Adsorption of polycyclic aromatic hydrocarbons at the air-water interface: molecular dynamics simulations and experimental atmospheric observations

Adsorption of benzene, naphthalene, anthracene, and phenanthrene at the aqueous surface is investigated by means of molecular dynamics simulations. Potentials of mean force, i.e., free energy profiles obtained when moving the studied molecules across an aqueous slab were evaluated. In all cases, deep surface free energy minima, corresponding to orders of magnitude of surface enhancement of the aromatic molecule, were located. This enhancement, which increases with the size of the solute, points to the importance of the aqueous surface for the chemistry of polycyclic aromatic hydrocarbons (PAHs). Supporting evidence in the atmospheric environment related to the heterogeneous chemistry of PAHs on water droplets and planar surfaces is summarized. There is good agreement between the hydration free energies computed from MD calculations and the experimentally determined values. Data pertaining to the importance of the air-water interface in the adsorption and transport of PAHs on micron sized water droplets are described. The relevant data on adsorption and reaction (ozonation and photochemical) at the air-water interface of planar surfaces and droplets are also summarized.