Adsorption of Phenanthrene on Natural Snow
Abstract
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 HPhen(T) = 2.88 × 1022 exp(−10660/T) Pa m2 mol-1, with T in Kelvin. The adsorption enthalpy is ΔHads = −89 ± 18 kJ mol-1. We also perform molecular dynamics calculations of phenanthrene adsorption to ice and obtain ΔHads = −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.