Distinctive Heterogeneous Reaction Mechanism of ClNO₂ on the Air-Water Surface Containing Cl

The heterogeneous reaction of nitryl chloride (ClNO₂) on the air-water surface plays a significant role in the chloride lifecycle. The air-water surface is ubiquitous on ice surfaces under supercooled conditions, affecting the uptake and heterogeneous reaction processes of trace gases. Previous studies suggest that ClNO₂ is formed on Cl-doped ice surfaces following the N₂O₅ uptake. Herein, a distinctive heterogeneous reaction mechanism of ClNO₂ is suggested on an air-water surface containing Cl under supercooled conditions using combined classic molecular dynamics (MD) and Born-Oppenheimer MD simulations. It is found that N₂O₅ dissociates into a NO₂⁺ and NO₃^- ionic pair on the top air-water surface. In the top layer of the surface containing barely any Cl^-, NO₂⁺ proceeds through hydrolysis and produces H₃O⁺ and HNO₃. Thus, surface acidification appears because of H₃O⁺ yields. With NO₂⁺ diffusion to the deep layer of the surface, NO₂⁺ reacts with Cl^- and forms ClNO₂. Note that ClNO₂ formation competes with NO₂⁺ hydrolysis, and the rate of ClNO₂ formation is 27.7[Cl^-] larger than that of NO₂⁺ hydrolysis. Afterward, the reaction of ClNO₂ with Cl^- becomes barrierless with the catalysis by H₃O⁺, which is not feasible on a neutral surface. Cl₂ is thus generated and escapes into the atmosphere (low solubility of Cl₂), contributing to the Cl radical. The proposed mechanism bolsters the current understanding of ClNO₂'s fate and its role in Cl chemistry in extremely cold environments like the Arctic and other high-latitude regions in wintertime.