Distinctive Heterogeneous Reaction Mechanism of ClNO 2 on the Air-Water Surface Containing Cl.

The heterogeneous reaction of nitryl chloride (ClNO 2 ) 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 ClNO 2 is formed on Cl-doped ice surfaces following the N 2 O 5 uptake. Herein, a distinctive heterogeneous reaction mechanism of ClNO 2 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 N 2 O 5 dissociates into a NO 2 + and NO 3 - ionic pair on the top air-water surface. In the top layer of the surface containing barely any Cl - , NO 2 + proceeds through hydrolysis and produces H 3 O + and HNO 3 . Thus, surface acidification appears because of H 3 O + yields. With NO 2 + diffusion to the deep layer of the surface, NO 2 + reacts with Cl - and forms ClNO 2 . Note that ClNO 2 formation competes with NO 2 + hydrolysis, and the rate of ClNO 2 formation is 27.7[Cl - ] larger than that of NO 2 + hydrolysis. Afterward, the reaction of ClNO 2 with Cl - becomes barrierless with the catalysis by H 3 O + , which is not feasible on a neutral surface. Cl 2 is thus generated and escapes into the atmosphere (low solubility of Cl 2 ), contributing to the Cl radical. The proposed mechanism bolsters the current understanding of ClNO 2 's fate and its role in Cl chemistry in extremely cold environments like the Arctic and other high-latitude regions in wintertime.