Molecular simulation study of the adsorption of naphthalene and ozone on atmospheric air/ice interfaces

The adsorption of gas-phase naphthalene and ozone on atmospheric air/ice interfaces was investigated using classical molecular dynamics (MD) simulations and potential of mean force (PMF) calculations. Naphthalene and ozone exhibit a strong preference to be adsorbed at the air/ice interface, rather than being dissolved into the bulk of the quasi-liquid layer (QLL) or incorporated into the ice crystals. When the air/ice interface is coated with increasing concentrations of naphthalene molecules, the QLL becomes thinner and surface adsorption of ozone is enhanced. Furthermore, ozone tends to adsorb on top of the naphthalene film, although significant penetration of ozone into this film is also observed. Naphthalene molecules tend to adopt a flat orientation on the air/ice interface. Less variation in the orientation was observed for lower concentrations of naphthalene, whereas variations in the ozone concentration do not affect the orientation of naphthalene molecules. However, as the concentration of ozone increases, most of the naphthalene molecules still prefer to stay close to the mobile water molecules in the QLL, but a significant fraction of the naphthalene molecules spends a considerable amount of time inside the thicker layer of ozone. We also monitored the number of contacts between naphthalene and ozone at the air/ice interface upon variations in the concentrations of these two species. These contacts were assumed to be proportional to the reaction rate between these two species. When the number of ozone molecules was held constant, the number of contacts showed a linear relationship to the number of naphthalene molecules. However, when the naphthalene concentration was held constant, for all systems we observed a linear relationship at low ozone concentrations and a plateau at high ozone concentrations.