Driving factors and photochemical impacts of Cl2 in coastal atmosphere of Southeast China

The elevated levels of molecular chlorine (Cl2) have been observed both during the daytime or nighttime, yet the key drivers influencing Cl2 formation remain unclear. In this study, we observed the distinct daytime and nighttime peaks of Cl2 in coastal atmosphere of Southeast China. Field observations combined with machine learning revealed that daytime Cl2 generation was driven by nitrate (especially ammonium nitrate) photolysis, and aerosol iron photochemistry, while the N2O5 uptake on aerosols containing chloride contributed to nighttime Cl2 formation. Around noon, alkane oxidation rates by Cl radicals generated from Cl2 photolysis surpassed those of OH radicals, leading to a 44% increment in RO2 radical levels and a 42% enhancement in net O3 production rates. This study offers new insights into the production and loss processes of Cl2 in the tropospheric atmosphere, emphasizing its significance in coastal photochemical pollution.