Product Study of the Gas-Phase BrO-Initiated ... - ACS Publications

Jul 2, 2004 - Sciences, McGill University, 801 Sherbrooke Street West,. Montreal ... and on wall deposits, and identified HgBr, HgBrO/HgOBr, and HgO a...
0 downloads 0 Views 211KB Size
Download PDF
Environ. Sci. Technol. 2004, 38, 4319-4326

Product Study of the Gas-Phase BrO-Initiated Oxidation of Hg0: Evidence for Stable Hg1+ Compounds FARHAD RAOFIE AND PARISA A. ARIYA* Departments of Chemistry and Atmospheric and Oceanic Sciences, McGill University, 801 Sherbrooke Street West, Montreal, Quebec, Canada H3A 2K6

Mercury is a key toxic environmental pollutant, and its speciation affects its bioavailability. BrO radicals have been identified as key oxidants during mercury depletion events observed in Arctic and sub-Arctic regions. We report the first experimental product study of BrO-initiated oxidation of elemental mercury at atmospheric pressure of ca. 0.987 bar and T ) 296 ( 2 K. We used chemical ionization and electron impact mass spectrometry, gas chromatography coupled to a mass spectrometer, a MALDITOF mass spectrometer, a cold vapor atomic fluorescence spectrometer, and high-resolution transmission electron microscopy coupled to energy dispersive spectrometry. BrO radicals were formed using visible and UV photolysis of Br2 and CH2Br2 in the presence of ozone. We have analyzed the products in the gas phase, on suspended aerosols and on wall deposits, and identified HgBr, HgBrO/HgOBr, and HgO as reaction products. Mercury aerosols with a characteristic width of ca. 0.2 µm were observed as products. We herein discuss the implications of our results to the chemistry of atmospheric mercury and its potential implications in the biogeochemical cycling of mercury.

Introduction The dominant form of mercury in the atmosphere is elemental mercury (Hg0(g)), and it is generally assumed to be long-lived (∼1-2 yr) (1). However, Schroeder et al. (1) reported surprisingly rapid depletion of gaseous mercury from the atmospheric boundary layer during spring ozone depletion events in the high Arctic at Alert (82.5° N, 62.3° W), Canada. Hg0(g) depletion is observed to be widespread in the Arctic (2), Antarctica (3), and the marine sub-Arctic (4) and is associated with enhanced mercury deposition in surface snow and ice (5). Interestingly, a near complete depletion of ozone in the boundary layer over large areas and evidence of reactive halogens have been observed during most mercury depletion events (MDEs) (6-9). Upon reaction with atmospheric oxidants, Hg0(g) can be transformed to oxidized forms, which are also more bioaccumulative than elemental mercury (10). Oxidized mercury species are more hygroscopic than Hg0(g) and are readily deposited on aerosols or ice, potentially being assimilated into the food chain upon snowmelt. Indeed, very toxic methyl mercury can be produced from inorganic Hg2+ precursors and can be biomagnified in the human food chain via natural biotic and abiotic processes (11, 12). * Corresponding author: phone: 514-398-6931; fax: 514-398-3797; e-mail: [email protected]. 10.1021/es035339a CCC: $27.50 Published on Web 07/02/2004

 2004 American Chemical Society

Substantial experimental work has been done on aqueous redox reactions of mercury species (e.g., refs 13-20). Some aqueous phase redox processes are considered reasonably well understood. However, there are key gaps in comprehending the reaction intermediates and understanding the reaction mechanisms. Unlike the reactions of Hg0 in solution, many gaseous reactions of mercury with atmospheric oxidants are difficult to investigate experimentally due to low concentrations of species at atmospheric conditions, low volatility of products, and the strong effect of water vapor and surface on kinetics (e.g., refs 21-23). Reactions of mercury with halogen oxide radicals drew major attention in light of the satellite “BrO” column measurement as well as simultaneous mercury and ozone depletion in the planetary boundary layer (24-27). It has become increasingly clear that mere physical processes such as adsorption and condensation of Hg0(g) on surfaces, including snow and ice-fog crystals, cannot elucidate the magnitude of Hg0(g) disappearance in the boundary layer (6). Although the reactions of BrO have been suggested to be key processes driving mercury depletion events (5), kinetic studies of BrO + Hg0 are very limited (28). Furthermore, there have been no previous product studies to identify the nature of the products of the BrO-initiated oxidation reaction of elemental mercury and hence the extent of bioaccumulation of mercury in aquatic systems. In aquatic systems mercury can be transformed into methyl mercury. The existence of stable mercurous ions, Hg22+, in addition to mercuric ions (Hg2+), in the aqueous phase has been frequently demonstrated (29-31). Mercurous ions have been obtained by reduction of mercuric salts. For instance, through a pulse radiolysis study, it was previously shown that the transient mercurous ions can lead to absorption peaks at 225 and 250 nm (31). However, to our knowledge, there is no previous laboratory evidence for stable Hg1+ species as the result of gas-phase reactions. It is commonly assumed that upon reaction of atmospheric oxidants with gaseous elemental mercury, mercuric ions are formed. The charge density and ionic diameter of Hg1+ and Hg2+ are different; therefore, one anticipates distinct hygroscopic behavior for Hg1+ in comparison with Hg2+. It is, hence, crucial to evaluate the speciation of mercury in atmospheric oxidation reactions in order to shed light on the potential bioaccumulation of atmospheric degradation products. We herein provide the first experimental product study of BrO-initiated oxidation of elemental mercury. We identified, in the course of BrO + Hg0(g) reactions, stable Hg1+ in the form of HgBr under tropospheric conditions. We discuss the implications of our results for the biogeochemical cycling of mercury.

Methodology Experiments were carried out at near atmospheric pressure (0.987 ( 0.001 bar) (98658.58 Pa) and room temperature (296 ( 2 K) in N2 and air. Reaction chambers were 2- and 3-L Pyrex double-wall flasks equipped with magnetic stirrers to ensure homogeneous mixing. The flask temperature was maintained at 296 ( 2 K by circulating water through the outer jacket using a circulator (Neslab RTE 111). The reaction flask was washed with acetone several times and dried at high temperature (T ) 398 K) overnight. The inside wall of the reaction flask was coated with halocarbon wax (Supelco) to prevent undesirable wall reactions (32). A vacuum system was used to prepare gaseous reactant mixtures. Gaseous substrates were introduced directly into the reaction mixture using a 10- or 250-µL gastight syringe (Hamilton series 1800 VOL. 38, NO. 16, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

9

4319

gastight). Liquid substrates were injected with a 2- or 10-µL syringe (Hamilton series 700). Ozone was produced using an ozone generator (model OL 100/DS, Ozone Services Inc.) by a silent discharge technique and was trapped on silica gel cooled to 200 K in a dry ice-acetone mixture. Ozone was then transferred to an evacuated flask (∼10-7 bar; ∼10-2 Pa), the and concentration of ozone was monitored by an ultraviolet-visible spectrometer (UV-vis, Varian Cary-50Bio). A given amount of ozone was injected into the reaction flask (2-3 L) using a gastight syringe. GC-MS Studies. Concentrations of mercury were monitored by MS detection (quadrupole MSD HP 5973) after separation on a gas chromatograph (HP 6890) equipped with a 0.2 mm i.d. × 30 m cross-linked phenyl-methyl-siloxane column (Hp 5-MS). The column was operated at a constant flow (1.5 mL min-1) of helium and was kept isothermal at 40 °C for 1 min. During the chromatographic runs, the oven temperature was then increased by 25 °C min-1 from 40 to 150 °C. At the beginning of each kinetic run, the first analysis was performed using the scanning mode. The product study was further performed using the single-ion monitoring (SIM) mode. Direct Mass Spectrometry. Reaction products were collected using two methods. First, the reaction products were passed through a 1.1 mm i.d. × 10 cm length Pyrex tube (Corning) immersed in liquid nitrogen. In the second method, we washed the wall of the reaction flask with concentrated HCl. A 50 µL amount of the collected products was transferred in a 1.1 i.d. × 10 cm length Pyrex tube, and a piece of glass wool was immediately placed over the samples. Extra HCl was evaporated by heating at 150 °C. Finally, the collected products in the tube were evaporated at stepwise elevated temperatures to the chemical and electron impact ion source of a Kratos MS25RFA mass spectrometer. CVAFS. Samples were prepared by washing the wall of the reaction chamber, Teflon filter (0.45 µm), and coiled Pyrex trap with a mixture of 20 mL of HNO3 and 0.5 mL of 30% H2O2, diluted to 50 mL. The diluted sample was heated to 350 K for 1 h to decompose extra H2O2 and again diluted to 100 mL in a volumetric flask and analyzed using a cold vapor atomic fluorescence spectrometer (Tekran 2600). Derivatization. This method is based on chemical transformation of Hg2+ to HgCl2 and then to a more volatile organomercury compound, n-Bu2Hg. Samples were prepared by washing the wall of the reaction flask with HCl, which transformed Hg2+ to HgCl2. The sample was heated to remove extra HCl, which resulted in the formation of a white residue. Derivatization was performed using a previously reported method (22, 33, 34). A 2 mL amount of toluene and 0.4 mL of 2 M n-butylmagnesiumchloride in tetrahydrofuran were added to the white residue. The mixture was then centrifuged at 0 °C for 10 min with occasional shaking. Subsequently, 0.4 mL of 0.6 M HCl was added to quench the excess derivatizing agent, the mixture was centrifuged, and the organic phase was collected for analysis.

Hg2+ + 2HCl f HgCl2 + 2H+

(1)

HgCl2 + 2n-BuMgCl f n-Bu2Hg + 2MgCl2

(2)

Transmission Electron Microscopy. Reaction products were collected by two different methods. The first technique involved placing grids on the surface of the reaction flask and collecting the grids upon completion of the reaction. The second method involved the collection of reaction products in a capillary Pyrex tube, which was immersed in liquid nitrogen. Collected products were analyzed using a high-resolution transmission electron microscope (JEOL 2000 FX TEM). The chemical composition was qualitatively determined by energy dispersive spectroscopy (EDS; JEOL 2000). 4320

9

ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 38, NO. 16, 2004

Matrix-Assisted Laser Desorption Ionization Time-ofFlight Mass Spectrometry (MALDI-TOF-MS). The reaction products were collected in a capillary Pyrex tube, which was immersed in liquid nitrogen. The collected products were analyzed using Kratos Kompakt MALDI-TOF-MS in reflectron mode. Dithranol and lithium bromide were used as matrix and cationization agent, respectively. The matrix has resonance absorption at laser energy (λ ) 337 nm) and thus absorbs the laser energy, causing rapid heating of the matrix. This rapid heating results in expulsion and soft ionization of the sample molecules without fragmentation (35). Materials. Mercury (99.99% purity) and n-butylmagnesium chloride (2 M solution in tetrahydrofuran) were supplied by Aldrich. Nitrogen (99.998%), argon (99.999%), helium (99.999%), nitric oxide (99.99%), and extradry oxygen were obtained from Matheson. Toluene (99.9%), hydrogen peroxide (30%), hydrochloric acid (trace metal grade), and nitric acid (trace metal grade) were purchased from Fisher. Bromine (>99%), dibromethane (>99.5%), and tin(II) chloride (99.999%) were obtained from Aldrich. Water used was (MilliQ) of 18.2 MΩ cm resistance.

Results and Discussion In the absence of irradiation, no change in the concentration of elemental mercury was observed, and no products were observed under our experimental conditions. BrO radicals were generated in situ upon visible photolysis of bromine (400 e λ e 700 nm) or UV photolysis (λmax = 250 nm) of dibromomethane in the presence of ozone

Br2 + hυ (400 e λ e 700 nm) f Br• + Br•

(3)

CH2Br2 + hυ (λ ≈ 250 nm) f CH2Br• + Br•

(4)

Br• + O3 f

BrO + O2 (k ) 1.2 × 10-12 cm3 molecule-1 s-1 (37)) (5)

Separate experiments were designed to positively identify BrO. Bromine oxide was detected by using NO as the reactive probe. NO2 peaks were monitored with m/z 14, 16, 30, and 46 in the SIM mode. The observed isotopic ratios for these ions corresponded to 9.6:22.3:100:37.0, confirming the presence of nitric oxide and consequently the presence of BrO in the reaction system according to eq 6 (36, 37).

BrO + NO f Br + NO2 (k ) 2.1 × 10-11 cm3 molecule-1 s-1 (38)) (6) The reaction of ozone with elemental mercury is slow ((7.49 ( 0.90) × 10-19 cm3 molecule-1s-1 (39-41)). Hence, the extent of reactions of ozone with mercury under our experimental conditions is assumed to be insignificant in light of our preliminary kinetic data on bimolecular reactions of BrO with elemental mercury (10-15 cm3 molecule-1 s-1 < kBrO < 10-14 cm3 molecule-1 s-1 (28)). In our previous studies of Br-initiated oxidation of elemental mercury (23), we observed the formation of HgBr2 in N2 diluent. No HgBr has been identified. In this study, we did not observe any HgBr2, indicating that the Br reactions were quenched completely in the presence of ozone and the reaction products were formed upon BrO-initiated oxidation of Hg0. However, we carried out a simple chemical modeling experiment using the ACUCHEM package (43) in which we incorporated reactions of mercury with O3, Br, Br2, BrO, NO, HO, HO2, O (3P), O(1D), and NO2. Under our experimental conditions, the predominant reaction process is seemingly BrO-initiated oxidation of elemental mercury. After each photolysis, we aged our samples in the dark and no reformation of mercury or formation of products was observed under our experi-

FIGURE 1. Mass spectra of products formed in the reaction of Hg0 with BrO radicals. Insets show theoretical mass calculations of (a) HgO, (b) HgBr, and (c) HgOBr. mental conditions and detection limits. However, we cannot rule out the possibility of heterogeneous reactions that we are unable to detect. Mercury ions were monitored with m/z 199, 200, 201, and 202 in the SIM mode using GC-MS. The observed isotopic ratios for these ions corresponded well to the expected m/z ratios of 56:78:44:100. The retention time of mercury ranged from 1.3 to 1.5 min, depending on the column temperature. The detection limit for Hg0 was 10 ppbv (1 ppbv ) 2.45 × 1010 molecule cm-3). It is noteworthy that the high-pressure gaseous mercury inside the bulb of the UV lamp (280-320 nm) acts as a very narrow efficient filter and removes the 253.6 nm mercury excitation line from radiated light (42). Hence, in the case of photolysis of dibromomethane, the formation of the excited mercury was avoided by using Oriel UV lamps. We identified the reaction products in the gas phase from the suspended aerosols and from the wall of the reaction flask. The chemical structure of the reaction mixture was identified using mass spectroscopy equipped with a chemical ionization ion source. The probe temperature was augmented to 430 K. We clearly identified the products HgOBr, HgBr, and HgO as shown in the mass spectrum in Figure 1. The isotopic ratio for HgO 33.5, 56.3, 77.4, 44.4, 100.0, and 23.0 corresponded well with the anticipated m/z ratios of 214, 215, 216, 217, 218, and 220, further supporting the identified mercury compounds. The isotopic ratio of 19.3, 32.0, 62.7, 56.6, 100.0, 24.8, 68.7, and 12.8 matched m/z ratios of 293, 294, 295, 296, 297, 298, 299, and 301, illustrating the presence of HgBrO or HgOBr. The presence of HgBr was confirmed using m/z ratios of 279, 280, 281, and 283, corresponding to

isotopic ratios of 62.8, 56.6, 100.0, and 24.7, respectively. It should be noted that NH3 was used as a reagent gas in the ion source. Hence, (M + 1)+ ions, as opposed to M+ ions, were observed (e.g., Figure 1). We also observed peaks corresponding to Br (m/z ) 81) (Figure 1). Note the Br radical lifetime in the reaction chamber is rather short and therefore cannot be probed by direct MS without a time-resolved system. Due to the fragmentation challenge of the chemical ionziation mass spectrometer, it is feasible that reaction products such as HgO or HgBr may be formed from HgOBr dissociation within the ion source of the MS. To ensure positive confirmation of reaction products, we used another complementary technique, MALDI-TOF-MS, in which fragmentation of HgOBr does not occur. The reaction products (gas and aerosols) were collected in a liquid N2 cooled capillary tube. An example of MALDI-TOF-MS spectrum is illustrated in Figure 2. The peak at mass 218, 283, and 297 m/z ratios are assigned to HgO, HgBr, and HgBrO/HgOBr, respectively. The peaks at 232 and 241 are raised from the matrix itself. Hence, we confirm the existence of HgBr and HgO as reaction products. Neither CI-MS nor MALDI-TOF-MS techniques can properly distinguish between HgOBr and HgBrO. The presence of Hg2+ on the wall of the reaction flask was confirmed by performing two different types of experiments. In the first method, Hg2+ was converted to the more volatile organomercury compound, as described in the Methodology section of the Experimental Section. The derivatized sample was analyzed using GC-MS. Figure 2 represents a chromatogram and mass spectrum of derivatized mercury. It is noteworthy that Hg0 and Hg1+ were not targeted in derivatization analysis, and thus, their existence cannot be evaluated VOL. 38, NO. 16, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

9

4321

FIGURE 2. MALDI-TOF mass spectrum of products formed in the reaction of Hg0 with BrO radicals

FIGURE 3. Mass spectrum of the chromatographic peak eluted at 7.55 min corresponding to derivatized mercury (di-n-butylmercury). (inset) Chromatogram of (di-n-buthylmercury). using the Grignard reagent. During the second set of experiments, Hg2+ was converted to HgCl2 by washing the wall of reaction flask with concentrated HCl. The collected sample was analyzed using direct MS with an electron impact 4322

9

ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 38, NO. 16, 2004

ion source (Figure 3). We, hence, conclude the existence of Hg2+ as a reaction product on the wall. Several experiments were carried out to quantitatively investigate mercury products observed in the gas phase in

FIGURE 4. Mass spectrum of HgCl2. (inset) Theoretical mass spectrum of HgCl2.

TABLE 1. Quantification of Mercury in the Reaction Vessel

mercury species yield (%) characteristic width (diameter)b (µm)

gas + aerosolsa

aerosols collected on 1 µm filters

wall deposits

(32 ( 1) 0.2

(29 ( 1) 0.2

(68 ( 2) 0.2-0.6

a Collected in capillary tube, gaseous mercury mixing ratio is estimated to be 3 ( 1. Identified using MS (chemical ionization and electron impact) and EDS. b Observed by TEM-EDS.

comparison to the aerosol phase and as wall deposits using CVAFS. Upon completion of the reaction (confirmed by disappearance of MS peak due to Hg0), the reaction flask was evacuated through a 0.45 µm Teflon filter to a residual pressure of ca. 0.0007 bar (66.67 Pa). The filter was treated with a HNO3 /H2O2 mixture. CVAFS analysis indicated that (29 ( 1)% of total mercury (relative to the amount of Hg0 loaded in the flask) was converted to mercury compounds in aerosols phase. In another set of experiments products were collected in a cooled coiled Pyrex trap, treated with HNO3/H2O2 mixture, and further analyzed using CVAFS upon reaction completion. By subtracting the total mercury collected from gas and aerosol mixtures and the amount of aerosol mercury collected on the filters, we obtained only ∼(3 ( 1)% of total mercury remains in the gas phase. This analysis recovered (32 ( 1)% of the mercury in the gasaerosol phase. Last, the reaction products were collected from the reaction flask wall by washing with a HNO3/H2O2 mixture and analyzed by CVAFS. Recovery of Hg from the wall was (68 ( 2)%. Depicted in Table 1, we hence demonstrate that the reaction products either were dominantly adsorbed on the reaction chamber wall as deposits or exist as aerosols. We attempted to characterize the shape, size, and composition of the reaction products in gaseous form, as aerosols, or on the wall of reaction chamber using HRTEM

coupled to EDS. We employed two sets of experiments. First, after completion of the reaction, products were collected in a liquid nitrogen cooled capillary tube. The air-dried specimens were prepared by transferring condensed materials from the capillary tube onto Formvar and carbon-coated Cu grids. High-resolution transmission electron microscopy was used to characterize the products onto grids by using a JEOL 2000FX TEM (Tokyo, Japon) operating at 200 kV in bright-field mode at Scheerzer defocus conditions. The chemical composition of products was qualitatively determined by energy dispersive spectroscopy (EDS). X-ray spectra were acquired with an electron beam size of 200 nm at 80 kV for 100 s. Figure 5 illustrates the TEM image of air-dried products which were collected from the gas-aerosol mixture. Particles in the gas-aerosol mixture were observed to be approximately 0.2 × 0.6 µm. Note that the blank carboncoated Cu grids show signals of copper, calcium, and chlorine along with weak oxygen peaks. Thus, these molecules do not represent any reaction products. The EDS spectrum of the chemical composition of the products in the gas-aerosol mixture revealed the gas-aerosol mixture should contain mercury and bromine. Due to the chemical composition of the reaction products, we believe that the observed aerosol can contain HgBr. In the second part of the experiment, the reaction products were collected from the wall of the reaction flask by placing the carbon-coated grid on the wall of the reaction flask. The collected products on several grids were analyzed using highresolution transmission electron microscopy (Figures 6 and 7). The size and chemical composition of aerosols collected from the gas-aerosol mixture exhibited similar features to those observed as wall deposits. All suspended mercury aerosols, as depicted in Figures 5 and 6, systematically show similar shape and size. However, wall deposit aerosols seemingly contain some other impurity from grid (Figure 7). As illustrated in Figure 6, mercury and bromine signals as well as particle size were clearly similar to those observed in the gas-aerosol phase (Figure 5). In Figure 7, in addition to VOL. 38, NO. 16, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

9

4323

FIGURE 5. (top) HRTEM image of air-dried product collected from the gas-aerosol, dispersed onto carbon-supported Cu grid. (bottom) EDS spectrum showing the chemical composition of the product collected from the gas-aerosol.

FIGURE 6. (top) HRTEM image of air-dried product collected from the wall of the reaction flask onto carbon-supported Cu grid. (bottom)EDS spectrum showing the chemical composition of the product collected from the wall of the reaction flask.

mercury and bromine signals, a relatively strong oxygen signal was observed. Our combined experiments revealed the existence of HgBr, HgO, and HgOBr (or HgBrO) as reaction products. These molecules can be potentially formed via reactions 7-9.

BrO + Hg0 f HgO + Br

(7)

BrO + Hg0 f HgBr + O

(8)

BrO + Hg0 f HgBrO

(9)

A very recent paper (44) has predicted that BrO-initiated oxidation of elemental mercury is considerably endoergic whether the product is HgO or HgBr. It has been noted that this conclusion is particularly valid for HgO, which is calculated to be unbound with respect to gas-phase Hg0 and O(3P) by 51.9 kJ/mol in free energy. HgBr was found to be stable but with a small (33.5 kJ/mol) amount of free energy (44). Recent theoretical studies (45, 46) have obtained the enthalpy of formation (∆Hr(0 K)) for the reactions 7-9. These theoretical studies included detailed treatment of corevalence correlation, scalar reactivity, and spin-orbit effect and have been calculated based on the CCSD (T) method. Using the heats of formation for Hg, BrO, and Br (47, 48), the value of ∆Hr(0 K) for reaction 7 has been shown to be 210 kJ/mol endothermic. Thus, HgO should not be formed through a direct BrO-initiated reaction of Hg0. It is noteworthy that these theoretical studies of reaction 7 are not in agreement with previous experimental results (49, 50), which report reaction 7 as being an exothermic pathway. To our knowledge, there is no experimental data on the enthalpy of formation of reaction 9. One theoretical study (46) evaluated 4324

9

ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 38, NO. 16, 2004

FIGURE 7. (top) HRTEM image of air-dried product collected from the wall of the reaction flask onto carbon-supported Cu grid. (bottom) EDS spectrum showing the chemical composition of the product collected from the wall of the reaction flask. enthalpies for HgBrO and evaluated (∆Hr (0 K)) ) - 84.6 kJ/mol for BrO-initiated reaction of gaseous mercury, indicating the possibility of formation of HgBrO. Regarding reaction 8, we did indeed observe the existence of HgBr using different types of mass spectrometry and by HRTEM-EDS.

However, the theoretical prediction for enthalpy of reaction 8 (∆Hr(0 K)) is 165 kJ/mol (46) endothermic, and hence, HgBr should not be produced from reaction 8. At this stage, based on the most recent theoretical studies, it is unlikely to account for observed products upon the simple one-step BrO-initiated reaction of Hg0(g). Although in this study, through six powerful complementary experimental techniques, we are confident in the positive identification of products, a caveat should be noted on potential heterogeneous reactions on surfaces and aerosols. We performed a series of aging experiments in which after photolysis samples remained in the dark. Moreover, we carried out experiments at different surface-to-volume ratios of reaction chambers and did not observe significant changes in products distribution. Yet, it is still conceivable that these identified products were formed partly by heterogeneous reactions due to the presence of aerosols in the system. Further detailed thermochemical and experimental studies on this system are desired. . In summary, we observed evidence for the existence of Hg1+ and Hg2+ upon BrO-initiated oxidation of Hg0. We positively identified HgBr, HgBrO/HgOBr, and HgO. These products are predominantly in the condensed phase (aerosols and deposits). The majority of mercury-containing products were identified as deposits; however, aerosols accounted for a substantial portion of products, with a characteristic width (diameter) of ∼0.2 µm. In field studies it is thus fundamental to selectively quantify various mercury species in mercury aerosols and deposits. In light of the present study, one should also consider stable Hg1+ species in addition to Hg2+ among oxidation products. It is noteworthy that we anticipate the possibility of transformation of Hg1+ to Hg2+ at high humidity levels. Since the hygroscopic nature of Hg1+ is different from Hg2+, one suspects the differences in methylation rates for Hg1+ in comparison to Hg2+ in aquatic systems that may potentially affect mercury bioaccumulation. All identified products are more hygroscopic than elemental mercury. Hence, their deposition to environmental surfaces and incorporation in the food chain should be considered.

Acknowledgments We thank Dr. H. Vali for his valuable insights and contributions in electron microscopy. We are grateful to Dr. Calvert and Dr. Paterson for sending us their manuscript and communicating their data prior to publication. We thank anonymous reviewers for their constructive comments. We thank the Natural Science and Engineering Research Council of Canada (NSERC), the Fond pour la Formation de Chercheurs et l’Aide a la Recherche (FCAR), the Canadian Foundation for Innovation (CFI), the COMERN project, CFCAS, and Environment Canada for financial support. We would also like to thank Ed Hudson, Clare Salustro, and Jackie Johnstone for proof reading the manuscript.

Literature Cited (1) Schroeder, W. H.; Anlauf, K. G.; Barrie, L. A.; Lu, J. Y.; Steffen, A.; Schneeberger, D. R.; Berg, T. Nature (London) 1998, 394, 331. (2) Lindberg, S. E.; Brooks, S.; Lin, C. J.; Scott, K. J.; Landis, M. S.; Stevens, R. K.; Goodsite, M.; Richter, A. Eviron. Sci. Technol. 2002, 36, 1245. (3) Ebinghaus, R.; Kock, H. H.; Temme, C.; Einax, J. W.; Loewe, A. G.; Richter, A.; Burrows, J. P.; Schroeder, W. H. Eviron. Sci. Technol. 2002, 36, 1238. (4) Poissant, L.; Pilote, M. Atmospheric mercury and ozone depletion events observed at low latitude along the Hudson Bay in northern Quebec (Kuujjuarapik: 55° N); International Conference on Mercury as a Global Pollutant. Minamata, Japan, Oct. 15-19, 2001, AT-23. (5) Lu, J. Y.; Schroeder, W. H.; Barrie, L. A.; Steffen, A.; Welch, H. E.; Martin, K.; Lockhart, L.; Hunt, R. V.; Boila, G.; Richter, A. Geophy. Res. Lett. 2001, 28, 3219.

(6) Barrie, L. A.; Bottenheim, J. W.; Schnell, R. C.; Crutzen, P. J.; Rasmussen, R. A. Nature 1998, 334, 138. (7) Jobson, B. T.; Niki, H.; Yokouchi, Y.; Bottenheim, J.; Hopper, F.; Leaitch, R. J. Geophys. Res. [Atmos.] 1994, 99, 23355. (8) Ariya, P. A.; Jobson, B. T.; Sander, R.; Niki, H.; Harris, G. W.; Hopper, J. F.; Anlauf, K. G. J. Geophys. Res. [Atmos.] 1998, 103, 13169. (9) Ariya, P. A.; Hopper, J. F.; Harris, G. W. J. Atmos. Chem. 1999, 34, 55064. (10) Gardfeldt, K.; Sommar, J.; Stromberg, D.; Feng, X. Atmos. Eviron. 2001, 35, 3039. (11) ATSDR. ToxFAQs for Mercury; Agency for Toxic Substances and Disease Registry, Division of Toxicology: Atlanta, GA, 1999. http://www.atsdr.cdc.gov/tfacts46.html. (12) AMAP. Arctic Pollution Issues: A State of the Arctic Environment Report; Arctic Monitoring and Assessment Programme, P.O. Box 8100 Dep., N-0032 Oslo, Norway, 1998. (13) Schroeder, W. H.; Yarwood, G.; Niki, H. Water Air Soil Pollut. 1991, 56, 653. (14) Bauer, D.; D’Ottone, L.; Campuzano-Jost, P.; Hynes, A. J. Photochem. Photobiol., A: Chem. 2003, 157, 247. (15) Tokos, J. J.; Hall, B.; Calhoun, J. A.; Prestbo, E. M. Atmos. Environ. 1998, 32, 823. (16) Lin, C. J.; Pehkonen, S. O. Atmos. Environ. 1997, 31, 4125. (17) Pehkonen, S. O.; Lin, C. J. AWMA 1998, 48, 144. (18) Gårdfeldt, K.; Sommar, J.; Stro¨mberg, D.; Feng X. Atmos. Environ. 2001, 35, 3039. (19) Xiao, Z. F.; Stromberg, D.; Lindqvist, O. Water Air Soil Pollut. 1995, 80, 789. (20) Amyot, M.; Mierle, G.; Lean, D.; McQueen, D. J. Geochim. Cosmochim Acta 1997, 61, 975. (21) Sommar, J.; Gardfeldt, K.; Stromberg, D.; Feng, X. Atmos. Environ. 2001, 35, 3049. (22) Miller, G. C.; Quashnick, J.; Hebert, V. Abstr. Pap. Am. Chem. Soc. 2001, 221, AGRO-016. (23) Ariya, P. A.; Khalizov, A.; Gidas, A. J. Phys. Chem. A 2002, 106, 7310. (24) Richter, A.; Wittrock, F.; Eisinger, M.; Burrows, J. P. Geophys. Res. Lett. 1998, 25, 2683. (25) Richter, A.; Wittrock, F.; Ladsta¨tter-Weissenmayer, A.; Burrows, J. P. Adv. Space Res. 2002, 29, 1667. (26) Mueller, R. W.; Bovensmann, H.; Kaiser, J. W.; Richter, A.; Rozanov, A.; Wittrock, F.; Burrows, J. P. Adv. Space Res. 2002, 29, 1655. (27) Van Roozendael, M.; Wagner, T.; Richter, A.; Pundt, I.; Arlander, D. W.; Burrows, J. P.; Chipperfield, M.; Fayt, C.; Johnston, P. V.; Lambert, J.-C.; Kreher, K.; Pfeilsticker, K.; Platt, U.; Pommereau, J.-P.; Sinnhuber, B.-M.; Tornkvist, K. K.; Wittrock, F. Adv. Space Res. 2002, 29, 1661. (28) Raofie, F.; Ariya, P. A. J. Phys. IV 2003, 107, 1119. (29) Wigfield, D. C.; Perkins, S. L. J. Appl. Toxicol. 1985, 5, 339. (30) Fujita, S.; Horii, H.; Mori, T.; Taniguchi, S. J. Chem. Phys. 1977, 66, 355. (31) Fujita, S.; Horii, H.; Taniguchi, S. J. Phys. Chem. 1973, 77, 2868 (32) Coquet, S.; Ariya, P. A. Int. J. Chem. Kinet. 2000, 32, 478. (33) Bulska, E.; Emteborg, H.; Baxter, D. C.; Frech, W.; Ellingsen, D.; Thomassen, Y. Analyst (Cambridge, U.K.) 1992, 117, 657. (34) Snell, J.; Qian, J.; Johansson, M.; Smit, K. Analyst (Cambridge, U.K.) 1998, 123, 905. (35) Karas, M.; Kru ¨ ger, R. Chem. Rev. 2003, 103, 427. (36) Poulet, G.; Lancar, I. T.; Laverdet, G.; Le Bras, G. J. Phys. Chem. 1990, 94, 278. (37) Bedjanian, Y.; Riffault, V.; Le Bras, G.; Poulet, G. J. Phys. Chem. A 2001, 105, 573. (38) Demore, W.; Sande, S. P.; Golden, D. M.; Hampson, R. F.; Kurylo, M. J.; Howard, C. J.; Ravishankara, A. R.; Kolb, C. E.; Molina, M. J. Chemical kinetics and photochemical data for use in stratospheric modeling, evaluation number; 1997; Vol. 12, JPL 9704. (39) Paul, B.; Ariya, P. A. J. Phys. IV 2003, 107, 189. (40) Paul, B.; Ariya, P. A. Phys. Chem. Chem. Phys. 2004, 6, 572. (41) Hall, B.; Water Air Soil Pollut. 1995, 80, 301. (42) The book of Photon Tools; Oriel Inc.: Stratford, CT, 2000. (43) Braun, W.; Heron, J. T.; Kahaner, D. K. Int. J. Chem. Kinet. 1988, 20, 51. (44) Tossell, J. A. J. Phys. Chem. A 2003, 107, 7804. (45) Shepler, B. C.; Peterson, K. A. J. Phys. Chem. A 2003, 107, 1783. (46) Balabanov, N. B.; Peterson, K. A. J. Phys. Chem. A 2003, 107, 7465. VOL. 38, NO. 16, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

9

4325

(47) Hu, A.; Otto, P.; Ladik, J. J. Mol. Struct. 1999, 468, 163. (48) Wilmouth, D. M.; Hanisco, T. F.; Donahue, N. M.; Anderson, J. G. J. Phys. Chem. A 1999, 103, 8935-8945. (49) Chase, M. W.; Jr.; Davies, C. A.; Downey, J. R., Jr.; Frurip, D. J.; McDonald, R. A.; Syverud, A. N. J. Phys. Chem. Ref. Data Suppl. 1985, 14, 1.

4326

9

ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 38, NO. 16, 2004

(50) Grade, M.; Hirschwald, W. Ber. Bunsen-Ges. Phys. Chem 1982, 86, 899.

Received for review December 2, 2003. Revised manuscript received May 12, 2004. Accepted May 27, 2004. ES035339A