F pollution in soil from the Antarctic, Arctic and Tibetan Plateau

Science of the Total Environment 497–498 (2014) 353–359

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Science of the Total Environment journal homepage: www.elsevier.com/locate/scitotenv

Comparative study on PCDD/F pollution in soil from the Antarctic, Arctic and Tibetan Plateau Shenglan Jia a, Qiang Wang b, Li Li a, Xuekun Fang a,c, Yehong Shi d, Weiguang Xu a, Jianxin Hu a,⁎ a State Key Joint Laboratory of Environmental Simulation and Pollution Control, College of Environmental Sciences and Engineering, Collaborative Innovation Center for Regional Environmental Quality, Peking University, Beijing 100871, PR China b School of Chemistry and Chemical Engineering, Central South University, Changsha 410083, PR China c Norwegian Institute for Air Research, P.O. Box 100, 2027 Kjeller, Norway d Beijing General Research Institute of Mining & Metallurgy, Beijing 100160, PR China

H I G H L I G H T S • The level of PCDD/Fs in soil at the Three Poles was reported. • PCDD/F congener profiles in soil from the Three Poles were compared. • Potential local and regional PCDD/F sources were conducted by FLEXPART simulation.

a r t i c l e

i n f o

Article history: Received 24 April 2014 Received in revised form 21 July 2014 Accepted 28 July 2014 Available online xxxx Editor: Adrian Covaci Keywords: PCDD/Fs Soil Three Poles FLEXPART model

a b s t r a c t The concentrations of polychlorinated dibenzo-p-dioxins (PCDDs) and dibenzofurans (PCDFs) in 35 soil samples collected from Fildes Peninsula in the Antarctic, Ny-Ålesund in the Arctic, and Zhangmu-Nyalam in the Tibetan Plateau were reported in this study. A comparison of the total concentration and TEQ of PCDD/Fs at the Three Poles was conducted. Both the total concentration and TEQ of PCDD/Fs demonstrates a decreasing trend in the order of Zhangmu-Nyalam (mean: 26.22 pg/g, 0.37 pg I-TEQ/g) N Ny-Ålesund (mean: 9.97 pg/g, 0.33 pg I-TEQ/g) N Fildes Peninsula (mean: 2.18 pg/g, 0.015 pg I-TEQ/g) (p b 0.05). In all samples, the congener and homologue profiles dominated with higher (seven and eight) chlorinated PCDD/Fs (more than 85% of the total mass percentage of PCDD/Fs) at the Three Poles. Finally, a FLEXPART backward simulation was used to preliminarily identify the potential local and regional anthropogenic sources of PCDD/Fs. The results imply that the air masses passing over surrounding regions with significant PCDD/F emissions might contribute to the occurrence of PCDD/Fs in both the Arctic and Tibetan Plateau. © 2014 Elsevier B.V. All rights reserved.

1. Introduction Polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofurans (PCDD/Fs) are persistent organic pollutants (POPs) regulated by the Stockholm Convention (http://chm.pops.int/TheConvention/ ThePOPs/ListingofPOPs/tabid/2509/Default.aspx). They are unintentional byproducts of combustion and chlorinating processes during both natural and human activities, such as wildfire and waste incineration (USEPA, 2000). Among all 210 congeners, 17 PCDD/Fs with chlorines at the 2, 3, 7, and 8 positions are dioxin-toxic, and pose a high risk to ecological systems and human health due to severe abnormality inductive, mutagenic, and carcinogenic effects (Wikoff et al., 2012). The Antarctic, the Arctic, and the Tibetan Plateau are collectively referred to as “Three Poles” of the earth. They share the ⁎ Corresponding author. Tel.: +86 10 62756593. E-mail address: [email protected] (J. Hu).

http://dx.doi.org/10.1016/j.scitotenv.2014.07.109 0048-9697/© 2014 Elsevier B.V. All rights reserved.

common characteristics of extremely harsh climate and fragile ecosystems. With few anthropogenic activities, the Three Poles were once thought to be free from man-made chemical contamination. There have been few reports of PCDD/Fs at the Three Poles, due to the extreme environment and the expense of sampling. However, the available data suggest that PCDD/Fs occur at the Three Poles: PCDD/Fs in soil (2.48–4.30 pg/g dry weight) from the eastern Tibetan Plateau were reported by Pan et al. (2013); PCDD/Fs were detected in the blubber of ringed seals from the Arctic (0.6–108.2 pg/g lipid weight) (Savinov et al., 2011); and PCDD/Fs in Antarctic penguins were measured (7–72 pg/g wet weight) (Corsolini et al., 2007). As listed above, most available reports of PCDD/F levels were based on the monitoring of biota and conducted to assess the ecological risk. However, because abiotic media are the transfer media and main receptors of POPs, they can be used to investigate the potential sources and transportation pathways of POPs (Hale et al., 2006). Among the various abiotic environmental media, soil is one of the first points

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of entry of PCDD/Fs into terrestrial ecological systems (Carballeira et al., 2006). We therefore selected soil to monitor PCDD/Fs in the Three Poles in this study. And the investigation of PCDD/F levels in soil from these regions could provide fundamental data for a range of academic applications, such as assessing the local ecological risk of PCDD/ Fs, as well as the effects of human disturbances on the environment. The identification of the potential PCDD/F sources of the Three Poles is essential for understanding the transport of PCDD/Fs. Previously, many studies have highlighted the importance of global long-range transport as the main source of POPs at the Three Poles (AMAP, 1998; Pienitz et al., 2004). For instance, Wania and his colleagues proposed the theory of global fractionation, which posited that remote regions tended to accumulate POPs through long-range transfer (Wania and Mackay, 1993; Wania, 2003). However, due to the increasing human activities at the Three Poles and the surrounding regions, the contributions of those local and regional sources to the PCDD/F occurrence can't be overlooked; thus, identifying the localized anthropogenic sources of PCDD/Fs at the Three Poles is needed. Simple backward trajectory models were often used to describe the neighboring sources of POPs at a simplified way (e.g., detailed atmospheric transport and chemical processes were ignored). The FLEXible PARTicle dispersion model (FLEXPART) considered not only atmospheric turbulence and convection, but also chemical removal of POPs by hydroxyl (OH) radicals during the backward simulation. In the previous study, it has been successfully applied in investigating the transport behavior of polychlorinated biphenyl (PCB) (Eckhardt et al., 2009). The soil samples from Fildes Peninsula in the Antarctic, Ny-Ålesund in the Arctic and Zhangmu-Nyalam in the Tibetan Plateau were collected in this study. The concentrations of PCDD/Fs and the congener profile at the Three Poles were compared to maximize spatial variation over a global geographic range. The potential local and regional anthropogenic sources of PCDD/Fs in the sampling sites were then investigated using a FLEXPART simulation.

2. Materials and methods 2.1. Sample collection The sampling locations were Fildes Peninsula in the Antarctic, NyÅlesund in the Arctic, and Zhangmu-Nyalam in the Tibetan Plateau (Fig. 1).

Fildes Peninsula is located on King George Island-the largest island of the South Shetland Islands and hosts nine scientific stations within the Antarctic Treaty System. The local temperature ranges from − 20.3 to 8.5 °C with an average of − 1.5 °C. From December 2007 to January 2008, fifteen topsoil (upper 5 cm) samples (62° S, 58° W) were collected. Ny-Ålesund is the northernmost permanent settlement, and is inhabited by ca. 30 scientists and technicians. The region is influenced by polar cyclones and the North Atlantic Ocean circulation. The average annual temperature is − 5.1 °C. From July to August in 2008, we collected topsoil samples from 20 sampling sites (78° N, 12° E). There are approximately 6000 residents of the Zhangmu-Nyalam region (28° N, 86° E), which includes both a warm and wet subtropical temperate zone, and a cold frigid zone. In September 2011, nine sampling sites were established from Zhangmu (located 2636 m above sea level, on the south side of the Himalayas, with an annual average temperature of 18 °C) to Nyalam (located 5129 m above sea level, on the north side of the Himalayas, with an average annual temperature of 2.1 °C, and a range of −19.1 to 22.1 °C).

2.2. Sample extraction/cleanup The soil samples were kept in aluminum foil and stored at −20 °C before extraction. Twenty-gram samples were freeze-dried, ground, passed through 80-mesh filters, and spiked with labeled standards as a recovery standard (13C-PCDD/Fs, Wellington Laboratories, Canada). They were then extracted five times by pressurized liquid extraction using toluene at 150 °C and 1500 psi (ASE-300, Dionex, USA). The crude extracts were concentrated by rotary evaporation, and then cleaned up with multi-step procedures, which included acid washing (98% H2SO4), multi-layer silica gel (from top to bottom in turn: 6 g Na2SO4, 0.9 g silica gel, 3 g basic silica gel with 3% KOH, 4.5 g acid silica gel with 44% H2SO4, 6 g acid silica gel with 22% H2SO4, 0.9 g silica gel and 3 g silica gel with 10% AgNO3), gel permeation chromatography (Bio-Beads SX3, Bio-Rad, USA), and an activated carbon–diatomite column. The final volume of the samples was adjusted to 20 μL by adding the internal standards (13C-1,2,3,4-TeCDD, Wellington Laboratories, Canada) before instrumental analysis. The reagents used in this study were pesticide-grade toluene, acetone, methylene chloride, decane, and hexane. Decane was obtained from Wako Pure Chemical Industries, Ltd. (Osaka, Japan),

Fig. 1. Locations of sampling sites in (a) Ny-Ålesund, (b) Fildes Peninsula, and (c) Zhangmu-Nyalam.

S. Jia et al. / Science of the Total Environment 497–498 (2014) 353–359

and the other reagents were purchased from Thermo Fisher Scientific (Waltham, MA, USA). A high-resolution gas chromatography (HP6890, Agilent Technologies, USA) coupled with a high-resolution mass spectrometer (JMS 700D, JEOL, Tokyo, Japan) was used, and was equipped with a DB-5MS column (60 m × 0.25 mm I.D., 0.25-μm thickness: J & W Scientific, Folsom, CA, USA) for the determination of TeCDD/Fs to HxCDD/Fs [the inlet temperature was 260 °C. The column temperature was programmed from 120 °C (1 min) to 200 °C at 20 °C/min, 2 °C/min to 260 °C (10 min)]. A DB-17MS column (15 m × 0.32 mm I.D., 0.25-μm thickness: J & W Scientific) was used for the determination of HpCDD/Fs and OCDD/Fs [the inlet temperature was 280 °C and the column temperature program: 100 °C (1 min), 20 °C/min to 280 °C (10 min)]. The flow rate was 1.2 mL/min, and the injection volume was 2 μL. MS operating conditions were as follows: ionization energy, 38 eV; ion current, 600 μA; accelerating voltage, 7.0 kV; detection mode, single ion monitoring (SIM); and mass resolution, 10,000. 2.3. QA/QC One field blank and one method blank were analyzed for each batch of samples, and no background level higher than limit of detection (LOD) was recorded. The LODs of the congeners ranged from 0.02 to 0.40 pg/g. The recovery efficiencies ranged from 54.47% to 104.35%, which was within the requirement of US EPA Method 1613B (EPA, 1994).

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source–receptor relationships is applicable for four to five chlorinated PCDD/Fs, but not six to eight chlorinated PCDD/Fs. However, most anthropogenic PCDD/F sources emit both four to five and six to eight chlorinated congeners (Table 2) (EPA, 1998); the identified local and regional emission areas for four to five chlorinated PCDD/Fs could be similar to primary emission sources for six to eight chlorinated PCDD/Fs. In this study, an air tracer was used as the released parcel for this backward simulation. The outputs of the FLEXPART backward simulation were 0.5° × 0.5° 3-h ES maps for each sampling site. And the 3-h ESs for each site were averaged over 1 year in this study. It should be noted that emission contributions were not calculated (multiplication of the footprint ES with the emission flux densities derived from an emission inventory) because of the lack of appropriate emission profile inventories. Therefore, the ES maps covered only the preliminary potential emission sources that were identified. These ESs were derived by simulating the atmospheric tracer concentrations above the ground at each sampling site, instead of the concentration in the soil as reported in this study. We assumed that the overall concentrations in the soil were correlated with the overall atmospheric concentrations at these sampling sites, as atmospheric transport and deposition were believed to be the main sources for POPs in surface soil at Pole regions (UNEP, 2002; AMAP, 2004). 3. Result and discussion 3.1. PCDD/F levels in soil: statistical descriptions

2.4. Data analysis Prior to statistical analysis, a Levene analysis (p N 0.05) was used to determine the homogeneity of variance of the dataset. Kruskal–Wallis H-tests were then used to evaluate datasets that did not conform to a normal distribution (p N 0.05). All statistical analyses were conducted using IBM SPSS v-20.0. A value of p b 0.05 was taken to indicate statistical significance. The Lagrangian particle dispersion model FLEXPART v-9.02 (Stohl et al., 1998, 2005; http://www.flexpart.eu) was used to establish the atmospheric source–receptor (sampling areas) relationships. As the potential emission sources outnumbered the sampling areas (receptors), “backward simulation” instead of “forward simulation” was used in this study. The backward model calculations were detailed by previous studies (Stohl et al., 2003; Seibert and Frank, 2004). Briefly, the backward simulation was driven with operational three-hourly meteorological data of 1° × 1° horizontal resolution and 91 vertical levels, from the European Center for Medium Range Weather forecast (ECMWF). Forty thousand particles (not real particle maters, but so-called ideal tracer parcels) were released every 3 h at ca. 20 m above model ground at each sampling site and traced backward for a twenty-day time to calculate the emission sensitivity (ES). Each grid cell has an ES value (in units of ns/kg). Providing a unit source strength (1 kg/s) in each grid cell, the ES value equals to the simulated concentration that the cell would produce at the receptor. The higher the ES value, the greater the contribution of the unit emission flux density from the source to the concentration at the receptor. FLEXPART is suitable for simulating atmospheric transport of POPs that are not strongly adsorbed on airborne particles (log KOA b 11 and log KAW N −5) (Meyer et al., 2005; Eckhardt et al., 2009). PCDD/Fs are present in both the gas phase and particle phase in the atmosphere. The four to five chlorinated PCDD/Fs are present mainly in the gas phase (N 50%), and have the above properties (i.e., log KOA b 11 and log KAW N −5). However, six to eight chlorinated congeners are mainly in the particle phase (N50%) and have log KOA N 11 (Wagrowski and Hites, 1998; Harner et al., 2000). Besides, for six to eight chlorinated congeners, primary emission sources are not the only effective sources to the atmosphere, and the secondary sources (six to eight chlorinated congeners contaminated soils, water, etc.) might also influence the measurement locations. Thus, the FLEXPART simulation of the

The PCDD/F concentrations are presented in Table 1. The highest concentration (26.22 pg/g dw; range 2.43–73.28 pg/g dw) was found in the soil of Zhangmu-Nyalam, followed by Ny-Ålesund (9.97 pg/g dw; range 3.55–16.60 pg/g dw), and Fildes Peninsula (2.18 pg/g dw; range 0.49–6.72 pg/g dw) (p b 0.05). The International Toxic Equivalent Quantity (I-TEQ) decreased in the same order, Zhangmu-Nyalam (0.37 pg I-TEQ/g; range 0.062–0.65 pg I-TEQ/g) N Ny-Ålesund (0.33 pg I-TEQ/g; range 0.016–0.65 pg I-TEQ/g) N Fildes Peninsula (0.015 pg I-TEQ/g; range 0.00070–0.062 pg I-TEQ/g) (p b 0.05). The identical order was apparent for the reported atmospheric concentrations of PCDD/F (both gas and particle phases) from the literature: those in the Arctic (4.2–59 fg/m3) was higher than those in the Antarctic (0.14–1.07 fg/m3) (Hung et al., 2002; Piazza et al., 2013). The PCDD/F concentrations in the soil of the Three Poles were compared with the literature data from various locations around the world (Table 1). The PCDD/F concentrations in this study were lower than those in remote or rural regions like Slovakia (Dömötörová et al., 2012), Australia (Gaus et al., 2001; Müller et al., 2004), Austrian (Weiss et al., 2000), Norway (Offenthaler et al., 2009), Swiss (Schmid et al., 2005), Italy (Baderna et al., 2013), Norwegian (Hassanin et al., 2005), and the UK (Rose et al., 1997; Hassanin et al., 2005). It can be seen from Table 1 that PCDD/F concentrations in soil collected from the Antarctic in this study were the lowest reported worldwide. The Arctic soil had the second-lowest PCDD/F levels, and the Tibetan Plateau the third lowest. This is consistent with a 1-year simulation of atmospheric dioxin deposition to land (Booth et al., 2013), which estimated that the deposition values in the Antarctic were the lowest worldwide (b 1 pg TEQ/km2), lower than those in the Arctic (between 1 pg and 0.11 mg TEQ/km2) and the Tibetan Plateau (0.04–2.2 mg TEQ/km2) regions. The monitoring data in this study confirmed the PCDD/F pollution at the Three Poles and provided key information regarding global dioxin pollution. Compared with PCDD/F concentrations elsewhere in the world, the Three Poles remain relatively unpolluted, which provide an opportunity to monitor the background PCDD/F levels. 3.2. Homologue and congener profiles Figure 2 shows the concentrations of individual PCDD/F congeners at each site. The results indicate that OCDD was the predominant congener

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Table 1 Comparison of PCDD/F concentrations in soils in this study with previously reported data from various locations (pg/g dw).

Soil

Country/region

Sampling type, sites and date

Concentration (pg/g dw)

I-TEQ (pg/g)

Ref

Ny-Ålesund, Arctic Fildes Peninsula, Antarctic Zhangmu-Nyalam, China Starina, Slovakia Alps, Norway Binningen, Swiss Lombardy, Italy Scotland, UK Queensland, Australia Australia United Kingdom

Tundra soil Tundra soil Grassland–forest land Background Spruce forest Agriculture Agrarian soil Sediment Sugarcane field Remote Grassland soil Woodland soil Woodland soil Background forest

9.97 (3.55–16.6) 2.18 (0.49–6.72) 26.22 (2.43–73.28) 22 (13–30) 207.62 (17.6–625.9) 188

0.33 (0.16–0.62) 0.02 (ND–0.06) 0.37 (0.06–0.65) 0.51 (0.38–0.71) 3.2 (0.14–10.15)a 2.4 2.06 (0.38–5.27)b 3.4 2.4 0.38 (0.00056–5.0) 15.25 (3.77–67.96) 50.44 (5.53–269.07) 30.87 (0.94–125.8) 4.0 (1.6–31.0)

This study This study This study Dömötörová et al. (2012) Offenthaler et al. (2009) Schmid et al. (2005) Baderna et al. (2013) Rose et al. (1997) Gaus et al. (2001) Müller et al. (2004) Hassanin et al. (2005)

Norwegian Austrian

a) Fildes Peninsula

3 2 1 0

b) Ny-Alesund

1 0.6 0.4 0.2 0.0

c) Zhangmu-Nyalam region

100

Ny-Alesund Fildes Peninsula Zhangmu-Nyalam

15

0

OCDF

20

HpCDFs

Fig. 2. Box-and-whisker plot of concentrations of total PCDD/Fs in soil samples from (a) the Fildes Peninsula, (b) Ny-Ålesund, and (c) Zhangmu-Nyalam. The upper and lower whiskers are the maximum and minimum concentrations. The upper and lower boundaries of the box are the 75th and 25th quantiles. The middle band and the hollow square represent the median and mean, respectively. The data for some congeners are missing due to the corresponding concentrations in soil samples being below the detection limit.

40

HxCDFs

1,2,3,4,6,7,8,9-OCDF

1,2,3,4,7,8,9-HCDF

2,3,4,6,7,8-HCDF

1,2,3,4,6,7,8-HCDF

1,2,3,7,8,9-HCDF

1,2,3,6,7,8-HCDF

2,3,4,7,8-PCDF

1,2,3,4,7,8-HCDF

2,3,7,8-TCDF

1,2,3,7,8-PCDF

1,2,3,4,6,7,8,9-OCDD

1,2,3,7,8,9-HCDD

1,2,3,4,6,7,8-HCDD

1,2,3,6,7,8-HCDD

1,2,3,4,7,8-HCDD

2,3,7,8-TCDD

1,2,3,7,8-PCDD

0

60

PeCDFs

5

80

TCDFs

10

OCDD

60 25 20

HpCDDs

8 6 4

HxCDDs

4

from Ny-Ålesund, 1,2,3,7,8-PCDD accounted for most of the PCDD/F I-TEQ (24.26% of the total PCDD/F I-TEQ); this was also true for Zhangmu-Nyalam (26.29%). In soil samples from the Fildes Peninsula, 1,2,3,4,6,7,8-HCDD accounted for 42.52% of the total PCDD/F I-TEQ. The homologue profiles (Fig. 3) were used to present the relative abundance of the PCDD/F group of isomers. Fig. 3 shows a notable presence of higher-chlorinated (seven and eight) PCDD/Fs: Fildes Peninsula (97.57% of the total PCDD/F mass percentage), Ny-Ålesund (88.47%), and Zhangmu-Nyalam (95.70%). There are two possible reasons for this distribution of lower and higher chlorinated PCDD/Fs. (1) Most of the PCDD/F emission sources emit a larger quantity (N50%) of higherchlorinated PCDD/Fs. Using the source profile from the USEPA database, the proportion of PCDD/F homologue profiles from the major sources are shown in Table 2 (EPA, 1998). Taking Europe as an example, it can be seen that 62% of PCDD/Fs were emitted from municipal solid waste incineration (higher-chlorinated PCDD/Fs account for 62% of the total PCDD/Fs), iron ore sinter plants (88%), medical waste incineration (70%), and the non-ferrous metal industry (75%). Thus, higherchlorinated PCDD/Fs were released into the environment more frequently than lower-chlorinated PCDD/Fs (UNEP, 1999). (2) Rapid gas-phase reactions occur between the hydroxyl radical and the PCDD/F congener (Kwok et al., 1995; Sinkkonen and Paasivirta, 2000). After release, the lower-chlorinated PCDD/Fs react more rapidly than

PeCDDs

Concentration (pg/g)

Concentration (pg/g)

on the Fildes Peninsula (Fig. 2a; 56.02% of the total PCDD/F mass), while 1,2,3,4,7,8,9-HpCDF (Fig. 2b; 48.55%) was highest at Ny-Ålesund, and PCDD/F concentrations at Zhangmu-Nyalam were dominated by OCDF (Fig. 2c; 61.13%). While, OCDD and OCDF were below the LODs at NyÅlesund, which is different from other pole regions. In soil samples

Concentration (pg/g)

Weiss et al. (2000)

WHO98-TEQ. WHO2005-TEQ.

TCDDs

b

1998 1998 1998 1993

488 1708.8 890 (0.31–15,000) 146.4 (46.33–460) 364.5 (64.24–1262) 208.7 (9.12–835) 319 (106–2676)

H o m o lo g u e P e r c e n t ( % )

a

2008 2007–2008 2011 2007 2004 2002 2011–2012 1970–1993

Fig. 3. Percentage contribution of the homologue profile to the total PCDD/Fs in soil samples from the Fildes Peninsula, Ny-Ålesund, and Zhangmu-Nyalam. The data for some congeners were missing due to the corresponding concentrations in soil samples being below the detection limit. Error bars indicate standard deviations.

S. Jia et al. / Science of the Total Environment 497–498 (2014) 353–359

357

Table 2 PCDD/F homologue profiles from typical environmental release sources in US (EPA, 1998). % Homologue group Chemical manufacturing and processing sources Combustion sources: waste incineration

Combustion sources: power/energy generation

Combustion sources: other high temperature sources

Combustion sources: minimally and uncontrolled combustion sources Metal smelting and refining sources

4D

5D

6D

7D

8D

4F

5F

6F

7F

8F

7 and 8D/F

Pulp Sludge Effluent Municipal solid waste incinerator Hazardous waste incineration Medical waste incineration Crematoria Tire combustion Sewage sludge incineration Diesel-fueled vehicles Leaded gas-fueled vehicles Unleaded gas-fueled vehicles Industrial wood combustion Residential/commercial oil combustion Utility sector and industrial boilers Utilities and industrial coal combustion Cement kilns burning hazardous waste Cement kilns not burning hazardous waste Kraft black liquid recovery boilers Petroleum refinery Combustion of landfill gas

0.08 0.06 0.01 0.00 0.00 0.00 0.00 0.02 0.00 0.01 0.01 0.01 0.00 0.27 0.00 0.00 0.01 0.00 0.00 0.00 0.00

0.00 0.00 0.00 0.02 0.00 0.01 0.01 0.00 0.00 0.00 0.03 0.00 0.01 0.00 0.01 0.00 0.03 0.01 0.00 0.01 0.00

0.04 0.03 0.05 0.06 0.01 0.04 0.05 0.02 0.01 0.04 0.04 0.02 0.06 0.00 0.07 0.00 0.11 0.04 0.01 0.04 0.02

0.00 0.00 0.00 0.12 0.03 0.09 0.16 0.06 0.04 0.14 0.04 0.07 0.08 0.00 0.15 0.13 0.10 0.14 0.08 0.07 0.03

0.61 0.65 0.85 0.17 0.06 0.12 0.26 0.64 0.23 0.65 0.11 0.52 0.36 0.32 0.65 0.30 0.03 0.23 0.73 0.04 0.17

0.24 0.22 0.03 0.02 0.04 0.01 0.02 0.05 0.45 0.01 0.12 0.03 0.07 0.26 0.00 0.06 0.13 0.24 0.03 0.01 0.54

0.00 0.01 0.00 0.07 0.08 0.05 0.05 0.03 0.15 0.02 0.10 0.03 0.09 0.00 0.04 0.05 0.27 0.11 0.04 0.09 0.06

0.00 0.00 0.00 0.21 0.27 0.19 0.16 0.05 0.07 0.04 0.16 0.04 0.13 0.00 0.04 0.10 0.26 0.11 0.02 0.27 0.10

0.00 0.01 0.00 0.25 0.29 0.24 0.21 0.00 0.03 0.04 0.30 0.14 0.13 0.00 0.05 0.26 0.06 0.05 0.02 0.39 0.05

0.03 0.02 0.05 0.07 0.21 0.25 0.07 0.12 0.02 0.04 0.08 0.15 0.07 0.15 0.00 0.09 0.01 0.08 0.08 0.08 0.02

0.64 0.68 0.91 0.62 0.60 0.70 0.70 0.83 0.32 0.88 0.54 0.87 0.64 0.47 0.85 0.78 0.20 0.49 0.90 0.58 0.27

Primary ferrous iron ore sintering Secondary aluminum smelters Secondary copper refinery Secondary lead smelters Ferrous foundries Scrap electric wire recovery Drum and barrel reclamation furnaces

0.02 0.00 0.00 0.05 0.02 0.00 0.02

0.04 0.01 0.00 0.02 0.04 0.00 0.00

0.11 0.04 0.02 0.05 0.02 0.00 0.00

0.05 0.06 0.05 0.04 0.04 0.00 0.00

0.04 0.07 0.17 0.09 0.00 0.55 0.38

0.37 0.07 0.02 0.23 0.24 0.00 0.37

0.25 0.14 0.04 0.22 0.30 0.00 0.00

0.10 0.27 0.32 0.20 0.21 0.00 0.00

0.02 0.21 0.13 0.07 0.09 0.00 0.00

0.01 0.11 0.25 0.03 0.03 0.45 0.23

0.11 0.46 0.60 0.24 0.16 1.00 0.61

the higher-chlorinated congeners. Empirical evidence has shown that the half-lives of PCDD/Fs increase with an increasing number of chlorine atoms in the benzene ring: 2,3,7,8-TCDD (14.20 days) b OCDD (197.44 days); 2,3,7,8-TCDF (42.26 days) b OCDF (1001.73 days) (Bruckmann et al., 2013). Therefore, a greater proportion of the higher-chlorinated PCDD/Fs than the lower chlorinated PCDD/Fs would remain in the atmosphere. 3.3. Preliminary analysis of potential sources Given the increasing human activities at the Three Poles and surrounding regions, special attention was paid to identify the potential local and regional anthropogenic PCDD/F sources. The footprint ES maps obtained from FLEXPART 20-day backward simulations averaged over 1 year for each sampling site are shown in Fig. 4. For all Three Poles, the highest ES areas (ES N 100 ps/kg, red areas in Fig. 4) surrounded the sampling sites, followed by neighboring countries or seas (5 ps/kg b ES b 100 ps/kg, yellow areas in Fig. 4). The red areas extended for hundreds of kilometers, thus we defined them as local areas; the yellow areas extended for up to thousands of kilometers, thus we defined them as regional areas. Merely the potential anthropogenic PCDD/F sources in those local and regional areas were concerned. At the Three Poles, incineration is considered to be the main source of PCDD/Fs among the various local activities. For Fildes Peninsula and Ny-Ålesund, combustion mainly occurs in the process of livelihood supporting for ~ 30 permanent researchers and limited tourists (Harris, 1991; Baker and Hites, 2000). Compared to the sparsely populated areas of the Fildes Peninsula and Ny-Ålesund, livelihood supporting for ~ 6000 inhabitants is the main combustion process in Zhangmu-Nyalam. Therefore, Zhangmu-Nyalam is likely to have received a greater amount of PCDD/F contamination from local anthropogenic sources. PCDD/F emissions in regional areas differed among the Three Poles. For example, the Antarctic is isolated from the other continents by an

extensive area of ocean surrounding the landmass. Yellow ES value areas appeared on the oceans (Fig. 4a). There are almost no anthropogenic sources, which suggests that the PCDD/Fs recorded in the Antarctic were not from regional anthropogenic sources. A previous study reported that the levels of POPs in the Antarctic are dependent on global long-range transport (UNEP, 2002). In the Arctic, Fig. 4b shows that the regions with yellow ES value include Denmark (5.8 × 10−4 g I-TEQ/km2 yr in 2010, per unit area releases of PCDD/Fs to the air) (Danish, 2012), Iceland (2.3 × 10−5 g ITEQ/km2 yr in 2004) (UNEP, 1999), Russia (7.0 × 10− 5 g I-TEQ/ km2 yr in 1994) (Lassen et al., 2003), Sweden (2.3 × 10−4 g I-TEQ/ km2 yr in 2005) (Swedish, 2012), Norway (5.4 × 10−5 g I-TEQ/km2 yr in 2009) (Norway, 2009) and Finland (5.8 × 10− 4 g I-TEQ/km2 yr in 2004) (UNEP, 1999). Thus, releases of PCDD/Fs from these countries may contribute to PCDD/F concentrations in the Arctic, which were higher than those measured in the Antarctic in this study (Fig. 2). Unlike the Arctic and the Antarctic, the Tibetan Plateau abuts densely populated countries, e.g., China in the north, and India, Pakistan and Nepal in the south (Wang et al., 2006). The air masses passing over these countries could transport PCDD/Fs directly to the Tibetan Plateau. In Fig. 4c, the regions of yellow-color ES are India (8.8 × 10− 4 g I-TEQ/km2 yr in 2010, per unit area releases of PCDD/Fs to the air) (India, 2011), followed by Bangladesh (1.3 × 10− 3 g I-TEQ/km2 yr in 2005) (Bangladesh, 2007), Pakistan (1.1 × 10− 3 g I-TEQ/km2 yr in 2003) (Pakistan, 2003), China (5.2 × 10− 4 g I-TEQ/km2 yr in 2004) (Zhao et al., 2011), Iran (6.5 × 10−4 g I-TEQ/km2 yr in 2006) (Iran, 2008) and Nepal (1.2 × 10−3 g I-TEQ/km2 yr in 2006) (Nepal, 2007). It is clear that the emission densities of PCDD/Fs approach an order of magnitude higher in these Asian countries than those in the Scandinavian countries in the Arctic region. Accordingly, Zhangmu-Nyalam would experience a high level of PCDD/Fs. Thus, Zhangmu-Nyalam suffered the highest occurrence of PCDD/Fs from both local and regional sources among the Three Poles, which is consistent with the data detected in this study. In addition, the Tibetan Plateau has also been shown to have the highest levels of

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Fig. 4. Footprint ES maps obtained from FLEXPART 20-day backward calculations based on ECMWF input data. The sample sites in (a) the Fildes Peninsula, (b) Ny-Ålesund, and (c) Zhangmu-Nyalam are indicated by white dots. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)

other POPs, e.g., dichloro-diphenyl-tricgloroethane (DDT), hexachlorocyclohexanes (HCHs) and endosulfan, among the Three Poles (Li et al., 2006).

highest level of PCDD/Fs among the Three Poles being recorded in Zhangmu-Nyalam, on the Tibetan Plateau. Acknowledgments

4. Conclusion (1) Data presented indicate the PCDD/F pollution situation at the Three Poles. The concentration of Σ17PCDD/Fs in soils increased in the order of Fildes Peninsula b Ny-Ålesund b ZhangmuNyalam. However, compared to other regions around the world, the Three Poles remain relatively unpolluted and could provide the global PCDD/F background. (2) The PCDD/F congeners and homologue profiles at the Three Poles were dominated by higher chlorinated PCDD/Fs, due to their larger quantity of emission and slower clearing-speed than lower chlorinated congeners. (3) The potential local and regional sources were identified in this study. The FLEXPART backward simulations revealed that: a) no obvious regional anthropogenic sources were present within the PCDD/F atmospheric transport pathway to the Antarctic; b) PCDD/Fs in Ny-Ålesund could be accounted for by emissions in countries surrounding the Arctic; and c) sources in the local and surrounding countries were likely to be responsible for the

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