Anthracene Photolysis in Aqueous Solution and Ice: Photon Flux

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Environ. Sci. Technol. 2010, 44, 1302–1306

Anthracene Photolysis in Aqueous Solution and Ice: Photon Flux Dependence and Comparison of Kinetics in Bulk Ice and at the Air-Ice Interface TARA F. KAHAN,† RAN ZHAO,† KLAUDIA B. JUMAA,† AND D . J . D O N A L D S O N †,‡,* Department of Chemistry, University of Toronto, 80 Saint George Street, Toronto, Ontario, Canada M5S 3H6 and Department of Physical and Environmental Sciences, University of Toronto, Scarborough, Ontario, Canada

Received October 19, 2009. Revised manuscript received December 15, 2009. Accepted December 21, 2009.

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 1013 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.

Introduction Organic pollutants such as polycyclic aromatic hydrocarbons (PAHs) are ubiquitous in urban centers, and have been detected in remote regions such as the Arctic (1-20). Understanding their chemical fate in the environment is important, because their oxidation products are often more toxic than the parent compounds (21). In the gas phase, reaction with hydroxyl radicals is the most important transformation process for PAHs, but PAHs present at the surfaces of environmental substrates can undergo rapid heterogeneous reactions (21). Snow and ice have received increasing attention in recent decades as important reaction media, both for inorganic and organic species. While most investigations have examined reactions involving inorganic species (ref (22) and references * Corresponding author phone: (416) 978-3603; fax: (416) 9788775; e-mail: [email protected]. † Department of Chemistry. ‡ Department of Physical and Environmental Sciences. 1302

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therein), a few recent studies (23-29) have shown that aromatic compounds can undergo photolysis in ice. Ice that is found at the Earth’s surface is of the type Ih, which consists of a highly ordered hexagonal matrix. Ice in the environment is polycrystalline, and ice matrices contain liquid water impurities in the form of pockets and veins (30). Near the air-ice interface, the structure of the ice matrix breaks down, and water molecules adopt more random orientations. This disordered region is often referred to as a quasi-liquid layer (QLL). Contaminants such as organic pollutants are expected to be concentrated in liquid regions within bulk ice, as well as in the QLL, due to exclusion during freezing (31-33) and gas-phase deposition to the ice surface. Therefore, chemical transformations in ice are thought to primarily occur in these two regions (22). The physical properties of the QLL are not well understood (see for example refs 22, 34), and neither is its role as a reaction medium (22). Because the QLL presents a very small volume compared to bulk ice, probing its physical attributes and monitoring reaction kinetics there is very difficult. Due to the difficulty of probing the QLL, most investigations of processes occurring there are performed by analyzing melted ice samples. This approach assumes that kinetics in the QLL and in liquid impurities within bulk ice are similar; however, the validity of this assumption has never been tested. The results of two recent studies suggest that for PAH photolysis, reaction rates in these two regions of ice might in fact be quite different: In 2007, we showed (24) that the PAHs anthracene and naphthalene photolyze much more rapidly at air-ice interfaces than in aqueous solution or at air-water interfaces. Contrarily, in 2009 Ram and Anastasio (25) reported photolysis rates for phenanthrene, fluoranthene, and pyrene which were similar in bulk ice and in aqueous solution. The discrepancies between the results of those two studies (24, 25), if not due to some artifact(s) associated with the different methods used, suggest that photolysis kinetics of aromatics could proceed at different rates in bulk ice and at air-ice interfaces. In the present work, we test the hypothesis that photolysis kinetics of aromatics could proceed at different rates in bulk ice and at air-ice interfaces, and find evidence to support it. Our results suggest that relating kinetics in melted ice samples to reactivity in the QLL may not always be appropriate. To determine whether the enhanced photolysis rates at ice surfaces compared to in bulk ice could be due to higher effective photon fluxes at the surface, we report photon flux dependences for anthracene and naphthalene photolysis. We find that at photon fluxes relevant to the Earth’s surface, the photolysis rates of aromatic compounds in aqueous solution and on ice are independent of photon flux. This surprising result suggests that common extrapolations from laboratory measurements may result in significant underpredictions of photolysis lifetimes of aromatic pollutants in the environment. Together, the results of these two different experiments indicate a need to question the way in which laboratory results are used to predict photochemical fates of aromatics in the environment.

Experimental Methods Bulk vs Surface. Samples were irradiated with the output of a 75 W xenon arc lamp which passed through a 295 nm long-pass cutoff filter. The lamp power was ∼0.5 W as measured by a power meter optimized between 250 and 400 nm. The distance between the lamp and the sample was ∼50 cm. Aqueous samples were irradiated in 10 mm path length 10.1021/es9031612

 2010 American Chemical Society

Published on Web 01/21/2010

quartz cuvettes at room temperature. Ice samples were irradiated in an aluminum reaction chamber with an opening in its roof to allow the output of the lamp to reach the sample. Copper tubes connected to a circulating chiller, maintained at approximately -15 °C, ran beneath the copper floor of the chamber. The temperature within the chamber was maintained at approximately -15 °C. Two types of ice sample were used: Ice cubes were prepared by freezing ∼5 mL aqueous anthracene solution in an ice cube tray and then transferring the frozen sample to the reaction chamber to undergo photolysis. The ice cubes were semispheres with radii of ∼1.5 cm. High surface area samples were prepared by crushing the ice cubes with a pestle on the chilled chamber floor prior to photolysis; the ice granules that were formed had radii of approximately 2 mm, and so their surface areato-volume ratios were approximately a factor of 4.4 greater than that of the cubes. Photon Flux Dependence. Samples were irradiated with the output of a xenon arc lamp which passed through a 295 nm long-pass cutoff filter. The distance between the lamp and the sample was ∼50 cm for aqueous samples, and ∼90 cm for ice samples. A 75 W lamp was used for anthracene samples and a 100 W lamp was used for naphthalene samples. The naphthalene rate constants obtained with the 100 W lamp were scaled to those we measured previously (24) using the 75 W lamp. Photon flux dependences were determined by measuring kinetics at various lamp powers. The lamp power was measured with a power meter optimized between 250 and 400 nm, and the power was varied by placing wire mesh in between the lamp and the sample. The photon flux at various lamp powers was determined by performing nitrite actinometry, which is discussed fully in the online Supporting Information to ref (35). Aqueous solutions were irradiated in 10 mm path length quartz cuvettes at room temperature, with the lamp output striking the cuvette horizontally such that the sample was illuminated uniformly. Ice samples were irradiated in a reaction chamber (24) which consisted of a Teflon box with a quartz window in the roof through which the lamp’s radiation entered the chamber. Copper tubes attached to a circulating chiller ran beneath the copper floor of the chamber. The temperature within the chamber was maintained at approximately -15 °C. Sample Preparation. Aqueous nitrite solutions were prepared by combining 1.2 × 10-3 mol L-1 benzene (ACP, g99.0% purity) and 3.5 × 10-5 mol L-1 NaNO2 (Sigma Aldrich, Reagent Plus) in 18 MΩ · cm deionized water. Aqueous PAH solutions were prepared by diluting saturated solutions (Sigma-Aldrich, 99% purity) in 18 MΩ · cm deionized water. Anthracene and naphthalene concentrations were varied between 10-8 to 10-7 mol L-1. For experiments in ice and at air-ice interfaces, anthracene solutions were frozen in the reaction chamber as described in ref (24). Fluorescence Detection and Kinetics Measurements. Nitrite photolysis kinetics were determined by using benzene as an OH trap and monitoring phenol growth by fluorescence detection. Full details of this technique are provided in the online Supporting Information to ref (35). Anthracene and naphthalene photolysis kinetics were measured by monitoring the loss of PAH fluorescence intensity as a function of irradiation time as described in ref (24). We have previously demonstrated (24) that anthracene photolysis kinetics at the surface of aqueous solutions are the same as those measured in solution by traditional offline fluorescence detection. Fluorescence and excitation spectra for most experiments were acquired using a commercial fluorimeter, with the sample contained in a 10 mm path length quartz cuvette. For the surface vs bulk experiments, ice samples were melted after a known time period of irradiation, then stored in brown

FIGURE 1. Logarithmic decay profiles for anthracene photolysis in room-temperature aqueous solution (solid red circles), in ice cubes (open green triangles), and in ice granules (open blue squares). Data from at least two trials are included in each profile. The straight lines are linear fits to the data, and indicate first-order decay.

TABLE 1. Observed First-Order Rate Constants for Anthracene Photolysis in Ice and Aqueous Solutiona medium

kobs (10-4 s-1)

aqueous solution (22 °C) ice cubes (-15 °C) ice granules (-15 °C) air-ice interfaceb (-15 °C)

2.5 ( 0.6 4(2 10 ( 3 10.4 ( 0.8

a Reported errors are standard deviations about the mean for at least three trials. b From ref (24).

glass bottles in the dark until analysis. Analysis was always performed the same day as photolysis. Anthracene fluorescence spectra were acquired by exciting the sample at 355 nm and monitoring emission between 370 and 470 nm, and photolysis kinetics were determined by monitoring the fluorescence intensity of the 380 nm emission peak as a function of irradiation time. Phenol excitation spectra were acquired by measuring emission at 318 nm and scanning the excitation wavelength from 245 to 285 nm. Phenol growth kinetics were measured by monitoring the intensity at 318 nm due to excitation at 271 nm as a function of irradiation time. The fluorescence intensity was converted to phenol concentration with a calibration curve. Anthracene photolysis kinetics at air-ice interface were measured using glancing-angle laser-induced fluorescence (LIF), as described elsewhere (24). We have previously demonstrated (24) that this technique is sensitive to reactions at air-ice interfaces, and that reactions in bulk ice do not contribute to the measured anthracene photolysis kinetics at air-ice interfaces. Naphthalene photolysis kinetics in aqueous solution were also measured in situ using LIF as described elsewhere (24).

Results and Discussion Figure 1 shows representative decay profiles of anthracene as a function of irradiation time in aqueous solution, in ice cubes, and in ice granules. The measured rate constants are summarized in Table 1. Anthracene decayed most slowly in aqueous solution, with a rate constant of (2.5 ( 0.6) × 10-4 s-1, which compares well with that reported for the water surface in ref (24) of (1.7 ( 0.4) × 10-4 s-1. In the ice cubes, the rate was slightly faster ((4 ( 2) × 10-4 s-1), although the increase was not statistically significant. When the ice sample VOL. 44, NO. 4, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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was crushed into granules prior to photolysis the rate increased to (1.0 ( 0.3) × 10-3 s-1; this is the same as the photolysis rate we have measured previously at air-ice interfaces of (1.04 ( 0.08) × 10-3 s-1 (24). The uncertainty associated with the data points for the ice cubes and ice granules is much greater than that associated with the aqueous samples and with the measurements acquired using glancing-angle LIF. This is because a separate sample was used to acquire each data point for anthracene decay profiles in ice cubes and granules, whereas the rate constants determined in aqueous solution and at air-ice interfaces used a single sample for each decay profile. As discussed in the Experimental Methods, the surfaceto-volume ratios of the crushed ice granules are about 4.4× greater than those of the ice cubes. If anthracene molecules were distributed evenly within the ice, then this suggests that the fraction of anthracene at the air-ice interface would be similarly enhanced in the granules compared to in the cubes. If anthracene molecules were concentrated in liquid pockets within the ice, then we still expect a significant enhancement in the surface concentration for the ice granules compared to the ice cubes. This is because ice is most likely to cleave at defect sites such as veins and pockets (36); when we crushed the ice cubes, the ice likely cleaved primarily at the sites of the liquid impurities, resulting in enhanced anthracene concentrations at the surface of the ice granules. Therefore, we are confident that the ice granules had higher relative surface concentrations of anthracene than did the ice cubes. This implies that kinetics measured in the ice granules likely have a greater contribution from reactions at the air-ice interface than do kinetics measured in the ice cubes. To test this, we calculated the relative rates expected in ice cubes and ice granules, assuming that the observed kinetics contain contributions from photolysis at the air-ice interface and from liquid regions within the ice. The surface area of the ice granule samples was approximately 4.4× greater than that of the ice cubes. Given that the rate constant we measure at the air-ice interface using glancing-angle LIF is a factor of 4 greater than that measured in aqueous solution, we calculated the expected relative photolysis rates in the ice granules and ice cubes for different anthracene distributions between the bulk and surface, ranging from anthracene being completely present in the bulk to it being excluded entirely to the surface. These calculations predict that rate constants measured in the granules will be at most ∼2.3× greater than those measured in the ice cubes if approximately 25% of the anthracene is at the air-ice interface. This estimate is in very good agreement with our measured enhancement of a factor of 2.5. Our results strongly support our hypothesis that PAH photolysis kinetics are enhanced at air-ice interfaces compared to in liquid regions within bulk ice. This suggests that bulk ice and ice surfaces should be treated as separate compartments when modeling the chemical fate of aromatic species. The question remains though as to why photolysis kinetics are enhanced at ice surfaces. We previously (24) compared anthracene photolysis kinetics at air-ice and air-water interfaces, and ruled out a number of reasons for the rate enhancements on ice, including differences in temperature and enhanced reagent concentrations on ice. In the following, we explore the possibility that photon fluxes could be enhanced at ice surfaces due to increased light scattering and reflection, resulting in faster photolysis there. Figure 2a shows absorption spectra of 4 × 10-5 mol L-1 nitrite and 2 × 10-7 mol L-1 anthracene in aqueous solution. Although different by orders of magnitude in their absorption cross sections, they absorb over a very similar wavelength range, which suggests that comparing the photolysis behavior of the two species as a function of photon flux is valid. Figure 1304

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FIGURE 2. (a) Absorption spectra of 3 × 10-5 mol L-1 nitrite (solid red trace) and 2 × 10-7 mol L-1 anthracene (dashed blue trace) in room-temperature aqueous solution; (b) Nitrite photolysis rate constants measured as a function of lamp power. Rate constants were determined from the rate of phenol formation, as described in the online Supporting Information to ref (35). Error bars represent the standard deviation about the mean of at least three trials; points without error bars indicate single measurements. The solid trace is a linear fit to the data. 2b shows nitrite photolysis rates measured as a function of lamp power for the 75 W lamp. The observed rate is linearly dependent on lamp power, as expected. On the basis of our actinometry measurements (35), the nitrite photolysis rates measured here can be used to determine photon fluxes from the lamp. Figure 3 shows degradation rate constants of anthracene in aqueous solution and at air-ice interfaces measured as a function of lamp power. Since the loss rate in the dark was minimal, the data point shown at the origin is the slowest rate constant we have measured using LIF; it thus represents an upper limit for anthracene loss in the dark. In the Figure, it is clear that the dependence of the photolytic loss rates on lamp intensity is nonlinear. At lamp powers above 0.1 W, the rates level off both in water and on ice. Figure 3 also shows naphthalene photolysis rates in aqueous solution at higher lamp powers; once again, the kinetics are independent of photon flux. We have also observed a similar nonlinear photon flux dependence for benzene photolysis at air-ice interfaces (37). To our knowledge, such nonlinear photon flux dependences have not previously been reported for direct photolysis reactions. They have been reported in some photocatalytic surface reactions, as discussed elsewhere (38). One possible explanation for the observation of saturation in the photolysis rate is that for these aromatic compounds at least, the

lifetime prediction of 1.4 s. This predicted lifetime is ∼500× shorter than the lifetime predicted from our photon flux dependence, which indicates that increasing the photon flux will not affect anthracene’s lifetime. The difference in the predicted lifetimes of anthracene in aqueous solution in Toronto in June is even greater: A linear extrapolation would underpredict the lifetime by a factor of 1000. These results highlight the need for caution in extrapolating laboratory measurements to environmental conditions for reactions involving aromatic pollutants. They also suggest that we need to better understand icesand especially the region near the air-ice interfacesas a reaction medium before we can confidently predict the chemical fates of species there.

Acknowledgments FIGURE 3. Photolysis rate constants measured as a function of lamp power: For anthracene in aqueous solution (closed blue circles) and at air-ice interfaces (open green diamonds); and for naphthalene in aqueous solution (closed red triangles), scaled down by a factor of 18 to account for the different lamp used. Error bars represent the standard deviation about the mean of at least three trials, except for those at 0 W, which reflect the lowest rate constant measured in our lab using LIF, and represent an upper limit for the rate of anthracene loss in the dark. The curved traces are hyperbolic fits to the data, and are included to guide the reader’s eye. The vertical arrows indicate the total photon fluxes in the wavelength range 295-410 nm at various lamp powers, based on nitrite actinometry; the units of the fluxes are photon cm-2 s-1. absorption cross section is large enough that at high photon fluxes the absorption step of the photolysis is no longer ratedetermining. Most studies of the photon flux dependence of environmental photolysis reactions have been performed on inorganic species which are relatively poor absorbers, such as nitrite; anthracene’s maximum absorption cross section in the actinic region is ∼360× greater than that of nitrite. The nonlinearity that we observe may be common in strong absorbers such as aromatic species. These results indicate that increased photon fluxes at air-ice interfaces are not responsible for the enhanced photolysis rates we observed there, as the surface vs bulk experiments were performed at lamp powers of ∼0.3 W, which is in the saturated region of the photon flux dependence. We therefore conclude that the faster photolysis rates at air-ice interfaces are likely due to differences in the physicochemical properties there compared to liquid water. Our results indicate a need to rethink the manner in which environmental lifetimes of organic pollutants in ice are predicted. We have demonstrated that the distribution of aromatic compounds in ice will greatly affect their photolysis lifetimes, with aromatic compounds located at air-ice interfaces reacting much more quickly than those sequestered within bulk ice. Unfortunately, there is currently no method of determining the distribution of organic species within environmental snow and ice. We have also demonstrated that the photolysis of aromatic compounds in aqueous solutions and at air-ice interfaces does not depend linearly on photon flux. This means that predictions of environmental lifetimes, which generally assume a linear relationship, may significantly underestimate actual lifetimes. To illustrate, extrapolating the anthracene photolysis rate constant measured at air-ice interfaces in our laboratory at a photon flux of ∼3 × 1013 photon cm-2 s-1 over the wavelength range 295-410 nm to the expected photon flux at Summit, Greenland at noon in late June over the same wavelength range (∼3 × 1016 photon cm-2 s-1) estimated using the TUV model (39) yields an atmospheric

The authors thank NSERC and CFCAS for funding for this research. T.F.K. thanks NSERC for a Canadian Graduate Doctoral Scholarship. R.Z. and K.B.J. thank NSERC for Undergraduate Student Research Awards.

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