Molecular atmospheric pollutant adsorption on ice: a theoretical survey

A theoretical survey of the adsorption of gas traces on liquid and solid water surfaces was carried out with an application to environmental physicochemistry. The state of the art regarding potential models used in the literature to describe water–water and pollutant–water interactions is presented in this paper. The structural and dynamical properties of the ice surface vs. temperature within the range 190–300 K are analyzed on the basis of potential calculations and available experimental data. Characteristic times of water motions at the ice interface are also discussed and compared with the experimental values. The adsorption of pollutant molecules N2, CO, CO2, HF, HCl, HOCl, NH3, CH2O, (CH3)2CO, CH3OH, HCOOH and CH3COOH, some of them having a dominant role in atmospheric processes (hydracids, organic acids, etc.) is first considered using semi-empirical potentials within the scheme of rigid ice surfaces. In this scheme, snapshots of thermally equilibrated ice samples issued from molecular dynamics simulations were used as ice substrates frozen at a chosen temperature. This provides basic information on the equilibrium sites, adsorption energies and H-bond formations of this set of pollutants on frozen ice surfaces, which are very different from the perfect ice geometry. Such information compares to that obtained from ab initio potentials and to the measurements of the adsorption energy. It also makes possible to discuss the relative adsorption, surface migration and desorption properties of pollutants within the available diffusion models. Then the adsorption/desorption/penetration mechanisms of some pollutant molecules on liquid water and dynamical ice are interpreted using results from classical molecular dynamics simulations in terms of free energy profile behavior vs. the pollutant–surface distance. Experimental and calculated uptake, mass accommodation and diffusion coefficients of pollutants are then compared. The analysis of the potential energy and entropy changes, which are responsible for the very different behavior of the free energy profiles depending on the pollutant and the temperature, is extensively discussed on the basis of theoretical approaches proposed in the literature and numerical data issued from the simulations. Residence times of the pollutants at the liquid and ice interfaces and transfer times from the interface to the bulk or to the gas phase are calculated and scenarios at the molecular scale regarding the behavior of the pollutant species with respect to the substrate interface are proposed. Molecular ionization and dissociation are only overviewed at the end of the paper for the specific case of HCl.