The effects of the gas–liquid interface and gas phase on Cl/ClO radical interaction with water molecules

Abstract

In the marine boundary layer (MBL), chlorine (Cl) and chlorine monoxide (ClO) are powerful oxidants with high concentrations. The gas–liquid interface is also ubiquitous in the MBL as a favorable site for atmospheric reactions. Understanding the role of water in Cl/ClO radical chemistry is essential for predicting their behavior in the atmosphere and developing effective strategies for mitigating their harmful effects. However, the research studies on the system of Cl/ClO radicals on the surface of water droplets are still insufficient. In previous studies, we have found unique results related to the hydroxyl radical at the interface using ab initio molecular dynamics (AIMD). In this work, we have used AIMD to investigate interactions between Cl/ClO radicals and water molecules at the gas–liquid interface. Radical mobility, radial distribution functions, coordination, and population analyses were conducted to investigate the surface preference, bonding pattern, and track Cl/ClO radicals in the water droplets. In addition, density functional theory (DFT) analysis was conducted to compare the results at the gas–liquid interface with those in the gas phase. We found that Cl/ClO radicals tend to remain near the gas–liquid interface in water droplet systems and outside of water clusters in gas phase systems. The ClO radical can form O*–H and Cl–O bonds with water molecules; however, neither the O*–O hemibond nor the Cl–H bond was detected in all systems. Different dominant structures were obtained for ClO in the interface and gas phase. The ClO radical can be bonded to one water molecule from its oxygen side, (H₂O)₀–Cl–O*–(H₂O)₁ at the interface, or to two water molecules from the chlorine and oxygen sides, (H₂O)₁–Cl–O*–(H₂O)₁ in the gas phase. Meanwhile, the Cl radical can only form a dominant structure like Cl*–(H₂O)₁ at the gas–liquid interface by making a Cl*–O hemibond. Providing a thorough explanation of the Cl/ClO radical behavior at the gas–liquid interface, this study will improve our understanding of the MBL's oxidizing capacity and pollution causes.