Distinctive Heterogeneous Reaction Mechanism of ClNO<sub>2</sub> on the Air–Water Surface Containing Cl
Weina Zhang, Jie Zhong, Ruijing Li, Liwen Li, Xiaohui Ma, Yuemeng Ji, Guiying Li, Joseph S. Francisco, Taicheng An
Abstract
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.