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...
Main Authors: | , , , , , , , , , , , , , , , , |
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Format: | Article |
Language: | English |
Published: |
Copernicus Publications
2016-12-01
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Series: | Atmospheric Chemistry and Physics |
Online Access: | https://www.atmos-chem-phys.net/16/14959/2016/acp-16-14959-2016.pdf |
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>
(> 350 pptv) were observed at most of the nights with low levels
of N<sub>2</sub>O<sub>5</sub> (< 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. |
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ISSN: | 1680-7316 1680-7324 |