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 (ClONO) plays a crucial role in ozone depletion. However, many of the physical and chemical properties of ClONO at the air-water interface or in bulk water are unknown or not understood on a microscopic scale. Here, the solvation and hydrolysis of ClONO at the air-water interface and in bulk water at 300 K were investigated by classical and molecular dynamics (AIMD) simulations combined with free energy methods. Our results revealed that ClONO prefers to accumulate at the air-water interface rather than in the bulk phase. Specifically, halogen bonding interactions (ClONO)Cl···O(HO) were found to be the predominant interactions between ClONO and HO. Moreover, metadynamics-biased AIMD simulations revealed that ClONO 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 ClONO hydrolysis at the air-water interface is reversible. When the temperature is lowered from 300 to 200 K, the activation barrier for the ClONO hydrolysis at the air-water interface is increased to ∼5.4 kcal/mol. These findings have important implications for the interpretation of experiments.