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 (φ(ClNO₂), 0.56 ± 0.20) inferred from the measurements. The φ(ClNO₂) value showed a positive correlation with the [Cl^-]/[H₂O] ratio, and its parameterization was improved 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 likely a co-product with ClNO₂ from N₂O₅ uptake on aerosols that contain acidic chloride, rather than being produced by ClNO₂ uptake, as previously suggested. The Cl₂ yield (φ(Cl₂)) derived from the N₂O₅ uptake hypothesis exhibited significant correlations with [Cl^-] and [H⁺], based on which a parameterization of φ(Cl₂) was developed. The derived parameterizations of φ(ClNO₂) and φ(Cl₂) can be used in models to quantify the nighttime production of ClNO₂ and Cl₂ and their impact on the next day’s photochemistry.