The passage of small pollutant molecules (HCl, CO2) through a thin water film supported on a MgO substrate at 300 K has been studied by constrained classical molecular dynamics simulations. The calculated free energy profile of the pollutants exhibits two minima, one at the gas/liquid film interface, and the other inside the film near the ionic substrate. Lifetimes of the pollutants in these two sites have been characterized by unconstrained simulations. The residence times in these sites are in the range of a few tens of picoseconds. The transfer times from one site to the other, and the times spent by the pollutants in the liquid and at the liquid/gas interface (similar to one hundred ps) are always twice longer for CO2 than for HCl. This difference is interpreted in terms of correlated dynamics of HCl and H2O due to hydrogen bond interactions with water. The duration of the hydrogen bond Cl-H ... O increases significantly (similar to 2 ps) at the film surface with respect to its value inside the film (< 1 ps).