Winter ClNO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; formation in the region of fresh anthropogenic emissions: seasonal variability and insights into daytime peaks in northern China

Men Xia,Yee Jun Tham,Jianmin Chen,Hong Li,Zhe Wang,Pengfei Liu,Chuan Yu,Tao Wang,Xiang Peng,Xinfeng Wang,Jian Gao,Chenglong Zhang,Weihao Wang,Yujing Mu,Hui Chen,Likun Xue

doi:10.5194/acp-21-15985-2021

Abstract

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, polluted rural, and polluted lower tropospheric (mountaintop) sites in the winter of 2017–2018 over the North China Plain (NCP). The nocturnal concentrations of ClNO2 were lower at the urban and polluted rural sites but higher at the polluted lower tropospheric 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 ratios of dinitrogen pentoxide (N2O5) to NO3 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 polluted rural, urban, and polluted lower tropospheric 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 %. Our results provide new insights into the ClNO2 processes in the lower troposphere impacted by fresh and intense anthropogenic emissions and reveal that ClNO2 can be an important daytime source of Cl radicals under certain conditions in winter.

Highlights

chlorine radical (Cl) is a potent atmospheric oxidant that reacts analogously to hydroxyl radicals (OH) with hydrocarbons (Simpson et al, 2015)
Overall, elevated levels of N2O5 and ClNO2 were observed with different patterns at each site
Observations of ClNO2 and related species were conducted at urban, polluted rural, and polluted lower tropospheric sites in the winter of 2017–2018 in the North China Plain (NCP), which suffers from severe winter haze pollution

Summary

Introduction

Cl is a potent atmospheric oxidant that reacts analogously to hydroxyl radicals (OH) with hydrocarbons (Simpson et al, 2015). Xia et al.: Winter ClNO2 formation in the region of fresh anthropogenic emissions (= OH + HO2 + RO2) via Reactions (R1)–(R4), which promotes O3 formation by converting nitric oxide (NO) to nitrogen dioxide (NO2) (Reactions R3 and R5). The net effect of the Cl chemistry is typically the depletion of O3 in the remote atmosphere, such as the stratosphere (Molina and Rowland, 1974) and remote oceans (Simpson et al, 2015; Wang et al, 2019), and an increase in O3 production in the polluted troposphere (Riedel et al, 2014; Xue et al, 2015)

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Results

Conclusion