Kinetics of the reaction of CO3^•−(H₂O)_n′ n = 0, 1, 2, with nitric acid, a key reaction in tropospheric negative ion chemistry

A significant fraction of nitrate in the troposphere is formed in the reactions of HNO₃ with the carbonate radical anion CO₃ ^- and the mono- and dihydrated species CO₃ ^-(H₂O)_1,2. A reaction mechanism was proposed in earlier flow reactor studies, which is investigated here in more detail by quantum chemical calculations and experimental reactivity studies of mass selected ions under ultra-high vacuum conditions. Bare CO₃ ^- forms NO₃ ^-(OH) as well as NO₃ ^-, with a total rate coefficient of 1.0 × 10^-10 cm³ s^-1. CO₃ ^-(H₂O) in addition affords stabilization of the NO₃ ^-(HCO₃) collision complex, and thermalized CO₃ ^-(H₂O) reacts with a total rate coefficient of 6.3 × 10^-10 cm³ s^-1. A second solvent molecule quenches the reaction, and only black-body radiation induced dissociation is observed for CO₃ ^-(H₂O)₂, with an upper limit of 6.0 × 10^-11 cm³ s^-1 for any potential bimolecular reaction channel. The rate coefficients obtained under ultra-high vacuum conditions are smaller than in the earlier flow reactor studies, due to the absence of stabilizing collisions, which also has a strong effect on the product branching ratio. Quantum chemical calculations corroborate the mechanism proposed by Möhler and Arnold. The reaction proceeds through a proton-transferred NO₃ ^-(HCO₃) collision complex, which can rearrange to NO₃ ^-(OH)(CO₂). The weakly bound CO₂ easily evaporates, followed by evaporation of the more strongly attached OH, if sufficient energy is available.