The dynamics of several important atmospheric reactions at the water/air interface is explored by Ab Initio Molecular Dynamics (AIMD) simulations. A point of major emphasis in the analysis are the rules of the hydrogen-bonded network of the water surface, and of the dielectric effect of the medium, upon the reaction mechanism. Results for specific examples, include: (1) HCl molecules from the gas phase, in impact upon NOx species adsorbed on water surfaces, react to produce ClNO and ClNO2 precursors of atomic chlorine in the atmosphere. The role of the water in the processes is found to be catalytic. Ionization of the HCl following impact is found to be a critical step, enabling a subsequent nucleophilic attack on the NOx species. (2) Collisions between vibrationally excited CO2 molecules and water cluster, or liquid water surfaces, lead to fast, direct formation of H2CO3. This constitutes a new mechanism for formation of carbonic acid. The mechanism involves charge separation of species in the transition state. (3) Impact of formic acid on a water surface leads to formation of a small percentage of ion pairs, essentially directly in the collision, on a picoseconds timescale. Analysis shows that the barrier for ionization of HCOOH at the surface is smaller than in the bulk. This result highlights the occurrence of direct collisional ionization even for weak acids. Several general conclusions are drawn from these and other different examples on the reaction mechanism of relevant molecular reactions at water surfaces.

Mechanisms and dynamics of atmospherically-relevant molecular reactions at liquid water surfaces

251st National Meeting (2016)

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