Significant concentrations of nitryl chloride sustained in the morning: investigations of the causes and impacts on ozone production in a polluted region of northern China

Nitryl chloride (ClNO<sub>2</sub>) is a dominant source of chlorine radical in polluted environment, and can significantly affect the atmospheric oxidative chemistry. However, the abundance of ClNO<sub>2</sub> and its exact role are not fully understood under different env...

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Bibliographic Details
Main Authors: Y. J. Tham, Z. Wang, Q. Li, H. Yun, W. Wang, X. Wang, L. Xue, K. Lu, N. Ma, B. Bohn, X. Li, S. Kecorius, J. Größ, M. Shao, A. Wiedensohler, Y. Zhang, T. Wang
Format: Article
Language:English
Published: Copernicus Publications 2016-12-01
Series:Atmospheric Chemistry and Physics
Online Access:https://www.atmos-chem-phys.net/16/14959/2016/acp-16-14959-2016.pdf
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Summary:Nitryl chloride (ClNO<sub>2</sub>) is a dominant source of chlorine radical in polluted environment, and can significantly affect the atmospheric oxidative chemistry. However, the abundance of ClNO<sub>2</sub> and its exact role are not fully understood under different environmental conditions. During the summer of 2014, we deployed a chemical ionization mass spectrometer to measure ClNO<sub>2</sub> and dinitrogen pentoxide (N<sub>2</sub>O<sub>5</sub>) at a rural site in the polluted North China Plain. Elevated mixing ratios of ClNO<sub>2</sub> (&gt; 350 pptv) were observed at most of the nights with low levels of N<sub>2</sub>O<sub>5</sub> (&lt; 200 pptv). The highest ClNO<sub>2</sub> mixing ratio of 2070 pptv (1 min average) was observed in a plume from a megacity (Tianjin), and was characterized with a faster N<sub>2</sub>O<sub>5</sub> heterogeneous loss rate and ClNO<sub>2</sub> production rate compared to average conditions. The abundant ClNO<sub>2</sub> concentration kept increasing even after sunrise, and reached a peak 4 h later. Such highly sustained ClNO<sub>2</sub> peaks after sunrise are discrepant from the previously observed typical diurnal pattern. Meteorological and chemical analysis shows that the sustained ClNO<sub>2</sub> morning peaks are caused by significant ClNO<sub>2</sub> production in the residual layer at night followed by downward mixing after breakup of the nocturnal inversion layer in the morning. We estimated that  ∼  1.7–4.0 ppbv of ClNO<sub>2</sub> would exist in the residual layer in order to maintain the observed morning ClNO<sub>2</sub> peaks at the surface site. Observation-based box model analysis show that photolysis of ClNO<sub>2</sub> produced chlorine radical with a rate up to 1.12 ppbv h<sup>−1</sup>, accounting for 10–30 % of primary RO<sub><i>x</i></sub> production in the morning hours. The perturbation in total radical production leads to an increase of integrated daytime net ozone production by 3 % (4.3 ppbv) on average, and with a larger increase of 13 % (11 ppbv) in megacity outflow that was characterized with higher ClNO<sub>2</sub> and a relatively lower oxygenated hydrocarbon (OVOC) to non-methane hydrocarbon (NMHC) ratio.
ISSN:1680-7316
1680-7324