Adsorption of Atmospherically Relevant Gases at the Air/Water Interface:  Free Energy Profiles of Aqueous Solvation of N₂, O₂, O₃, OH, H₂O, HO₂, and H₂O₂

Abstract

Free energy profiles associated with moving atmospheric gases or radicals across the air/water interface were calculated as potentials of mean force by classical molecular dynamics simulations. With the employed force field, the experimental hydration free energies are satisfactorily reproduced. The main finding is that both hydrophobic gases (nitrogen, oxygen, and ozone) and hydrophilic species (hydroxyl radical, hydroperoxy radical, or hydrogen peroxide) have a free energy minimum at the air/water interface. As a consequence, it is inferred that atmospheric gases, with the exception of water vapor, exhibit enhanced concentrations at surfaces of aqueous aerosols. This has important implications for understanding heterogeneous chemical processes in the troposphere.