(178n) Ice Growth From Benzene, Naphthalene and Phenanthrene/Super-Cooled Water Solutions

Polycyclic aromatic hydrocarbons (PAHs) consist of two or more carbon-hydrogen ring compounds in which at least one ring has an aromatic structure. PAHs are known to have important carcinogenic and mutagenic effects. Furthermore, these compounds can undergo oxidation and nitration reactions with reactive oxygen species (ROSs) [e.g., ozone (O₃) and radicals such as singlet oxygen, hydroperoxy (HO₂), hydroxyl (OH) and nitrate (NO₃)], yielding oxy- and nitro-PAHs that are even more toxic. PAHs and ROSs can be adsorbed at the surfaces of water droplets, atmospheric aerosols, fog and mist, and ice and snow. This process consists of adsorption to the air/water interface and dissolution in the bulk water The processes taking place at atmospheric air/ice therefore have a profound impact on the fate and transport of PAHs and other trace gases in the atmosphere.

In this study, the adsorption of gas-phase benzene and phenanthrene on atmospheric air/ice interfaces and ice growth from benzene, naphthalene or phenanthrene/super-cooled water solutions was investigated using classical molecular dynamics (MD) and potential of mean force (PMF) calculations. From PMF calculations, both benzene and phenanthrene 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 crystalline ice. Ice growth from benzene, naphthalene or phenanthrene/super-cooled water solutions resulted in a thicker QLL at the air/ice interface compared to the QLL thickness formed from freezing pure super-cooled water. Naphthalene and phenanthrene molecules are excluded from the ice lattice and migrate to the QLL close to the air/ice interface during the freezing process at both 270 K and 260 K. Benzene molecules are also excluded from the ice crystals and displaced to the QLL during the freezing process at 270 K, but a fraction of the benzene molecules become incorporated in the ice crystal during the freezing process at 260 K. Benzene, naphthalene and phenathrene molecules preferred to adopt a flat orientation at the air/ice interface with a considerable ability to rotate around the molecular axis. The preference to stay flat at the interface increased as the temperature decreases and also with the increasing number of aromatic rings for aromatic hydrocarbons.