Mechanistic Insights into the Reactive Uptake of Chlorine Nitrate at the Air–Water Interface

It is well-known that the aqueous-phase processing of chlorine nitrate (ClONO2) plays a crucial role in ozone depletion. However, many of the physical and chemical properties of ClONO2 at the air–water interface or in bulk water are unknown or not understood on a microscopic scale. Here, the solvation and hydrolysis of ClONO2 at the air–water interface and in bulk water at 300 K were investigated by classical and ab initio molecular dynamics (AIMD) simulations combined with free energy methods. Our results revealed that ClONO2 prefers to accumulate at the air–water interface rather than in the bulk phase. Specifically, halogen bonding interactions (ClONO2)Cl···O(H2O) were found to be the predominant interactions between ClONO2 and H2O. Moreover, metadynamics-biased AIMD simulations revealed that ClONO2 hydrolysis is catalyzed at the air–water interface with an activation barrier of only ∼0.2 kcal/mol; additionally, the difference in free energy between the product and reactant is only ∼0.1 kcal/mol. Surprisingly, the near-barrierless reaction and the comparable free energies of the reactant and product suggested that the ClONO2 hydrolysis at the air–water interface is reversible. When the temperature is lowered from 300 to 200 K, the activation barrier for the ClONO2 hydrolysis at the air–water interface is increased to ∼5.4 kcal/mol. These findings have important implications for the interpretation of experiments.