Anthracene Photolysis in Aqueous Solution and Ice: Photon Flux Dependence and Comparison of Kinetics in Bulk Ice and at the Air−Ice Interface

Abstract

We report an investigation of the photolysis kinetics of polycyclic aromatic hydrocarbons (PAHs) in aqueous solution, frozen in ice, and at air-ice interfaces. Measurements of PAH photolysis rates in aqueous solution and at air-ice interfaces as a function of lamp power show that the kinetics depend nonlinearly on photon flux. In both media, the rates do not increase when lamp powers are above 0.1 W, which corresponds to total photon fluxes lower than 10(13) photon cm(-2) s(-1) in the actinic region. This suggests that extrapolating laboratory-measured rates to expected atmospheric photon fluxes may not yield accurate lifetimes for some species. In the plateau region of the photon flux dependence, anthracene located within the ice matrix (or in liquid pockets or veins in the ice) undergoes photolysis at a similar rate to that in room temperature aqueous solution, but the rate of anthracene photolysis at air-ice interfaces is five times greater. This indicates that in order to accurately predict the lifetimes of aromatic pollutants in snow and ice, the quasi-liquid layer (QLL) must be treated separately from bulk ice.

A review of air–ice chemical and physical interactions (AICI): liquids, quasi-liquids, and solids in snow

“…Those experiments indicated that the apparent reaction rate can be enhanced up to a factor of ten (Grannas et al, 2007a;Kahan and Donaldson, 2008;Weber et al, 2009;Kahan and Donaldson, 2010). Other chemical systems displayed similar apparent reaction rates in frozen and liquid samples (Dubowski and Hoffmann, 2000;Klánová et al, 2003a, b;Ružička et al, 2005;Matykiewiczová et Galbavy et al, 2010;Kahan et al, 2010c;Gao and Abbatt, 2011). In addition, a third kind of systems showed up to 10-times slower reactions in snow than in liquid samples for the same total concentration of reactants (Klánová et al, 2003a;Ružička et al, 2005;Matykiewiczová et al, 2007a;Anastasio and Chu, 2009;Bartels-Rausch et al, 2011;Gao and Abbatt, 2011).…”

“…The faster loss of benzene and naphthalene are due at least in part to their aggregation to form dimers at ice surfaces, where they experience a redshift in the absorption spectra as compared to aqueous solutions, but aggregation does not explain the photolysis behaviour of anthracene and harmine at ice surfaces Kahan et al, 2010a). Under the same experimental conditions, anthracene photolysis in bulk ice proceeded at the same apparent rate as in aqueous solution (Kahan et al, 2010c). The observed rates did not depend on temperature in aqueous solutions (274-297 K) or at the ice surface (257-271 K) .…”

“…A particular study showed that local concentrations were enhanced by 10 6 in frozen samples upon slowly freezing aqueous solutions (Heger et al, 2005). High concentrations can enhance the reactivity, such as the photolytic reaction between pnitroanisole and pyridine (Grannas et al, 2007a), the photodegradation of organophosphorus pesticides (Weber et al, 2009), the photolysis of polycyclic aromatic hydrocarbons (Kahan et al, 2010c), or the photoreduction of ferric ions in ice by organic molecules (Kim et al, 2010). The chemistry in Fig.…”

“…Enhanced local concentrations of aromatic compounds such as benzene and naphthalene account for faster apparent photolysis rates at ice surfaces Kahan et al, 2010c). Considering that unimolecular reactions are not per se concentration-dependent, therefore, in such cases rather the absorption spectrum changes.…”

Organics in environmental ices: sources, chemistry, and impacts

“…Some (but not all) of this increase may be attributed to spectral shifts giving rise to increased overlap with the excitation source Donaldson, 2007, 2010). The rate increase is clearly associated with compounds present in the interface region, as demonstrated by a rate increase as the surface/volume ratio of the ice sample is increased (Kahan et al, 2010c). Additionally, Kahan et al (2010a) showed that the rate of reaction could be "tuned" from that at the air-pure ice interface (fast) to that at the water interface (slow) by freezing salt water solutions of successively greater concentration.…”