Environ. Sci. Technol. 2010, 44, 588–592
Trends and Sources of Perchlorate in Arctic Snow VASILE I. FURDUI* AND FRANK TOMASSINI Ontario Ministry of the Environment, 125 Resources Road, Toronto, ON, M9P 3 V6
Received July 24, 2009. Revised manuscript received November 2, 2009. Accepted November 16, 2009.
Samples from the Devon Island ice cap (Nunavut, Canada) were used to calculate the annual input of atmospheric formed perchlorate. Depth samples collected in the spring of 2006 were dated between 1996 and 2005. An optimized ion chromatography tandem mass spectrometry (IC-MS/MS) method with direct injection allowed detection of perchlorate in all analyzed samples. Concentrations varied between 1 and 18 ng L-1, showed seasonality, and were correlated with the total ozone levels from the area. A significant correlation was observed between chloride and perchlorate only for data sets corresponding to peak perchlorate concentrations. Data available suggests that perchlorate from the Arctic snow was formed in the atmosphere following two different mechanisms. Stratospheric chlorine radicals reacted with ozone year around, producing concentrations of perchlorate correlated with the total ozone level. The second pathway was specific to the summer months, when the amounts of perchlorate were correlated with the chloride concentrations, suggesting a possible tropospheric formation. Analysis of a deep ice core sample confirmed that perchlorate was present in precipitation at similar concentration more than 2000 years ago. Perchlorate ion represents a sink for the stratospheric chlorine, being removed via precipitation. The estimated amount of perchlorate that reached the Arctic in 2005 was 41-86 t.
Introduction Perchlorate (ClO4-) is a widespread contaminant of drinking water and food (1-3), with influence on the iodide uptake and hormone production in the thyroid gland (4). Although a strong oxidizer, perchlorate anion is quite stable in aqueous solutions, due to a large kinetic barrier to reduction (5). Perchlorate contamination in groundwater is the result of anthropogenic and natural sources. Anthropogenic contamination resulted from the extensive use of perchlorate in solid rocket propellant, blasting agents, explosives, fireworks, and road flares, and as an impurity present in sodium chlorate and bleach solutions (6). Natural sources of perchlorate include its presence as an impurity in Chilean nitrate (5, 7, 8) and atmospheric sources (9, 10). The extensive use of Chilean nitrate fertilizer in the 19th century in the United States was documented as a significant source of contamination in the environment (5), with stable isotope analysis recently confirming its contribution to the total concentration of perchlorate in groundwater (11). The atmospheric source of perchlorate has not been completely characterized. Although not detected, Jaegle´ et * Corresponding author e-mail:
[email protected]. 588
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al. (12) modeled the existence of HClO4 in the stratosphere in 1996, explaining a significant discrepancy between the total inorganic chlorine and the sum of measured HCl, ClONO2, and HOCl. Perchlorate was identified in stratospheric aerosols in 2000 (13) and rain samples in 2005 (9), suggesting a potential atmospheric formation. Measurement of oxygen isotope composition was used successfully to differentiate between natural and anthropogenic perchlorate (10). Correlations between the concentrations of chloride and perchlorate were observed recently in soils and sediments (14), but not in precipitation. Although chlorine oxidation is well understood, the final atmospheric oxidation steps leading to perchlorate are not well characterized. Two homogeneous mechanisms of formation were proposed considering the ozone and HOx as atmospheric oxidants and chlorine radicals (5, 9). A first model was suggested more than three decades ago by Simonaitis-Heicklen considering ClO4- a better stratospheric sink for chlorine radicals than HCl (15), with the following key steps: Cl· + O3 + M f ClO3· + M ClO3· + ·OH f HClO4 A second model was proposed later by Prasad-Lee (16), claiming no dependence on ClO3 · and · OH: ClO· + O2 + M f ClO · O2 + M ClO · O2 + O3 f ClO · O3 + O2 ClO3· + ·OH f HO2· + ClO2· ClO · O3 + HO2· f HClO4 + O2 Dasgupta et al. proposed an expanded Simonaitis-Heicklen pathway considering · ClO3 and ClO2 as intermediates that react with · OH to produce HClO4 (9). Perchlorate can be formed from heterogeneous reactions on aerosols (5). Martin et al. (17) reported HClO4 as a minor product from the reaction of Cl · on H2SO4-coated Pyrex. Kang et al. produced ClO4- by oxidizing Cl- from sand and glass surfaces with ozone (18). Although the ozone concentration was much higher than the normal atmospheric level, the study identified ClO2- as an intermediate, excluding ClOand ClO3- as possible intermediates. Ice caps situated in the High Arctic provide long-term records for atmospheric deposition of different persistent organic pollutants such as polycyclic aromatic hydrocarbons (19) and perfluorinated acids as the layers of accumulated snow are well-defined temporally, with no significant chemical and physical change (20). Dating snow pit samples can be achieved by visual inspection of the well-defined ice-fern layers (20) or based on the seasonal variations of common inorganic ions (21). The objectives of this study were to distinguish the atmospheric source of perchlorate, identify correlations with other anions, find details about the sources of perchlorate in the Arctic, and estimate the yearly fluxes received by precipitation. This is the first report of a temporal trend of perchlorate in precipitation. Samples were collected from Devon Island ice cap and previously analyzed for their content of perfluorinated acids (20). A direct injection ICMS/MS method was optimized for low level detection of perchlorate and other anions, including ClO3-, ClO2-, Cl-, BrO3-, Br-, IO3-, and I-.
Experimental Section Sample Collection. In April 2006 depth samples were collected from the Devon Ice Cap, Devon Island, Nunavut 10.1021/es902243b
2010 American Chemical Society
Published on Web 12/07/2009
FIGURE 1. Density-corrected concentrations of perchlorate, chlorate, and chloride in snow from Devon Ice Cap. (75° 20N, 82° 40W at 1797 m altitude). The snow pit was located close to the highest point of the ice cap, 2.2 km N from the nearest temporary research site. A stainless steel scraper was used to remove the surface layer of the snow pit wall. Duplicate samples were taken horizontally at 25-cm (0-3 m depth) or 20-cm (3-6.8 m depth) intervals, using a stainless steel corer of 8.1 cm diameter and stored in new polypropylene bottles at -20 °C prior to analysis. Field blanks were taken from HPLC grade water, transported in new polypropylene bottle and opened at the sampling location for 10 min. Densities were calculated by determining the mass of known volume samples collected at 10-cm intervals. Surface samples were collected in the spring of 2006 from three other locations in the Canadian Arctic (map in Figure S1, SI): Melville Ice Cap (Melville Island, Northwest Territories, 75° 27N, 114° 59W), Agassiz Ice Cap (Ellesmere Island, Nunavut, 80° 07N, 73° 01W), and Meighen Ice Cap (Meighen Island, Nunavut, 79° 55N, 99° 08W). Deep ice core water collected from Agassiz Ice Cap (Nunavut, Canada) in 1977 was obtained from Geological Survey of Canada (Ottawa, ON) and corresponds to precipitation received more than 2000 years ago (20).
Results and Discussion Dating Arctic Snow. Arctic snow was dated based on density, conductivity, ion concentrations (Cl-, SO42-, NO3-), visual inspection of ice layers, and considering also the historical precipitation data in the area (21, 22). These parameters show seasonal variation each year and were previously used to identify annual markers along a snow depth profile (20, 23). The maximum ion concentrations were used for ice dating, particularly SO42- with a broad peak corresponding to January-March, NO3- with a peak in June, and Cl- with a large peak in January and a small peak in October (21). The yearly precipitation profile (Figure S5, SI) was also considered for dating depth perchlorate concentrations between the surface and a 340 cm depth. A higher level of uncertainty limits a precise monthly dating below this depth. In 2004 and 2005 Resolute Bay received 95% of the total yearly precipitation during 9 months, between March and November, with 66% of the precipitation received during 6 months, between April and September (24). Precipitation was almost absent in December and January and peaked in July and August, when more than 30% of precipitation was received. While different precipitation levels were received on Devon Island and Resolute Bay, similar yearly trends were assumed for both locations. The 6.8 m deep pit corresponded to precipitation received on Devon Island between 1996 and early 2006. Based on the chloride concentration trend (Figure 1) and the estimated
FIGURE 2. Yearly fluxes of perchlorate, chlorate, and iodate on Devon Ice Cap. 8-22% of the yearly precipitation received in the first three months of each year in Resolute Bay in 2004 and 2005 (Figure S5, SI), the top 25 cm corresponds to the snow received in 2006. Resolute Bay represents the closest location to Devon Island for which historical temperature and precipitation data were available (24). Temporal Trend of Perchlorate in the Arctic Snow. All measured ions showed seasonal variation, with maxima not occurring simultaneously. The mean perchlorate concentration was 5.5 ( 3.9 ng L-1 (Figure S2, SI), almost three times lower than the 14.1 ng L-1 mean concentration determined in precipitation from midlatitudes and similar pattern with a perchlorate peak occurring in July (3).The mean chlorate concentration was 14.0 ( 3.6 ng L-1. A lack of significant correlation (Figure S6, SI) can be caused by formation of ClO3- and ClO4- at different altitudes, formation from different precursors, or independent mechanisms of formation. The chloride content was at ppb level (µg L-1) with chloride to perchlorate ratios varying between 2400 and 132 000, with an average value of 14 900 (Table S3, SI). Similar ratios were previously reported in rain (3, 9) and groundwater (25, 26) from Texas and New Mexico and no correlation between perchlorate-chloride and chlorate-chloride concentrations (Figures S7-S8, SI). Concentrations in the snow samples are presented in Figure 1 for perchlorate, chlorate, and chloride and were calculated using the snow densities measured at sampling time. Bromate was detected in 16% of the samples at levels between 4 and 18 ng L-1 (LOD ) 0.9 ng L-1). Chlorite was not detected in any sample (LOD ) 140 ng L-1). Bromate and chlorite were excluded from this discussion because of the limited quantitative information obtained. Higher fluxes of perchlorate were observed in 1996 and 1997, with a decreasing trend until 1999 and a slower increase after 2004 (Figure 2). As previously reported, volcanic eruptions inject in the atmosphere large amounts of aerosols with significant S and Cl content (27). Elevated aerosol presence in the stratosphere induced in the past an immediate strong cooling effect of the Earth climate system, with temperature anomalies observed globally in the following years (28-30). A substantial stratospheric chlorine injection is limited by HCl removal in condensed supercooled water (31), a possible explanation for lower chloride fluxes observed in this study when compared with the sulfate and nitrate fluxes. Sulfate was a marker of major volcanic activity in ice core samples (32, 33). Mount Pinatubo (15° 08′ N, 120° 21′ E, Philippines) had a powerful eruption in 1991 and a buildup of sulfate aerosols occurred in the following months in the lower stratosphere (34). As previously reported some aerosols were transported to midlatitudes and the poles. In the Arctic, the aerosols were removed during the winter, with more aerosols introduced from midlatitudes during spring VOL. 44, NO. 2, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 3. Yearly fluxes of sulfate, nitrate, and chloride on Devon Ice Cap. and summer, after the breakdown of the polar vortex (30). A higher annual flux for sulfate corresponding to 1996 and 1997 suggests a volcanic source, probably Mount Pinatubo with a series of eruptions continuing until 1995 (35, 36). Elevated sulfate levels in Antarctica between 1991 and 1994 were related to the Pinatubo eruption (37) and proved that the aerosols traveled large distances and remain in the atmosphere much longer than previously thought. Based on balloon-borne aerosol measurements, Watanabe et al. (38) calculated a decaying trend of sulfate mass mixing ratio in the Arctic stratosphere between 1994 and 1998. The midlatitude sulfate mass was smaller before 1998, but similar between 1998 and 2000, suggesting a time delay between the Pinatubo aerosol arrival at midlatitudes and the Arctic (38). In our study the yearly flux of sulfate shows a similar decreasing trend until 1999. Perchlorate and chloride showed the same decreasing trend between 1996 and 1999 (Figure 2), suggesting that the elevated fluxes observed in 1996 and 1997 are related to the increased presence of stratospheric aerosols of volcanic origin. Other volcanic events with significant SO2 injections occurred between 1996 and 2005, including eruptions from Popocate´petl (Mexico) with 9 Mt total SO2 released in 1996-1997 (39) and Miyakejima (Japan) with 18 Mt total SO2 released in 2000-2003 (40). For comparison, the 1991 eruption of Mount Pinatubo released an estimated 20 Mt SO2 (33). An increase in yearly flux of sulfate and perchlorate after 2002 may be related to Miyakejima volcano emissions. However the increase in yearly flux for perchlorate is observed only after 2003. By 2005 the yearly flux is similar to the level observed in 1998. An almost constant yearly flux of chlorate between 1996 and 2005 suggests that the chlorate level was not affected by the volcanic activity and the amount of aerosols injected in the stratosphere. The yearly nitrate deposition (Figure 3) varied between 20 and 70 mg m-2, showing a good agreement with the predicted values of less than 50 mg m-2 at 80° N latitude (41). Sources of Perchlorate in Arctic Precipitation. There are no known anthropogenic sources of perchlorate extensively used in the Arctic. Since the Arctic ice caps receive contamination solely from the atmosphere it is assumed that all perchlorate is formed in the atmosphere following homogeneous or heterogeneous mechanisms, and removed by precipitation. The deep ice core sample analyzed in this study corresponded to water trapped for over 2000 years and had a ClO4- concentration (7.5 ng L-1) similar to the mean of all 1996-2006 samples (5.5 ( 3.9 ng L-1). No correlation was observed between Cl- and ClO4- (Figure S8, SI), but selecting data pairs corresponding to peak ClO4- concentrations (Figure S2, SI), a statistically significant correlation was obtained between Cl- and ClO4-, as shown in Figure 4. This indicates that at peak concentrations, corresponding to the 590
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FIGURE 4. Variation of chloride with perchlorate at peak perchlorate concentrations.
FIGURE 5. Variation of total ozone (Resolute Bay) with perchlorate (Devon Island) between 2002 and 2006. Data pairs for peak perchlorate concentration were excluded. summer season, ClO4- was formed predominantly from Cl-, with differences caused by the yearly variation of atmospheric chloride concentrations. The seasonal variation observed suggests a photochemical formation of ClO4-. It is known that total ozone shows seasonal variation in any Arctic location, including Resolute Bay (Canada), the closest location to Devon Island with measured concentrations of ozone (42). Regression analysis suggests a possible positive correlation (p < 0.05) between the concentrations of perchlorate and the total ozone (r ) 0.53, N ) 70, p ) 2 × 10-6 at 95% confidence level; Figure S12, SI). The correlation between perchlorate and total ozone is more evident after excluding data pairs corresponding to peak perchlorate values, as presented in Figure 5 (r ) 0.78, N ) 45, p ) 2 × 10-10 at 95% confidence level). Stratospheric origin of ClO4- is strongly suggested by the similar variation of the ozone profile in the Arctic. A significant correlation between total ozone and perchlorate is possible only if the ozone plays a key role in perchlorate formation and the reactions take place at elevations characterized by significant seasonal ozone variation, which are stratospheric altitudes as observed in Figure 6. The intercept of the plot from Figure 4 suggested also that each summer a constant small amount of ClO4- (1 ng L-1) did not result from chloride ions, but another precursor which may be Cl · reacting with ozone. The complex atmospheric chlorine-ozone chemistry was studied extensively over the last few decades (43). Atmospheric Cl · can result from natural and anthropogenic chlorine reservoirs such as HCl, HClO, ClO, ClO2, ClONO2, CH3Cl, and chlorofluorocarbon compounds (CFCs) (43, 44). The reactions leading to ClO4- consider a Cl · precursor, with ClO · , ClO3 · , and ClO2 as intermediates from reaction with O3 (9, 15). Finally ClO3 · can react with hydroxide radical
FIGURE 6. Ozone profiles from Resolute Bay for fall (solid lines) and spring months (42) with proposed pathways and estimated contributions. ( · OH) producing HClO4. This photochemical pathway appears to be the predominant pathway formation between March and November, with higher ClO4- concentrations observed during the months with higher ozone level. As precipitation between December and February represents less than 5% of the yearly total (Figure S5, SI), from the current study the information about ClO4- formation is limited for these three months. The peak perchlorate level observed during the summer season is the result of at least two different mechanisms, with one responsible each year for variable amounts of ClO4from Cl-, and the second mechanism producing a constant amount of ClO4- (1 ng L-1) from stratospheric Cl · (Figure 6). Previously it was reported that starting with chloride as a precursor, perchlorate was produced from NaCl aerosols in the presence of electrical discharges (9) and also by oxidation in the presence of high ozone levels (18). In the current study a production of perchlorate from chloride ions in the presence of electrical discharges is favored, as no correlation was determined between total ozone and ClO4- at peak values (summer season). The electrical discharges are more probable in the troposphere, where the ozone does not show seasonal variation (Figure 6). Higher concentrations determined in this study for ClO3- than for ClO4- suggested also a much reduced ClO4- formation from ozone oxidation in the absence of electrical discharges (18). The annually averaged lightning flash frequency has a maximum in North America at 30° N latitude near the Gulf of Mexico, decreasing substantially with latitude and reaching a minimum at 70° N, with most flashes occurring also during the Northern Hemisphere summer (41). Reduced lightning activity characterizes the High Arctic and explains also the lower ClO4- concentrations observed in this study compared with a relatively higher level observed in precipitation from the continental United States (3, 9). In this situation tropospheric formation from Cl- and O3 would be possible without any significant correlation with total ozone, as the lightning activity is more localized. A strong chloride-ozone correlation would be expected if ClO4- is formed mainly by oxidizing Cl- with O3 in the absence of electrical discharges. The yearly deposition of perchlorate from Cl · precursor was estimated assuming a constant value of 1 ng L-1 (corresponding to zero concentration of Cl-, Figure 4) for all peak perchlorate concentrations, and the results are presented in Figure 7. The yearly depositions of perchlorate with Cl- precursor were calculated as the differences between total perchlorate yearly fluxes and the estimated flux of perchlorate with Cl · precursor. In the future, analysis of deep ice core samples might be able to estimate the contribution of anthropogenic chlorine compounds to atmospheric ClO4formation by quantifying the past contribution of ClO4- with Cl- precursor to the total ClO4- concentration.
FIGURE 7. Estimated yearly fluxes of perchlorate formed from Cl- and Cl · precursors. Chlorate concentrations were not correlated with neither Cl- (Figure S7, SI) nor O3 (r ) 0.09, N ) 14, p ) 0.75 at 95% confidence level), suggesting different mechanism for the production of ClO3- and ClO4-, with Cl- and ClO- as probable ClO3- precursors. The yearly chlorate fluxes showed little changes, contrasting with the trend observed for perchlorate. Yearly Depositions of Perchlorate and Chlorate to the Arctic. To identify the yearly Arctic deposition, the fluxes were multiplied by the calculated total area (24.1 mil km2 between 65° N and 90° N). In early 2006 the average surface ClO4- concentration from all four sampled ice caps was 7.4 ( 2.4 ng L-1, a value 25% higher than for Devon ice cap. The lowest level of ClO4- was on Melville ice cap (5.6 ng L-1) and the highest was on Meighen ice cap (10.8 ng L-1). Based on Devon ice cap data the yearly deposition of perchlorate over the Arctic in 2005 is estimated at 44 t, with 41 t as minimum based on the Melville ice cap data and 86 t as maximum based on the Meighen ice cap data. This compares with a 51 t perchlorate yearly flux calculated for the continental United States (3). In 2005 about 24 t ClO4- with Cl · precursor were deposited over the Arctic, representing 54% of the total perchlorate received from precipitation. While atmospheric perchlorate formation removes atmospheric chlorine, the amount removed is relatively small compared with the total CFCs emissions (Figure S13, SI) which averaged 5 × 105 t between 1994 and 2000 (45).
Acknowledgments Special thanks to Dan Walsh (Environment Canada) for organizing the field trip to Devon Island. Many thanks to Scott Mabury and Cora Young (University of Toronto) for making this study possible (20). We also thank our colleague Adrian Tencic.
Supporting Information Available Separation method details, LOD values, and additional statistical information. This material is available free of charge via the Internet at http://pubs.acs.org.
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ES902243B
