Observations of Dinitrogen Pentoxide and Nitryl Chloride at two inland sites in North China: Abundances, Origins, and Impact on Photochemistry

Abstract:

Dinitrogen pentoxide (N₂O₅) and nitryl chloride (ClNO₂) are key players in the nocturnal tropospheric chemistry, and also have potential to perturb the next-day’s photochemistry. We here present the first ambient measurements of N₂O₅ and ClNO₂ in inland regions of northern China, which is suffering from severe photochemical smog and haze pollution. A chemical ionization mass spectrometer (CIMS) was deployed at a semi-rural site in Wangdu, Hebei and on the top of Mt Tai, Shandong (1500 m a.s.l.) during the summer of 2014. At Wangdu, significant levels of ClNO₂ were observed persistently throughout the campaign, with the maximum concentration of up to 2.1 ppbv (1-min data). N₂O₅ were generally in small concentrations but on few occasions reached up to hundreds of pptv. Clear variation of ClNO₂ and N₂O₅ from night to night suggests the variability of N₂O₅ heterogeneous reactivity under different conditions over the region. On the mountain-top, elevated ClNO₂-laden plumes were frequently observed around mid-night with a 1-min maximum of 2.1 ppbv, whilst N₂O₅ was always in very low levels indicating a fast N₂O₅ hydrolysis. The elevated ClNO₂ levels at both locations were significantly influenced by the high NOx-saturated urban plumes and non-oceanic sources of chloride like biomass burning and coal-fired power plants in the region. MCM (Master Chemical Mechanism) modeling analyses indicate the significance of ClNO₂ photolysis to the daytime radical and ozone production. Our study implies that the N₂O₅ reactivity and chlorine activation are significant in North China, and should be also important in other non-marine regions of China where NOx and particle chloride are in great abundances.