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

摘要

Nitryl chloride (ClNO) is a dominant source of chlorineradical in polluted environment, and can significantly affect the atmosphericoxidative chemistry. However, the abundance of ClNO and its exact roleare not fully understood under different environmental conditions. Duringthe summer of 2014, we deployed a chemical ionization mass spectrometer tomeasure ClNO and dinitrogen pentoxide (NO) at a rural sitein the polluted North China Plain. Elevated mixing ratios of ClNO( 350 pptv) were observed at most of the nights with low levelsof NO ( 200 pptv). The highest ClNO mixing ratioof 2070 pptv (1 min average) was observed in a plume from a megacity(Tianjin), and was characterized with a faster NO heterogeneous loss rateand ClNO production rate compared to average conditions. The abundantClNO concentration kept increasing even after sunrise, and reached apeak 4 h later. Such highly sustained ClNO peaks after sunrise arediscrepant from the previously observed typical diurnal pattern.Meteorological and chemical analysis shows that the sustained ClNOmorning peaks are caused by significant ClNO production in theresidual layer at night followed by downward mixing after breakup of thenocturnal inversion layer in the morning. We estimated that  ∼  1.7–4.0 ppbv of ClNO would exist in the residual layer in order tomaintain the observed morning ClNO peaks at the surface site.Observation-based box model analysis show that photolysis of ClNOproduced chlorine radical with a rate up to 1.12 ppbv h, accountingfor 10–30 % of primary RO production in the morning hours. Theperturbation in total radical production leads to an increase of integrateddaytime net ozone production by 3 % (4.3 ppbv) on average, and with alarger increase of 13 % (11 ppbv) in megacity outflow that wascharacterized with higher ClNO and a relatively lower oxygenated hydrocarbon (OVOC) to non-methane hydrocarbon(NMHC) ratio.