCP. The heterogeneous N2O5uptake coefficient (γ) and ClNO2yield (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 N2O5reactions dominated the nocturnal NOxloss in the residual layer over this region and contributed to substantial nitrate formation of up to 17 μgm-3. The estimated nocturnal nitrate formation rates ranged from 0.2 to 4.8 μgm-3h-1in various plumes, with a mean of 2.2 ±1.4 μgm-3h-1. The results demonstrate the significance of heterogeneous N2O5reactivity and chlorine activation in the NCP, and their unique and universal roles in fine aerosol formation and NOxtransformation, and thus their potential impacts on regional haze pollution in northern China.