Adsorption of Gas-Phase Phenanthrene on Atmospheric Water and Ice Films

Abstract

The adsorption of gas-phase phenanthrene on atmospheric water and ice films was investigated in a flow-tube reactor with a view to understanding the processing of semi-volatile organic compounds by fog and snow. Air-water (ice) interface partition constants were obtained by measuring the mass uptake of phenanthrene vapor on thin water and ice films with varying thickness. Adsorption enthalpies and entropies were obtained from the temperature dependence of the interfacial partition constants. The surface adsorption is the predominant mechanism for the uptake of phenanthrene in water and ice films with small film thickness or at low temperature. The adsorption of phenanthrene to ice resembles that to sub-cooled water and there's no significant difference between the adsorption of phenanthrene to water and that to quasi liquid layer (QLL) if we take into account the uncertainties on the thermodynamic quantities measured. Molecular dynamics simulations of phenanthrene at air/water and air/ice interfaces support these experimental observations. The interfacial air-water and air-ice partition constants of phenanthrene increased greatly in the presence of surface-active substances, indicating that surface active materials effectively enhanced the uptake of organic compounds by atmospheric water and ice films.

Key Words:

• adsorption
• air-ice partitioning
• air-water partitioning
• molecular dynamics simulations and thermodynamics
• phenanthrene

Acknowledgments

This work was supported by National Science Foundation under grant numbers ATM 0355291 and AGS 0907261. Jing Chen would like to acknowledge the financial support by National Natural Science Foundation of China (40905054) and Ministry of Science and Technology of China (2005CB422207 and 2010CB429003). Thilanga P. Liyana-Arachchi and Francisco R. Hung acknowledge support from a Pfund grant from the Louisiana Board of Regents. High performance computational resources for this research were provided by High Performance Computing at Louisiana State University (http://www.hpc.lsu.edu), and the Louisiana Optical Network Initiative (http://www.loni.org).