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

Abstract

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 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 NO2+ 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 (φ (Cl2)) was derived using ambient data. φ (Cl2) 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 evaluate the nighttime production of ClNO2 and Cl2 and their impact on the next day's photochemistry.