High Levels of Daytime Molecular Chlorine and Nitryl Chloride at a Rural Site on the North China Plain

Molecular chlorine (Cl2) and nitryl chloride (ClNO2) concentrations were measured using chemical ionization mass spectrometry at a rural site over the North China Plain during June 2014. High levels of daytime Cl2 up to ∼450 pptv were observed. The average diurnal Cl2 mixing ratios showed a maximum around noon at ∼100 pptv. ClNO2 exhibited a strong diurnal variation with early morning maxima reaching ppbv levels and afternoon minima sustained above 60 pptv. A moderate correlation (R2 = 0.31) between Cl2 and sulfur dioxide was observed, perhaps indicating a role for power plant emissions in the generation of the observed chlorine. We also observed a strong correlation (R2 = 0.83) between daytime (10:00-20:00) Cl2 and ClNO2, which implies that both of them were formed from a similar mechanism. In addition, Cl2 production is likely associated with a photochemical mechanism as Cl2 concentrations varied with ozone (O3) levels. The impact of Cl2 and ClNO2 as Cl atom sources is investigated using a photochemical box model. We estimated that the produced Cl atoms oxidized slightly more alkanes than OH radicals and enhanced the daily concentrations of peroxy radicals by 15% and the O3 production rate by 19%.