Significant production of ClNO₂ and possible source of Cl₂ from N2O5 uptake at a suburban site in eastern China

ClNO₂ and Cl₂ can affect atmospheric oxidation and thereby the formation of ozone and secondary aerosols, yet their sources and production mechanisms are not well understood or quantified. In this study we present field observations of ClNO₂ and Cl₂ at a suburban site in eastern China during April 2018. Persistent high levels of ClNO₂ (maximum: - 3:7 ppbv; 1 min average) were frequently observed at night, due to the high ClNO₂ yield (' (ClNO₂), 0:560:20) inferred from the measurements. The ' (ClNO₂) value showed a positive correlation with the [Cl-] / [H2O] ratio, and its parameterization was improved at low to median yields (0-0.75) by the incorporation of [Cl-] / [H2O] and the suppression effect of aerosol organics. ClNO₂ and Cl₂ showed a significant correlation on most nights.We show that the Cl₂ at our site was more likely a co-product with ClNO₂ from N2O5 uptake on acidic aerosols that contain chloride than being produced by ClNO₂ uptake as previously suggested.We propose a mechanism in which NOC 2 can react with Cl- to produce Cl₂ and ClNO₂ simultaneously. Under a new framework which regards Cl₂, ClNO₂, and nitrate as products of N2O5 uptake, the Cl₂ yield (' (Cl₂)) was derived using ambient data. ' (Cl₂) exhibited significant correlations with [Cl-] and [HC], based on which a parameterization of ' (Cl₂) was developed. The derived parameterizations of ' (ClNO₂) and ' (Cl₂) can be used in models to evaluate the nighttime production of ClNO₂ and Cl₂ and their impact on the next day's photochemistry.