(611d) A Molecular Simulation Study of the Adsorption of Aromatic Hydrocarbons and Reactive Oxygen Species On Atmospheric Ice Films

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 photo-chemically induced 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, where the kinetics of their reactions can be much faster than in the gas phase. Therefore, 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. However, and despite their relevance, a fundamental understanding of the adsorption and heterogeneous reactions between PAHs and ROSs at the air/ice interface is still lacking. Here we report a molecular simulation study where we first attempted to elucidate molecular-level details of the adsorption mechanism of gas-phase naphthalene and ozone species onto air/ice interfaces that are either bare or coated with different surfactant species (1-octanol, 1-hexadecanol, or 1-octanal). The surface adsorption of both naphthalene and ozone onto surfactant-coated air/ice interfaces is enhanced when compared to bare air/ice interface. Both naphthalene and ozone tend to stay dissolved in the surfactant layer present at the air/ice interface rather than adsorbing on top of the surfactant molecules. In the second part of our simulation study, we modeled ice growth from super-cooled water containing PAHs dissolved in it. We aimed at understanding the fate of the already dissolved PAHs after the freezing process (i.e., if they end up at the air/ice interface, bulk QLL or in the crystalline structure of ice). Such knowledge can lead to an improved understanding of the multiple ways PAHs and ROSs can undergo photochemical reactions in atmospherically-relevant systems. Freezing of supercooled water solutions containing either benzene, naphthalene or phenanthrene / resulted in a thicker QLL at the air/ice interface. Naphthalene and phenanthrene molecules are excluded from the ice lattice and migrated into the QLL near the air/ice interface during the freezing process at both 270 K and 260 K. Likewise, benzene molecules migrate to the QLL during the freezing process at 270 K, but at 260 K a significant fraction of benzene molecules incorporated into the crystalline structure of ice during the freezing process.