Atmospheric Oxidation Kinetics of Monochloramine by Hydroxyl Radical, Carbonyl Oxide, and Sulfur Trioxide Catalyzed by Water

Monochloramine (NH 2 Cl) serves as a reservoir for atmospheric chlorine, contributing to radical chemistry and indirectly affecting ozone formation and the growth of secondary organic aerosols. We investigated the nonphotolytic atmospheric removal of NH 2 Cl by the key oxidants OH, CH 2 OO, and SO 3 by using dual-level kinetics calculations based on high-level quantum chemical calculations and density functional theory. The higher level of electronic structure (HL) is coupled cluster theory with single, double, triple, and quadruple excitations and noniterative pentuple excitations with a complete basis [CCSDTQ(P)/CBS] for the OH reaction and CCSDT(Q)/CBS for the CH 2 OO and SO 3 reactions. The kinetics calculations are direct dynamics calculations that combine conventional transition state theory based on coupled cluster electronic structure and variational transition state theory with small-curvature tunneling based on density functional theory. The results reveal that beyond-CCSD(T) contributions can be as large as 0.8 kcal/mol for determining enthalpies of activation. Reaction-specific anharmonicity effects were found to enhance the rate constant at 190 K by a factor of 14 for NH 2 Cl + OH and a factor of 8.5 for NH 2 Cl···SO 3 + H 2 O; these factors decrease to 6.2 and 4.4, respectively, at 280 K. Anharmonic effects are much smaller (factors ≤1.16) for NH 2 Cl + CH 2 OO. Tunneling was found to enhance the rate constant at 190 K by a factor of 4.5 for NH 2 Cl + OH and a factor of 20 for NH 2 Cl···SO 3 + H 2 O; these factors decrease to 2.0 and 2.3, respectively, at 280 K. The recrossing effects reduce the NH 2 Cl + OH rate coefficient by 0.46 at 190 K and 0.67 at 280 K. The contributions of the various sinks for atmospheric NH 2 Cl─photolysis, reaction with OH, multiphase processes, and the reaction with SO 3 catalyzed H 2 O─depend on the conditions.