Xianming
Zhang
a,
Torsten
Meyer
a,
Derek C. G.
Muir
*b,
Camilla
Teixeira
b,
Xiaowa
Wang
b and
Frank
Wania
a
aDepartment of Physical and Environmental Sciences, University of Toronto Scarborough, 1265 Military Trail, Toronto, Ontario M1C 1A4, Canada
bAquatic Contaminants Research Division, Environment Canada, Burlington, Ontario L7R 4A6, Canada. E-mail: derek.muir@ec.gc.ca; Tel: +1-905-319-6921
First published on 7th October 2013
Current use pesticides (CUPs) have been detected in the Arctic, even though there are no direct sources and their long range atmospheric transport potential is generally lower than that of legacy pesticides. Data on the deposition of CUPs in the Arctic are required to assess the impact of their global usage and emission. In this study, selected CUPs were measured in the layers of a snow pit sampled on the Devon Ice Cap, Nunavut, Canada. The oldest sampled layers correspond to deposition from the early 1990s. Dacthal and endosulfan sulfate were most frequently detected, with peak deposition fluxes of 1.0 and 0.4 pg cm−2 per year. While endosulfan sulfate was more abundant than its parent compounds in most years, endosulfan (sum of α and β isomers) was predominant in 2003 and 2006, which together with air mass backward trajectories suggests a possible origin from ongoing use in Eurasia. The interannual variation in CUP deposition fluxes could not be explained with annual variations in the extent of air mass origin over agricultural lands, suggesting that other factors, such as the interannual variation in pesticide use, play a role in affecting the long range transport of CUPs to the Arctic. The very high variability in the concentrations of CUPs in the horizontal layers of Arctic ice caps is most plausibly explained by the highly episodic nature of long range atmospheric transport and deposition. While this strong influence of rare events limits the suitability of ice caps as reliable records of historical trends in Arctic contaminant deposition with annual resolution, the presence of concentration peaks in the ice record is proof of the possibility of such transport and deposition.
Environmental impactQuantitative information on the deposition of current use pesticides (CUPs) in the Arctic is important for assessing the impact of their global usage and emission. Snowpack acts as a reservoir for atmospherically deposited organic contaminants in the Arctic and plays an important role in the environmental fate and impact of the contaminants. In this study, using a snow pit sampled on the Devon Ice Cap, Nunavut, Canada, we inferred the atmospheric deposition history of semivolatile organic compounds from the early 1990s to the mid 2000s. Large interannual variations of the deposition fluxes indicate a highly episodic nature of long range atmospheric transport and deposition. |
The pesticides can deposit on Arctic surfaces via diffusive gas exchange and together with rain, snow and atmospheric particles.11 Snow, due to its highly porous structure and low prevailing temperatures, can effectively scavenge both gas phase and particle-bound chemicals from the air12 and is thus believed to contribute significantly to the atmospheric deposition of semivolatile contaminants in the Arctic.11 After deposition, the snowpack can act as a reservoir for the contaminants. Depending on the physicochemical properties and facilitated by physical changes in the snow structure during snow ageing and snowmelt, contaminants accumulated in surface snow may be re-emitted to the atmosphere or transferred deeper into aged snowpack and possibly released to ecosystems.13–15 Therefore, snow plays an important role in the environmental fate and impact of atmospherically deposited organic contaminants in the Arctic.16–18
The presence of pesticide residues in snow from remote regions has been used as evidence of global scale long range atmospheric transport for a long time, beginning with the detection of DDT in Antarctic snow more than 40 years ago.19,20 Since then, pesticides have been regularly detected in snow from the Arctic6,16,21 and mid-latitude mountains.22–24 In the 1990s, snow pits dug on the ice caps on Ellesmere Island, Nunavut, Canada;25,26 on Greenland;27,28 and in the Canadian Rocky Mountains29 were used to infer the atmospheric deposition history of semivolatile organic compounds. More recently, similar approaches have been used in Svalbard2,30,31 and Antarctica.32 The Devon Ice Cap has been used to infer the deposition of antimony,33 perfluoroalkyl acids34 and brominated flame retardants.18 Depth profiles of semivolatile organic contaminants in snow pits from Arctic ice caps generally display very high variability between the layers corresponding to different deposition years; also, time trends, if apparent at all, tend not to correlate very well with time trends in the global usage of the analyzed compounds.
While the contaminant concentration in fresh snow directly reflects atmospheric deposition, concentrations in aged snow and firn may be affected by post-depositional processes such as re-volatilization to the air and transfer to deeper firn layers caused by wind-pumping, seasonal temperature increase and melt.18,35,36 The effect of such post-depositional processes depends on compound and snow properties; volatile compounds in permeable snow are more likely to re-volatilize while water soluble compounds are more likely to move with the meltwater.13,15 Even in Arctic ice caps at high altitude, where seasonal snow melt is limited, the relocation of chemicals in the snowpack due to post-depositional processes makes it highly uncertain to retrieve the atmospheric deposition history at a very high temporal resolution. However, layers with an approximately annual resolution can be inferred from the chemical vertical profiles measured in snow and ice cores.18,29,32
In this study, snow cores were sampled from the Devon Ice Cap, Nunavut, Canada in 2008 and analyzed for a suite of CUPs in commerce in North America and Europe (Table S1†). The CUPs were selected based on their known use and on their analyzability by gas chromatography-negative chemical ionization mass spectrometry (GC-NCI-MS). A similar suite was analyzed by Ruggirello et al.31 Whereas results for brominated flame retardants have been presented previously,18 we present here the concentrations and historical deposition fluxes of selected CUPs. With the vertical concentration profiles of the chemicals in the snow core, we aim to assess the historical trends of the chemicals deposited on the Arctic surface and importance of source regions.
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Fig. 1 Map showing the sampling site on the Devon Island Ice Cap and the areas with agriculture activities (data from digital map of land use systems of the world by Food and Agriculture Organization, http://www.fao.org/geonetwork/srv/en/main.home). |
The columns were sequentially extracted with methanol (CH3OH) and dichloromethane (CH2Cl2) and the combined extracts were washed with 3% NaCl and dried on anhydrous Na2SO4. The extract was concentrated to a small volume under vacuum in a rotary evaporator and added to a small column of 10% H2O-deactivated silica-gel and eluted with 10% CH3OH–CH2Cl2. Acetone–isooctane (2:
1) was added and the sample was concentrated to a final volume of 0.2 mL. All CUPs were analyzed using GC-NCI-MS (low resolution MS). A 1 μL aliquot of each extract was injected into a GC (pulsed splitless injection at 250 °C) onto a 30 m × 0.25 mm diameter capillary column (5% diphenyl-/95% dimethylsiloxane liquid phase) (0.25 mm film thickness) at an initial GC oven temperature of 80 °C. 13C-mirex was added to each XAD column prior to elution as a method recovery standard. Mirex recoveries (N = 14 samples) averaged (± standard deviation, SD) 90 ± 12% and no recovery correction was made. Procedural blanks (N = 4) consisting of XAD and all reagents were analyzed with every 4 snow samples. XAD resin blanks (N = 4) consisting of the CH3OH–CH2Cl2 elution were also analyzed. Method detection limits (MDL) were calculated from the combined blank results (3 × SD, N = 8) or where blanks were non-detectable, using the instrument detection limit (IDL). All results were blank corrected using the average blank (N = 8). The full list of analytes, as well as MDLs and IDLs, is given in the ESI, Table S1.†
Dacthal levels (water equivalent) in Devon Ice Cap snow were comparable with concentrations of 10–70 pg L−1 measured in the water of Canadian Arctic lakes (>70°N) sampled from 1999–2003,42 but were lower than the peak concentration of 300 pg L−1 (corresponding to the deposition year of 1979–1986) reported for an ice core from the Austfonna Ice Cap, Svalbard, Norway.2 While endosulfan sulfate concentrations in the snow were comparable to the concentrations in Arctic lake water (<10 pg L−1), the peak concentrations in the snow samples corresponding to the deposition years of 1993–1996 and 2002–2004 were higher than in Arctic lake water.42
Based on the annual accumulation of water equivalents (kg cm−2 per year) inferred from the density and thickness of each snow segment, the concentrations were converted to net atmospheric deposition fluxes (Fig. 2). These fluxes in units of pg cm−2 per year ranged from <0.002 to 0.04 for trifluralin, <0.01 to 0.6 for PCNB, <0.01 to 1.0 for chlorothalonil, <0.005 to 1.1 for metribuzin, 0.04 to 1.0 for dacthal, <0.002 to 0.6 for α-endosulfan, <0.003 to 0.2 for β-endosulfan, <0.02 to 0.4 for endosulfan sulfate, and <0.005 to 0.6 for quizalofop ethyl.
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Fig. 2 Annual net deposition fluxes of current use pesticides inferred from measured concentrations in snow segments from a snow pit dug on the Devon Island Ice Cap in 2008. |
The peak deposition flux of 1 pg cm−2 per year for dacthal was lower than the 1995–2005 deposition flux of 5 pg cm−2 per year inferred from ice core records at Holtedahlfonna, Svalbard, Norway.31 Fluxes of α-, β-endosulfans and endosulfan sulfate are lower than those at Holtedahlfonna by one order of magnitude.31 Fluxes of the endosulfan-group compounds were also lower than fluxes derived from concentrations in fresh snow from the Canadian Arctic,5 which could be caused by a significant fraction of previously deposited endosulfan undergoing re-volatilization due to strong katabatic winds occurring on the Devon Ice Cap.37
The herbicide trifluralin has a near-continuous profile in the snow core (Fig. 2), which is similar to the observation in the Holtedahlfonna ice core.31 However, the peak deposition flux of trifluralin at the Devon Ice Cap recorded for 2003 was 50 times lower than the flux measured at Holtedahlfonna.31 Fungicides PCNB and chlorothalonil were not continuously detected in the snow segments (Fig. 2). Quizalofop ethyl, a post-emergence herbicide for the selective control of annual and perennial grass weeds in broad-leaved crops, was detected in the snow deposited before the early 2000s at a level of ∼20 pg L−1 (Table S1†). In contrast, in the ice core from Holtedahlfonna, no quizalofop ethyl could be detected.31 The herbicide metribuzin was only detected in snow deposited in 2000–2001 and 2007–2008.
Despite its much lower production volume and more limited geographical use area,43,45 dacthal concentrations were higher than those of the sum of endosulfan sulfate and its parent compounds (α- and β-endosulfan) in all snow samples. The only exception is the sample corresponding to the year 2003, in which both endosulfan isomers had levels about one order of magnitude higher than in other years. Generally higher concentrations of dacthal compared to the endosulfans are consistent with measurements in Arctic lake water, but contrast with what has been observed in the water of remote lakes at lower latitude (40–50°N).42 These observations provide empirical evidence for earlier model predictions46 that dacthal has a higher potential for long range atmospheric transport than the endosulfans. The efficiency of atmospheric deposition of dacthal and endosulfan sulfate should be similar because their physicochemical properties are similar.5,46 The higher potential of dacthal for long range transport can thus be attributed to a predicted atmospheric half-life (860 h) that is much longer than that of endosulfan (50 h) and endosulfan sulfate (50 h).31
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Fig. 3 Relative abundance of α- and β-endosulfan and endosulfan sulfate in snow segments from a snow pit dug on the Devon Island Ice Cap in 2008. |
In the environment, endosulfan is subject to biotic and abiotic degradation that results in the formation of endosulfan sulfate. Therefore, the relative abundance of endosulfan sulfate serves as an indicator for the level of environmental degradation of the technical product. Except for the samples corresponding to years 2003 and 2006, endosulfan sulfate was more abundant than its parent compounds. This is in contrast to the pattern in the ice core from Holtedahlfonna, Svalbard, where endosulfan sulfate was less abundant than endosulfan. Because no trend in the relative abundance of endosulfan sulfate with time was observed, we infer that firn records the endosulfan composition at the time of deposition, i.e. no transformation has occurred after deposition. The higher abundance of the parent compounds in 2003 and 2006 was probably due to deposition of recently used endosulfan. Judging from the interannual variations of the airsheds (Fig. 4), these two years are quite unusual in that some air masses originated from Western Russia. Hermanson et al.2 also argued that deposition of some pesticides to ice caps in Svalbard could be accounted for by event-based long range atmospheric transport from agricultural areas in northern Eurasia. Possibly, the elevated deposition of trifluralin, PCNB and chlorothalonil in 2003 can also be related to the occurrence of air mass origin from Western Russia.
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Fig. 4 Airsheds of the Devon Island Ice Cap sampling site during selected summer periods obtained by plotting the density of air mass backward trajectory endpoints (hot color represents high density; cold color represents low density). Stronger influence from Western Russia was observed in 2003 and 2006. |
The annual variation of BDE-209 deposition was explained by the fraction of trajectory endpoints originating from densely populated North America. A similar approach was adopted to assess whether air mass origin over agricultural lands, where the majority of CUPs presumably have been used, can explain the variations in the CUP deposition fluxes. Fig. 5 displays the annual variations of the agriculture contribution index (ACI), a semi-quantitative indicator for the level of agricultural influence on the air masses arriving at the sampling site, which is based on the airsheds assembled from the backward trajectories (Fig. S2†). Air mass origin and therefore also ACI vary from year to year. In 1994 and 1999, the 5-day trajectories reached further into temperate latitudes and thus both the fraction of trajectory endpoints passing densely populated area in North America, which was used as an indicator for the contribution of PBDE sources in a previous study,18 and the ACI are higher in these years. Efforts to relate the annual variations in air mass origin with the variations in measured CUP concentrations were not successful. For example, most of the CUPs had peak concentrations in the layer corresponding to the year 2003 but the ACI for 2003 was one of the lowest among the years. This is likely because the ACI relies on generic classifications of agricultural lands rather than geographically explicit data on CUP use. Also, while we considered only air mass origin during summer, pesticide application is sometimes confined to much shorter time periods.
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Fig. 5 Agriculture contribution index (ACI) for air masses arriving at the sampling site at the Devon Ice Cap. ACI is defined as the total area of agricultural land within 10 km of each of the 7.5 × 104 trajectory endpoints based on 6-hour interval air mass backward trajectories from May to September (inclusive) of each year. The contributions of endpoints located in North America and Eurasia to ACI are distinguished. |
Geographically explicit annual use data for dacthal and endosulfan in the United States from 1992–2009 are available from the Pesticide National Synthesis Project of the U.S. Geological Survey (Fig. S3 and S4†).49 Although pesticide use in regions other than the US can contribute to the CUP deposition in the Arctic, the peak in the deposition of dacthal and endosulfan to the Devon Ice Cap in 1996 and in 2002–2004 can partially be explained by the annual dacthal and endosulfan use maps in the US together with the airshed maps (Fig. S2†). In 1996, a higher portion of the trajectory endpoints overlapped with the agricultural land stretching from the southwest of the Great Lakes to the Canadian Prairies; dacthal and endosulfan happened to be used more in this area in 1996 than in 1995 and 1997. In 2002–2004, the annual use of dacthal in the same region was higher than in 2001 and after 2005, which could have caused the elevated dacthal deposition at Devon during these years.
Measurements of pesticides in precipitation at reasonably high temporal resolution generally do indeed show a highly episodic character.57–59 In source regions, the occurrence of high concentrations in rain depends on the timing of pesticide application and precipitation. As a result, even over extended time periods, deposition fluxes can be dominated by a few events. For example, concentrations of HCHs measured in precipitation of southern Norway at a weekly temporal resolution varied by as much as two orders of magnitude.60 A few episodic events with high concentrations contributed the majority of the total wet HCH deposition over the course of a whole year.60 If this is the case at sampling sites in close proximity to regions with pesticide application, episodicity of pesticide deposition will only be more pronounced in remote Arctic regions. Even though they may be influenced by many more source regions, very few air masses that picked up pesticides during an application event will find their way to the Arctic without encountering precipitation along the way.
This high episodicity of pesticide deposition on Arctic ice caps also questions their suitability as archives recording temporal trends in the transport and deposition of contaminants to the Arctic. The very high interannual variability compromises their use as reliable indicators of pesticide use on a hemispheric scale, at least at the annual resolution. Drilled snow cores, that are deeper than the pits investigated here and thus cover much longer time periods, may be suitable for determining long term trends in pesticide usage over the decadal time scale. The primary value of relatively shallow snow pits lies in providing proof of the atmospheric transport and deposition of a contaminant in the Arctic.
Footnote |
† Electronic supplementary information (ESI) available: Figures showing concentrations in the snow segments in snow water equivalent, airsheds of the Devon Island Ice Cap sampling site for all investigated deposition years, and maps for the use of dacthal and endosulfan in the USA during the investigated deposition years. See DOI: 10.1039/c3em00433c |
This journal is © The Royal Society of Chemistry 2013 |