Heterogeneous NO uptake coefficient and production yield of ClNO in polluted northern China: Roles of aerosol water content and chemical composition

17 pags, 7 figs, 2 tabs.Heterogeneous uptake of dinitrogen pentoxide (N2O5) and production of nitryl chloride (ClNO2) are important nocturnal atmospheric processes that have significant implications for the production of secondary pollutants. However, the understanding of N2O5 uptake processes and ClNO2 production remains limited, especially in China. This study presents a field investigation of the N2O5 heterogeneous uptake coefficient (3(N2O5)) and ClNO2 production yield (•) in a polluted area of northern China during the summer of 2014. The N2O5 uptake coefficient and ClNO2 yield were estimated by using the simultaneously measured ClNO2 and total nitrate in 10 selected cases, which have concurrent increases in the ClNO2 and nitrate concentrations and relatively stable environmental conditions. The determined 3(N2O5) and • values varied greatly, with an average of 0.022 for 3(N2O5) (±0.012, standard deviation) and 0.34 for • (±0.28, standard deviation). The variations in 3(N2O5) could not be fully explained by the previously derived parameterizations of N2O5 uptake that consider nitrate, chloride, and the organic coating. Heterogeneous uptake of N2O5 was found to have a strong positive dependence on the relative humidity and aerosol water content. This result suggests that the heterogeneous uptake of N2O5 in Wangdu is governed mainly by the amount of water in the aerosol, and is strongly water limited, which is different from most of the field observations in the US and Europe. The ClNO2 yield estimated from the parameterization was also overestimated comparing to that derived from the observation. The observation-derived • showed a decreasing trend with an increasing ratio of acetonitrile to carbon monoxide, an indicator of biomass burning emissions, which suggests a possible suppressive effect on the production yield of ClNO2 in the plumes influenced by biomass burning in this region. The findings of this study illustrate the need to improve our understanding and to parameterize the key factors for 3(N2O5) and • to accurately assess photochemical and haze pollution.The authors thank Steven Poon, Qiaozhi Zha, Zheng Xu, and Hao Wang for the logistics support; Liming Cao and Lingyan He for providing the aerosol mass spectrometer data; and to Li Zhang for scientific discussion. This work was funded by the National Natural Science Foundation of China (91544213, 41505103 and 41275123), National Key Research and Development Program of China (2016YFC0200500), Research Grants Council of Hong Kong (15265516), PolyU Project of Strategic Importance (1-ZE13), and the Research Institute for Sustainable Urban Development (RISUD). The Peking University team acknowledges support from the National Natural Science Foundation of China (21190052) and the Strategic Priority Research Program of the Chinese Academy of Sciences (XDB05010500). The Leibniz Institute for Tropospheric Research team acknowledges funding from the Sino German Center (no. GZ663).Peer reviewe