ARTICLE pubs.acs.org/est
Determination of the Specific Surface Area of Snow Using Ozonation of 1,1-Diphenylethylene Debajyoti Ray,† Romana Kurkova,† Ivana Hovorkova,† and Petr Klan*,†,‡ †
Research Centre for Toxic Compounds in the Environment, Faculty of Science, Masaryk University, Kamenice 3, 62500 Brno, Czech Republic ‡ Department of Chemistry, Faculty of Science, Masaryk University, Kamenice 5/A8, 62500 Brno, Czech Republic
bS Supporting Information ABSTRACT: We measured the kinetics of ozonation reaction of 1,1-diphenylethylene (DPE) in artificial snow, produced by shock freezing of DPE aqueous solutions sprayed into liquid nitrogen. It was demonstrated that most of the reactant molecules are in direct (productive) contact with gaseous ozone, thus the technique produces snow with organic molecules largely ejected to the surface of snow grains. The kinetic data were used to evaluate the snow specific surface area (∼70 cm2 g1). This number is a measure of the availability of the molecules on the surface for chemical reaction with gaseous species. The experimental results were consistent with the LangmuirHinshelwood type reaction mechanism. DPE represents environmentally relevant compounds such as alkenes which can react with atmospheric ozone, and are relatively abundant in natural snow. For typical atmospheric ozone concentrations in polar areas (20 ppbv), we estimated that halflife of DPE on the surface of snow grains is ∼5 days at submonolayer coverages and 15 °C.
’ INTRODUCTION A number of (semi)volatile natural and anthropogenic organic compounds, including various aromatic hydrocarbons, have been detected in natural snow13 because noncovalent interactions and the porous character of the snow crystal/grain surface allows for their adsorption.46 It has been demonstrated that some of the hydrophobic aromatic compounds tend to self-associate on ice/snow surface even at submonolayer coverages.710 The chemical reactivity of snow contaminants is determined by many factors, such as light absorption properties, phase properties of the host matrix, temperature, and the presence of other (reactive) species.11,12 Laboratory studies have confirmed that they can undergo direct or indirect photochemical reactions,3,8,9,1114 or reactions with gas-phase hydroxyl radicals,7,1518 nitrogen oxides,7,17 and ozone.1922 Indeed, surface ozone plays an important role in the chemistry of the polar boundary layer.2326 Low concentrations of snow contaminants as well as the complexity of the system being studied are a great challenge to scientists who want to study the physical and chemical processes occurring in natural snow. However, artificial snow, produced by a shock-freezing preparation technique,2730 may represent a readily available study matrix that can be used to emulate snow chemistry in the laboratory,9,10,13 although natural and laboratory ices may differ in many ways, especially at the molecular (nanoscopic) rather than macroscopic/mesoscopic level.31 The specific surface area (SSA) of snow, that is, the surface area accessible to gases per unit mass, is one of the important physical properties of snow.5,3234 Various methods were used to evaluate the SSA of natural or artificial snow, such as BET35 CH4 adsorption (natural snow, SSA in the order of 10 103 cm2 g1 5,32,33,3639), BET N2 adsorption (natural snow, SSA = 616 102 cm2 g1;40 artificial snow produced by a r 2011 American Chemical Society
nebulizer, SSA ∼2.5 104 cm2g1 41), BET Kr adsorption (artificial snow, SSA = 580680 cm2 g1 42), frontal chromatography (natural snow, SSA = 513 102 cm2 g1 43), or valerophenone (VP) photochemistry (artificial snow, 400440 cm2 g1;9 and artificial snow produced by a nebulizer, ∼104 cm2 g1 10). In addition, the SSA of snow has also been determined using X-ray tomography,44 near-infrared photography,45 and infrared reflectance.46,47 Ozonolysis is a well-established method for the oxidative cleavage of alkenes.48 In general, the primary ozonides, resulting from a 1,3-dipolar cycloaddition of ozone on a CdC bond, fragment to carbonyl oxide (the Criegee intermediate) and the corresponding aldehyde or ketone.49 The intermediates then undergo a 1,3-dipolar cycloaddition to give a trioxolane intermediate, which can be reduced to give aldehydes or ketones in the subsequent step.49,50 1,1-Diphenylethene (DPE), the substrate used in this work, thus produces benzophenone (BP) and formaldehyde in the presence of ozone (Scheme 1). For example, DPE was shown to undergo an ozonation reaction in aqueous acetone to form benzophenone (BP) in 54% isolated chemical yield.51 In this case, the ozonation reaction leads to the final products without the need to isolate or decompose ozonides. A higher BP yield (87%) was obtained in a flow-through chemistry apparatus, in which contact between a DPE solution in methanol and ozone is facilitated by a semipermeable membrane.52 Almost quantitative yield of formaldehyde (as a second product, Scheme 1) was observed to occur upon DPE ozonation in xylene.53 Received: August 21, 2011 Accepted: October 28, 2011 Revised: October 20, 2011 Published: November 09, 2011 10061
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Scheme 1. Ozonolysis of 1,1-Diphenylethene
In this work, the ozonation kinetics of DPE deposited on artificial snow grains, produced by shock freezing of the corresponding aqueous solution sprayed into liquid nitrogen, was utilized to evaluate the specific surface area of this snow. This method allowed us to follow not the accessibility of the snow surface to gases but the availability of the molecules on the surface for a chemical reaction with gaseous O3. In addition, we investigated whether the ozonation kinetics follows a LangmuirHinshelwood mechanism,15,54,55 as was observed by Kahan and Donaldson in the case of phenanthrene ozonation on an ice surface.21 DPE represents environmentally relevant snow contaminants which can undergo an ozonation reaction.
’ EXPERIMENTAL SECTION Preparation of Artificial Snow Samples. Artificial snow samples were produced by spraying pressurized aqueous solutions (organic solvents were avoided) of DPE (c = 1.8 1073.3 105 mol L1) through a hollow cone brass nozzle9,13 into a large vessel containing liquid nitrogen. The size of the individual snow grains formed using this technique (50300 μm) was determined in our previous work.9 Each sample was divided into four homogeneous parts and stored at 15 °C for 1 h before the experiment started. The first part was analyzed directly; the remaining parts were exposed to ozone and analyzed in the same way (Supporting Information (SI)). Ozonolysis. A homogenized snow sample (50 g; 0.5 or 3 cm snow layer thickness) was spread on a Petri dish (of a 7 or 12 cm diameter) situated on a porous porcelain plate inside a glass desiccator. The desiccator was placed inside a stainless-steel freezer, as described elsewhere.9,13 The freezer temperature was kept at 15 ( 3 °C. Ozone was produced in an oxygen stream by UV irradiation using a homemade variable ozone generator. The flow rate of the incoming oxygen was adjusted by a flow meter. An O3/O2 mixture was led through the bottom of a desiccator; the unreacted ozone was directed into an aqueous KI solution via a glass tube outlet located on the top. The snow samples were exposed to ozone for a given time. The flow tube ozone concentrations, determined iodometrically using UVvis spectrometry (SI),56,57 were 8.1 10131.6 1016 molecules cm3 (3.2 500 ppmv). In the control experiments, selected samples were exposed to pure oxygen or nitrogen (a flow of 0.25 L min1).
’ RESULTS AND DISCUSSION Surface coverage (the fraction of the adsorption sites occupied by molecules) of snow grains can be determined from the Langmuir adsorption isotherm,5,58 or calculated from the known matrix surface area and the area occupied by an adsorbed molecule.9,10 The magnitude of the area accessible to gases per unit mass, i.e., the specific surface area (SSA), is then calculated from this number. In our previous work, we employed a VP
Figure 1. Relative concentrations of DPE (9) and BP (b) during ozonolysis of DPE in a snow sample prepared from the corresponding aqueous solution sprayed into liquid nitrogen: c0DPE = 3.6 107 mol kg1 (submonolayer concentration, vide infra); cO3 = 1016 molecules cm3; T = 15 °C; sample layer thickness = 3 cm; all measured data are shown.
photochemistry method to determine the SSA of snow, based on comparison of the reaction rates obtained for different surface loads under the same irradiation conditions.9 When multiple layers of VP molecules are formed, the surface photoactive molecules undergo a photoreaction, but at the same time, act as an internal optical filter for the remaining molecules in the close vicinity, causing a decrease in the overall reaction efficiency. In this work, a modified SSA determination technique was utilized. We evaluated the amount of a hydrophobic compound (DPE) located on a snow surface not as a function of its partial pressure or concentration but as a function of its specific ability to react with a gas-phase reagent ozone. The surface DPE molecules were expected to react with a different rate constant (more rapidly) than those overlaid by another molecular layer of DPE, as was demonstrated for example in a study of the heterogeneous ozonation of a benzo[a]pyrene adsorbed on soot aerosol particles.59 Ozonation kinetics of DPE deposited on the surface of snow grains should thus principally distinguish surface loadings which correspond to either sub- or above monolayer coverage, as a VP photochemistry method did before.9,10 However, the major difference between these two methods is that UV light penetrates the entire snow sample, including micropockets or microveins (where the compounds are unavailable to gases),9,60 whereas ozone reacts with the molecules at the snow/air interface only. Snow samples containing DPE were prepared by shock freezing of DPE aqueous solutions sprayed into liquid nitrogen. This technique was described in detail in our preceding articles, which revealed that small organic hydrophobic molecules are ejected to the surface of snow grains.9,10 The snow samples were placed into a reaction vessel (desiccator) and exposed to an O3/O2 mixture, pure oxygen, or pure nitrogen (the gas flow was always the same). In the first set of experiments, DPE was found to be converted to BP by the reaction with ozone (Scheme 1). The concentrations of both the starting material (DPE) and the product (BP) were followed by GC-MS after the compounds were extracted from the melted samples by dichloromethane. The availability of DPE deposited on the surface of snow grains to undergo an ozonation reaction was tested by prolonged 10062
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Environmental Science & Technology exposure of contaminated snow to ozone at 15 °C. Figure 1 shows the plot of the DPE degradation/BP formation in a 3 cm thick layer of snow, produced from a DPE aqueous solution (the initial DPE concentration, c0DPE, was 3.6 107 mol kg1), as a function of time. The flow tube ozone concentration was constant (cO3 = 1016 molecules cm3), which guaranteed that the reaction can be described by pseudofirst order kinetics (vide infra). The DPE concentration leveled off at ∼90% DPE conversion in 2 h; about 65% of BP was recovered from the sample at the highest conversion. These data demonstrate that most of the reactant molecules are in direct (productive) contact with gaseous ozone/oxygen. A shock-freezing technique thus produces snow with organic molecules largely ejected to the surface of snow grains, as was suggested before.9,10,13,28,29,61 Less polar DPE has a boiling point (270 °C) close to that of BP (305 °C). Because DPE and BP losses by evaporation from the snow samples could interfere with the ozonation kinetics, they were tested by flowing nitrogen through a desiccator containing the contaminated snow samples. For DPE (cDPE = 3.2 106 mol kg1) the loss was below 15% after 2 h, below 5% after 15 min; for BP (cBP = 5 106 mol kg1) the loss was below 18% after 2 h, below 5% after 15 min. As a result, the lower BP isolated yields partly reflect evaporation losses. Furthermore, sample exposure to pure oxygen was carried out in order to find whether oxygen can also convert DPE to BP. The extent of this oxidation reaction was always below 10% (excluding the losses by evaporation) compared to that of ozonation in all cases. All data shown in this study are corrected for these losses determined in the corresponding control experiments. The reaction conversions were kept low (below 25%) in all kinetic measurements in this work in order to retain relatively constant DPE surface loads during the experiment, minimize the DPE loss by evaporation, and avoid interference from the products. In addition, we expected that the outermost grains of a snow sample are in a direct contact with ozone, whereas the reaction on the subjacent grains can be restricted by ozone diffusion through the top layers. Indeed, a more extensive overall ozonation reaction was observed when the snow sample layer was thinner (SI Figure S1). The SSA determination method utilized in this work aimed to distinguish monolayer from multilayer coverage. Because the shock-freezing technique affords in principle uniform DPE distribution9,10,13,28,29,61 in the snow sample (bulk), uneven ozone exposure should not affect the calculated SSA magnitude when the loss by evaporation is kept low. Figure 2 shows the observed pseudofirst order rate constants (kobs) of DPE ozonation obtained at different initial (prefreezing) DPE concentrations (c0DPE; this value is closely related to the ice surface DPE loadings provided that DPE is largely ejected to the grain surface). These rate constants were determined by plotting ln(cDPE/c0DPE) vs reaction time using the least-squares method, where cDPE are the DPE sample concentrations in the course of ozonation (SI Figure S2). The plots were linear at DPE conversions lower than ∼70%, that is, the degradation followed the first-order kinetics, which is characteristic for heterogeneous ozonation of aromatic hydrocarbons under the flow reactor conditions, where cO3 is constant.62 At higher conversions, a slower kinetic component appeared (one example is shown in SI, Figure S3), which may be related to the reaction of DPE molecules overlaid by another molecular layer of DPE or buried in the snow matrix (ozone must diffuse through the top layers). However, this interpretation is
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Figure 2. Ozonation kinetics of DPE in snow as a function of c0DPE. Snow was prepared by a shock-freezing method; cO3 = 1016 molecules cm3; T = 15 °C; sample layer thickness = 3 cm; exposure to O3: 15 min; all measured kobs values and the standard deviations are shown; the values are corrected for losses caused by evaporation and O2 oxidation.
ambiguous with regard to the measurement error incurred at low cDPE due to heterogeneity of the samples and the methods of their preparation. Figure 2 shows a dependence of the pseudofirst order rate constants on the initial DPE concentrations. For concentrations below c0DPE = ∼2 105 mol kg1, kobs was approximately 1.6 ((0.4) 102 s1. With increasing surface load, the rate constants decreased to kobs ∼3 103 s1 at c0DPE = 3.2 105 mol kg1, and this part of the data set can be fit with linear regression. We were unable to increase this (maximum) concentration because of the DPE solubility limits. We assume that the statistical dispersion of the data is caused by the heterogeneous nature of the samples. Alebic-Juretic, Cvitas, and their co-workers observed different rate constants for reactions of the adsorbed molecule (polycyclic aromatic compound) at sub- and above monolayer coverages on silica gel (and other) solid surfaces.63,64 They assumed that the primary oxidation products quickly desorb, and interpreted the increased rate constants for submonolayer coverages in terms of enhanced reactivities on the acidic silica gel surface.63 However, P€oschl and co-workers noted that the corresponding oxidation products are less volatile than the parent compounds and thus cannot easily desorb.59 In their experiments, they observed that the pseudofirst order rate constants of ozonation of benzo[a]pyrene on soot aerosol particles exhibited Langmuir-type dependencies on ozone concentrations. For submonolayer surface coverages the rate constants were found to be practically independent of the degree of coverage. At the coverage corresponding to two molecular layers, the rate constant was 30% lower. Analogous to the P€oschl’s work,59 we concluded that DPE molecules on snow grains are ozonized at approximately the same rate constant on the surface covered by a monolayer or less. A monolayer surface concentration (designated as cLDPE ∼2 105 mol kg1 in Figure 2; L as a Langmuir layer) is then identified by sudden decrease of kobs with increasing c0DPE. When an additional molecular layer is formed at higher DPE surface loads, the outermost DPE layer undergoes the reaction, leaving a relatively nonvolatile product (BP) in the same location. The overall rate constants are decreased because the subjacent layers of DPE are much less available to ozone exposure. 10063
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reaction of two species adsorbed on a surface.15 Similar results were also observed by Kahan and Donaldson in their study of phenanthrene ozonation on the surface of ice.21 One of the molecules (ozone) is in rapid equilibrium between the gas and solid phases. The experimental data fits well into the Langmuir Hinshelwood equation: kobs ¼ kmax
Figure 3. Pseudofirst-order rate constants (kobs) as a function of gasphase ozone concentration for the reaction of DPE and ozone on an artificial snow grain surface at 15 °C. The error bars represent the standard deviations. Fitting of the LangmuirHinshelwood equation to the data gave the mean values of KO3, kmax, and their standard errors.
Analogous to our previous studies,9,10 the cLDPE value (2 10 mol kg1; Figure 2) and the area of a single DPE molecule (calculated from the structure optimized with a DFT method9 (∼6 1019 m2)) were used to determine the SSA of artificial snow (see also SI). The calculated SSA value of approximately 70 cm2 g1 was smaller than that obtained using VP photochemistry (SSA ∼ 400 cm2 g1)9 for snow produced by the same technique and processed at the same temperature. We made sure that only a negligible amount (below 5%) of both DPE and BP was lost through desorption during the experiment (vide supra). The smaller SSA can be justified by the fact that it relates to the molecules which are in a physical contact with ozone molecules, while VP molecules are photolyzed in the entire sample volume and inherently cannot act as an ideal optical filter on a porous snow grain surface (the molecular layer geometries are complex; radiation is scattered). This means that the VP photochemistry method overemphasizes the number of outermost molecules, thus providing an SSA higher than the ozonation technique presented here. In addition, DPE is more hydrophobic than VP; therefore, the ice surface could be specifically altered by its presence. Kahan and Donaldson determined the equilibrium surface concentration of phenanthrene to be 5 1013 molecules cm2.21 Using a simple molecular model, the area of a single phenanthrene molecule is almost the same as that of DPE. For the SSA of snow equal to 70 cm2 g1, the phenanthrene concentration would be 5.8 106 mol kg1, which is in a good agreement with the DPE concentration corresponding to a monolayer coverage (at cLDPE = 2 105 mol kg1), determined for this work. In addition, the observed rate constants (kobs) of DPE ozonation were measured as a function of the gas-phase concentration of ozone at 15 °C (Figure 3). Using the cLDPE value, the representative snow samples of sub- (1.8 107 and 3.6 107 mol kg1, corresponding to cLDPE 0.009 and cLDPE 0.02, respectively) and above monolayer coverages (2.1 105 and 2.8 105 mol kg1, corresponding to cLDPE 1.1 and cLDPE 1.4, respectively), were prepared. The kobs dependences on cO3 were not linear. The kobs values leveled off at high ozone concentrations, which is consistent with the Langmuir Hinshelwood type reaction mechanism describing the bimolecular 5
KO3 ½O3 1 þ KO3 ½O3
where kmax is the maximum pseudofirst order rate constant at saturated surface concentration of ozone, KO3 is the adsorption equilibrium constant of ozone, and [O3] is the gas phase ozone concentration (cO3).55 The calculated values are shown in Figure 3. The course of a heterogeneous reaction on snow depends, among other things, on the static and dynamic properties its surface.9,10 A sufficiently flexible disordered QLL surface of ice65 was shown to allow hydrophobic organic molecules to move or diffuse at relatively higher temperatures at submonolayer concentrations,9 and its physical nature is influenced by the presence of ice impurities as well as temperature.66,67 It has been demonstrated that ice surface coverage of adsorbed ozone under stratospheric conditions is not significant, and that the flux of water molecules from/to the surface does not trap ozone efficiently.68 Abbatt and co-workers demonstrated that surface quality affects the ozone partitioning irrespective of the organic species on the aerosol substrate.69 However, the number of reactant monolayers has been shown to influence reaction rates.59,62 For example, the KO3 values ranged from 1013 to ∼0.5 1015 cm3 for heterogeneous ozonation of anthracene on phenylsiloxane oil thin film69 and water,70 respectively. Interestingly, no temperature effect on the ozonation kinetics of phenanthrene at the airice interface was found in the range of 30 and 10 °C.21 In our study, the equilibrium constant KO3 values were found higher for above-monolayer coverages by an order of magnitude (KO3 = ∼2.5 1014 cm3) than those for submonolayer coverages (KO3 = ∼2 1015 cm3). It means that ozone has a higher affinity to a surface covered by DPE molecules than that to a polar (premelted) ice surface. This is in agreement with the conclusions of Hung and co-workers who have recently reported a simulation study of the adsorption of gas-phase naphthalene and ozone on atmospheric air/ice interfaces.71 They have demonstrated that when the airice interface is coated with increasing concentrations of naphthalene molecules, the surface adsorption of ozone is enhanced. Using the data from Figures 2 and 3, higher and practically invariant rate constants kobs and kmax (∼103 s1) are observed for submonolayer DPE concentrations, compared to abovemonolayer concentrations. The kmax rate constant values decreased with increasing DPE loading above c0DPE = ∼2 105 mol kg1, which, however, approaches the DPE solubility limits. Nevertheless, the data in Figure 3 are complementary to those in Figure 2 and differentiate the surface coverage. Environmental Implications. The area occupied by DPE is similar to that of many common small organic molecules within an order of magnitude, and the SSA of artificial snow produced in this work (70 cm2 g1) was found to be comparable to that of natural snow (10103 cm2 g1 5,32,33,3639). This study supports our previous conclusions, according to which artificial snow may have properties found in natural snow. A simple sample 10064
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Environmental Science & Technology preparation and subsequent utilization are undoubtedly the major advantages of using artificial snow for laboratory study of the chemical processes of organic contaminants occurring in polar areas. DPE represents environmentally relevant compounds such as alkenes which are relatively abundant in natural snow,12 and can undergo ozonolysis. The experimental ozone concentrations used in this work (3.2500 ppmv) were considerably higher than those of tropospheric ozone in polar areas (for example, the projected median annual ozone mixing ratios were ∼2050 ppbv at the Northern Hemisphere stations in 200526). Using our results shown in Figure 2, we estimated the half-life of DPE on the surface of artificial snow grains to be 5.1 and 2.2 days at cO3 = 20 and 50 ppbv, respectively, submonolayer DPE loadings, and 15 °C. This number excludes other possible chemical transformations and physical processes that can occur on the surface. Our estimates are well in accord with that of phenanthrene halflife on the ice surface at 50 ppbv ozone concentration (5 days) reported by Kahan and Donaldson.21
’ ASSOCIATED CONTENT
bS
Supporting Information. Experimental details: materials; instrumentation; determination of ozone concentration; sample analyses; ozonation of snow samples of different layer thickness; determination of the rate constants. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*Phone: +420-549494856; fax: +420-549492443; e-mail: klan@ sci.muni.cz.
’ ACKNOWLEDGMENT The project was supported by the Grant Agency of the Czech Republic (P503/10/0947), the Czech Ministry of Education, Youth and Sport (MSM0021622412), 7FP EU ArcRisk (226534), and the project CETOCOEN (CZ.1.05/2.1.00/01.0001) granted by the European Regional Development Fund. We express our thanks to Dana Nachtigallova for the calculation of the area of a single DPE molecule, and Petr Kukucka for helping with the GC-MS analyses. ’ REFERENCES (1) Jaffrezo, J. L.; Clain, M. P.; Masclet, P. Polycyclic aromatic hydrocarbons in the polar ice of Greenland - Geochemical use of these atmospheric tracers. Atmos. Environ. 1994, 28, 1139–1145. (2) Masclet, P.; Hoyau, V.; Jaffrezo, J.; Legrand, M. Evidence for the presence of polycyclic aromatic hydrocarbons in the polar atmosphere and in the polar ice of Greenland. Analusis 1995, 23, 250–252. (3) Domine, F.; Shepson, P. B. Air-snow interactions and atmospheric chemistry. Science 2002, 297, 1506–1510. (4) Wania, F.; Hoff, J. T.; Jia, C. Q.; Mackay, D. The effects of snow and ice on the environmental behaviour of hydrophobic organic chemicals. Environ. Pollut. 1998, 102, 25–41. (5) Domine, F.; Cincinelli, A.; Bonnaud, E.; Martellini, T.; Picaud, S. Adsorption of phenanthrene on natural snow. Environ. Sci. Technol. 2007, 41, 6033–6038. (6) Herbert, B. M. J.; Halsall, C. J.; Villa, S.; Jones, K. C.; Kallenborn, R. Rapid changes in PCB and OC pesticide concentrations in Arctic snow. Environ. Sci. Technol. 2005, 39, 2998–3005.
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