Winter ClNO2 formation in the region of fresh anthropogenic emissions: seasonal variability and insights into daytime peaks in northern China

Nitryl chloride (ClNO 2 ) is an important chlorine reservoir in the atmosphere that affects the oxidation of volatile organic compounds (VOCs) and the production of RO x radicals and ozone (O 3 ). This study presents measurements of ClNO 2 and related compounds at urban, polluted rural, and polluted lower tropospheric (mountaintop) sites in the winter of 2017–2018 over the North China Plain (NCP). The nocturnal concentrations of ClNO 2 were lower at the urban and polluted rural sites but higher at the polluted lower tropospheric site. The winter concentrations of ClNO 2 were generally lower than the summer concentrations that were previously observed at these sites, which was due to the lower nitrate radical (NO 3 ) production rate ( P (NO 3) ) and the smaller N 2 O 5 uptake coefficients ( γ (N 2 O 5) ) in winter, despite the higher ratios of dinitrogen pentoxide (N 2 O 5 ) to NO 3 in winter. Significant daytime peaks of ClNO 2 were observed at all the sites during the winter campaigns, with ClNO 2 mixing ratios of up to 1.3 ppbv. Vertical transport of ClNO 2 from the residual layers and prolonged photochemical lifetime of ClNO 2 in winter may explain the elevated daytime concentrations. The daytime-averaged chlorine radical (Cl) production rates ( P (Cl)) from the daytime ClNO 2 were 0.17, 0.11, and 0.12 ppbv h −1 at the polluted rural, urban, and polluted lower tropospheric sites, respectively, which were approximately 3–4 times higher than the campaign-averaged conditions. Box model calculations showed that the Cl atoms liberated during the daytime peaks of ClNO 2 increased the RO x levels by up to 27 %–37 % and increased the daily O 3 productions by up to 13 %–18 %. Our results provide new insights into the ClNO 2 processes in the lower troposphere impacted by fresh and intense anthropogenic emissions and reveal that ClNO 2 can be an important daytime source of Cl radicals under certain conditions in winter.