Adsorption of Phenanthrene on Natural Snow FLORENT DOMINE,* ,† ALESSANDRA CINCINELLI, ‡ ELODIE BONNAUD, † TANIA MARTELLINI, ‡ AND SYLVAIN PICAUD § CNRS, Laboratoire de Glaciologie et Ge ´ophysique de l’Environnement, BP 96, 38402 Saint-Martin d’He `res Cedex, France, Dipartimento di Chimica, Universita ` degli Studi di Firenze/Polo Scientifico, Via della Lastruccia 3, 50019 Sesto Fiorentino, Firenze, Italy, and CNRS, Institut UTINAM, UMR 6213, Universite ´ de Franche-Comte ´, 25030 Besanc ¸ on cedex, France The snowpack is a reservoir for semivolatile organic compounds (SVOCs) and, in particular, for persistent organic pollutants (POPs), which are sequestered in winter and released to the atmosphere or hydrosphere in the spring. Modeling these processes usually assumes that SVOCs are incorporated into the snowpack by adsorption to snow surfaces, but this has never been proven because the specific surface area (SSA) of snow has never been measured together with snow composition. Here we expose natural snow to phenanthrene vapors (one of the more volatile POPs) and measure for the first time both the SSA and the chemical composition of the snow. The results are consistent with an adsorption equilibrium. The measured Henry’s law constant is H Phen (T) ) 2.88 × 10 22 exp(-10660/ T) Pa m 2 mol -1 , with T in Kelvin. The adsorption enthalpy is ΔH ads )-89 ( 18 kJ mol -1 . We also perform molecular dynamics calculations of phenanthrene adsorption to ice and obtain ΔH ads )-85 ( 8 kJ mol -1 , close to the experimental value. Results are applied to the adsorption of phenanthrene to the Arctic and subarctic snowpacks. The subarctic snowpack, with a low snow area index (SAI ) 1000), is a negligible reservoir of phenanthrene, but the colder Arctic snowpack, with SAI ) 2500, sequesters most of the phenanthrene present in the (snow + boundary layer) system. Introduction Numerous recent studies have pointed to the role of snow in the transfer of semivolatile organic compounds (SVOCs), in general, and persistent organic pollutants (POPs), in particular, from the atmosphere to the surface (1-5). The efficiency of snow as an air-to-surface vector of organic constituents is thought to be largely because falling snow has a large surface area that allows efficient scavenging of both vapors and particles from the atmosphere during precipitation (4). Once on the ground, the snowpack also provides a large surface area for SVOC adsorption, quantified by the snow area index, SAI. The SAI is defined, by analogy to the leaf area index (LAI, ref 6), as the surface area of snow available for gas adsorption per unit surface area of ground (7). Typical values are 1000-3000 m 2 /m 2 (7, 8). Furthermore, the snowpack acts as an efficient particle filter that enriches the surface layer in particulate contaminants (9, 10). Efforts have therefore been made to incorporate snow as a compartment into environmental models. SVOCs are often treated as species that adsorb to snow surfaces, and quantifying their amounts in snow requires the knowledge of snow surface area and of the air/snow partition coefficients (4, 5, 11). However, the explicit or implicit hypothesis that the incorporation of SVOCs to the snowpack is via an adsorption mechanism has never been fully tested, neither in the field nor in the laboratory. In fact, Roth et al. (12) mention that the sorption mechanism could be by adsorption to the surface, absorption in the quasi-liquid layer (QLL) at the ice surface, or incorporation into solid ice. Since intuition commands that the large SVOC molecules are unlikely to be incorporated into the ice, accommodation at or near the surface forms the basis of most modeling studies and a verification of such an important assumption is needed. Domine et al. (13) discuss why there is limited basis and interest to distinguish between surface adsorption and absorption into the QLL, and we subsequently refer to adsorption onto snow crystals as the process by which a molecule is incorporated on the surface or within the QLL and remains available for rapid exchange with the atmosphere or snow interstitial air. If a SVOC is incorporated in the snowpack by adsorption and if its surface coverage remains significantly less than a monolayer, its concentration in snow, [SVOC] snow, can be expressed as a function of the partial pressure of the SVOC, PSVOC, and of temperature, by where HSVOC(T) is the surface Henry’s Law constant at the snow temperature T and is expressed in Pa m 2 mol -1 , while [SVOC]snow is in mol kg -1 . SSA is the specific surface area, that is, the surface area per unit mass expressed in m 2 kg -1 . A complete test of the adsorption equilibrium hypothesis therefore requires the measurement of the specific surface area (SSA) of snow. This is a tedious and delicate enterprise, as stressed by Legagneux et al. (14) and Domine et al. (15), who published compilations of 176 and 345 snow SSA measurements, and as confirmed by other authors (12), who relied on the first compilation to obtain SSA estimates. Here we perform cold room experiments where we expose snow to phenanthrene vapors. To perform the first test of the adsorption equilibrium hypothesis for a SVOC in snow, we measure simultaneously its concentration in snow and the SSA of the snow. Phenanthrene was chosen because it is a persistent organic pollutant (POP) of strong environ- mental relevance as it is among the species monitored in the study of polar contaminants (16). Moreover, it is among the more volatile POPs and is therefore not expected to partition strongly to organic aerosols, thus facilitating the observation and interpretation of air-snow equilibria. It also has a low toxicity, allowing facile manipulation in the laboratory. Our results are consistent with an adsorption equilibrium and yield a Henry’s law constant that allows the calculation of phenanthrene concentrations on snow surfaces, knowing the value of its partial pressure. * Corresponding author e-mail: florent@lgge.obs.ujf-grenoble.fr; phone: +(33) 476 82 42 69. † Laboratoire de Glaciologie et Ge ´ophysique de l’Environnement. ‡ Universita ` degli Studi di Firenze/Polo Scientifico. § Universite ´ de Franche-Comte ´. [SVOC] snow ) P SVOC × SSA/H SVOC (T) (1) Environ. Sci. Technol. 2007, 41, 6033-6038 10.1021/es0706798 CCC: $37.00  2007 American Chemical Society VOL. 41, NO. 17, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 9 6033 Published on Web 07/31/2007