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

Abstract

Abstract. Nitryl chloride (ClNO2) is a dominant source of chlorine radical in polluted environment, and can significantly affect the atmospheric oxidative chemistry. However, the abundance of ClNO2 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 ClNO2 and dinitrogen pentoxide (N2O5) at a rural site in the polluted North China Plain. Elevated mixing ratios of ClNO2 (> 350 pptv) were observed at most of the nights with low levels of N2O5 (< 200 pptv). The highest ClNO2 mixing ratio of 2070 pptv (1 min average) was observed in a plume from a megacity (Tianjin), and was characterized with a faster N2O5 heterogeneous loss rate and ClNO2 production rate compared to average conditions. The abundant ClNO2 concentration kept increasing even after sunrise, and reached a peak 4 h later. Such highly sustained ClNO2 peaks after sunrise are discrepant from the previously observed typical diurnal pattern. Meteorological and chemical analysis shows that the sustained ClNO2 morning peaks are caused by significant ClNO2 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 ClNO2 would exist in the residual layer in order to maintain the observed morning ClNO2 peaks at the surface site. Observation-based box model analysis show that photolysis of ClNO2 produced chlorine radical with a rate up to 1.12 ppbv h−1, accounting for 10–30 % of primary ROx 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 ClNO2 and a relatively lower oxygenated hydrocarbon (OVOC) to non-methane hydrocarbon (NMHC) ratio.

Production of N<sub>2</sub>O<sub>5</sub> and ClNO<sub>2</sub> in summer in urban Beijing, China

“…The average (±1σ ) ratio of ClNO 2 /N 2 O 5 was 6.9 ± 7.4, consistent with previous studies in NCP, for example 0.4-131.3 in Jinan and Wangdu (X. Tham et al, 2016). However, the ratios are substantially higher than those measured in other megacities, e.g., Hong Kong (0.1-2.0; Wang et al, 2016), London (0.02-2.4;Bannan et al, 2015), and Los Angeles, California (0.2-10.0; Mielke et al, 2013).…”

“…High levels of ClNO 2 from anthropogenic chloride sources were also reported in some inland areas (Thornton et al, 2010;Mielke et al, 2011;Phillips et al, 2012Phillips et al, , 2016Bannan et al, 2015). More recently some studies in Hong Kong Brown et al, 2016a;Wang et al, 2016) and in the North China Plain (NCP; Tham et al, 2016;Wang et al, 2018) observed consistently high mixing ratios of N 2 O 5 and ClNO 2 . In particular, ClNO 2 can be rapidly formed in the plumes of coal-fired power plants in the NCP, which serves as an important source of chloride in non-ocean regions.…”

“…Recently, the CIMS using iodide reagent ions (I-CIMS) with an unheated inlet configuration allowed for direct measurements of N 2 O 5 (Kercher et al, 2009;Tham et al, 2014Tham et al, , 2016Wang et al, 2016). The I-CIMS is also widely used to measure ClNO 2 in both laboratory and field studies (Thornton and Abbatt, 2005;McNeill et al, 2006;Osthoff et al, 2008;Tham et al, 2014Tham et al, , 2016Wang et al, 2016). A large amount of ClNO 2 was first observed in polluted coastal regions owing to the abundant chloride from sea salt aerosol, for example in the Gulf of Mexico and the Los Angeles basin Riedel et al, 2012;Kercher et al, 2009).…”

“…Simultaneous indirect measurements of N 2 O 5 and NO 3 can be implemented using thermal dissociation-chemical ionization mass spectrometer (TD-CIMS) with high sensitivity and time resolution (Stutz et al, 2004), although the interference of m/z 62 (NO 3 ) from the thermal decomposition of peroxy acetyl nitrate (PAN) and other related species needs to be considered . Recently, the CIMS using iodide reagent ions (I-CIMS) with an unheated inlet configuration allowed for direct measurements of N 2 O 5 (Kercher et al, 2009;Tham et al, 2014Tham et al, , 2016Wang et al, 2016). The I-CIMS is also widely used to measure ClNO 2 in both laboratory and field studies (Thornton and Abbatt, 2005;McNeill et al, 2006;Osthoff et al, 2008;Tham et al, 2014Tham et al, , 2016Wang et al, 2016).…”

Observations of fine particulate nitrated phenols in four sites in northern China: concentrations, source apportionment, and secondary formation

“…Water-soluble ions were analyzed by a gas aerosol collector-ion chromatography (GAC-IC) system. More information on the site and instruments may be found in Tham et al (2016). The fourth sampling site was located near the summit of Mt.…”