08 Mar 2021

08 Mar 2021

Review status: this preprint is currently under review for the journal ACP.

Winter observations of ClNO2 in northern China: Spatiotemporal variability and insights into daytime peaks

Men Xia1, Xiang Peng1, Weihao Wang1,8, Chuan Yu1,2, Zhe Wang6, Yee Jun Tham7, Jianmin Chen4, Hui Chen4, Yujing Mu5, Chenglong Zhang5, Pengfei Liu5, Likun Xue2, Xinfeng Wang2, Jian Gao3, Hong Li3, and Tao Wang1 Men Xia et al.
  • 1Department of Civil and Environmental Engineering, The Hong Kong Polytechnic University, Hong Kong SAR, China
  • 2Environment Research Institute, Shandong University, Ji’nan, Shandong, China
  • 3Chinese Research Academy of Environmental Sciences, Beijing, China
  • 4Department of Environmental Science and Engineering, Fudan University, Institute of Atmospheric Sciences, Shanghai, China
  • 5Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing, China
  • 6Division of Environment and Sustainability, Hong Kong University of Science and Technology, Hong Kong SAR, China
  • 7Institute for Atmospheric and Earth System Research/Physics, University of Helsinki, Helsinki, Finland
  • 8Hangzhou PuYu Technology Development Co., Ltd, Hangzhou, Zhejiang, China

Abstract. Nitryl chloride (ClNO2) is an important chlorine reservoir in the atmosphere that affects the oxidation of volatile organic compounds (VOCs) and the production of ROx radicals and ozone (O3). This study presents measurements of ClNO2 and related compounds at urban, rural, and mountain sites in the winter of 2017–2018 over the North China Plain (NCP). The nocturnal concentrations of ClNO2 were lower at the urban and rural sites but higher at the mountain site. The winter concentrations of ClNO2 were generally lower than the summer concentrations that were previously observed at these sites, which was due to the lower nitrate radical (NO3) production rate (P(NO3) and the smaller N2O5 uptake coefficients (γ(N2O5)) in winter, despite the higher dinitrogen pentoxide (N2O5) to NO3 ratios in winter. Significant daytime peaks of ClNO2 were observed at all the sites during the winter campaigns, with ClNO2 mixing ratios of up to 1.3 ppbv. Vertical transport of ClNO2 from the residual layers and prolonged photochemical lifetime of ClNO2 in winter may explain the elevated daytime concentrations. The daytime-averaged chlorine radical (Cl) production rates (P(Cl)) from the daytime ClNO2 were 0.17, 0.11, and 0.12 ppbv h−1 at the rural, urban, and mountain sites, respectively, which were approximately 3–4 times higher than the campaign-averaged conditions. Box model calculations showed that the Cl atoms liberated during the daytime peaks of ClNO2 increased the ROx levels by up to 27–37 % and increased the daily O3 productions by up to 13–18 %.

Men Xia et al.

Status: open (until 12 May 2021)

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Men Xia et al.


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Short summary
ClNO2 is an important precursor of chlorine radical that affects photochemistry. However, its production and impact are not well understood. Our study presents field observations of ClNO2 at three sites in northern China. These observations provide new insights into nighttime processes that produce ClNO2, and the significant impact of ClNO2 on secondary pollutions during daytime. The results improve the understanding of photochemical pollution in the lower part of the atmosphere.