Nitrogen dioxide at the air–water interface: trapping, absorption, and solvation in the bulk and at the surface

Abstract

The interaction of NO₂ with water surfaces in the troposphere is of major interest in atmospheric chemistry. We examined an initial step in this process, the uptake of NO₂ by water through the use of molecular dynamics simulations. An NO₂–H₂O intermolecular potential was obtained by fitting to high-level ab initio calculations. We determined the binding of NO₂–H₂O to be about two times stronger than that previously calculated. From scattering simulations of an NO₂ molecule interacting with a water slab we observed that the majority of the scattering events resulted in outcomes in which the NO₂ molecule became trapped at the surface or in the interior of the water slab. Typical surface-trapped/adsorbed and bulk-solvated/absorbed trajectories were analyzed to obtain radial distribution functions and the orientational propensity of NO₂ with respect to the water surface. We observed an affinity of the nitrogen atom for the oxygen in water, rather than hydrogen-bonding which was rare. The water solvation shell was less tight for the bulk-absorbed NO₂ than for the surface-adsorbed NO₂. Adsorbed NO₂ demonstrated a marked orientational preference, with the oxygens pointing into the vacuum. Such behavior is expected for a mildly hydrophobic and surfactant molecule like NO₂. Estimates based on our results suggest that at high NO₂ concentrations encountered, for example, in some sampling systems, adsorption and reaction of NO₂ at the surface may contribute to the formation of gas-phase HONO.