Atmospheric transport and accumulation of organochlorine compounds on the southern slopes of the Himalayas, Nepal

Environmental Pollution 192 (2014) 44e51

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Atmospheric transport and accumulation of organochlorine compounds on the southern slopes of the Himalayas, Nepal Ping Gong a, Xiao-ping Wang a, *, Sheng-hai Li a, Wu-sheng Yu a, Jiu-le Li a, Dambaru Ballab Kattel a, c, Wei-cai Wang a, Lochan Prasad Devkota b, Tan-dong Yao a, Daniel R. Joswiak a a Key Laboratory of Tibetan Environmental Changes and Land Surface Process, Institute of Tibetan Plateau Research, Chinese Academy of Sciences, P.O. Box 2871, Beijing 100101, China b Central Department of Hydrology and Meteorology, Tribhuvan University, Kathmandu 44618, Nepal c Department of Meteorology, COMSATS Institute of Information Technology, Islamabad, Pakistan

a r t i c l e i n f o

a b s t r a c t

Article history: Received 29 January 2014 Received in revised form 2 May 2014 Accepted 4 May 2014 Available online xxx

Studies have been devoted to the transport and accumulation of persistent organic pollutants (POPs) in mountain environments. The Himalayas have the widest altitude gradient of any mountain range, but few studies examining the environmental behavior of POPs have been performed in the Himalayas. In this study, air, soil, and leaf samples were collected along a transect on the southern slope of the Himalayas, Nepal (altitude: 135e5100 m). Local emission occurred in the lowlands, and POPs were transported by uplift along the slope. During the atmospheric transport, the HCB proportion increased from the lowlands (20%) to high elevation (>50%), whereas the proportions of DDTs decreased. The largest residue of soil POPs appeared at an altitude of approximately 2500 m, and may be related to absorption by vegetation and precipitation. The net deposition tendencies at the airesoil surface indicated that the Himalayas may be a ‘sink’ for DDTs and PCBs. © 2014 Elsevier Ltd. All rights reserved.

Keywords: Persistent organic pollutants (POPs) Altitudinal distribution Hierarchical clustering analysis (HCA) Cold-trapping Airesoil exchange

1. Introduction Persistent organic pollutants (POPs), such as organochlorine pesticides (OCPs), hexachlorobenzene (HCB), and polychlorinated biphenyls (PCBs), are a class of organic compounds exhibiting high toxicity, environmental persistence, and a potential for bioaccumulation (Lallas, 2001). There are growing concerns regarding the potential for long-range atmospheric transport (LRAT) of POPs arising from detection of POPs in remote regions (Daly and Wania, 2005; Hung et al., 2010). Of particular interest in this regard are the transport and accumulation of POPs in mountain environments. Mountains have steep environmental gradients on a small spatial scale. Due to the sharp variations in temperature and precipitation, atmospheric POPs are prone to accumulate at high elevations (Daly and Wania, 2005). Mountains, therefore, have been considered ‘traps’ for LRAT of POPs (Vighi, 2006). With respect to the accumulation in mountain environments, both the variable concentrations and the relative compositions of

* Corresponding author. E-mail address: [email protected] (X.-p. Wang). http://dx.doi.org/10.1016/j.envpol.2014.05.015 0269-7491/© 2014 Elsevier Ltd. All rights reserved.

POPs have been strongly correlated along altitudinal gradients. Many studies have addressed these patterns along the slopes of high mountains, such as the European Alps (Shen et al., 2009; van Drooge et al., 2004), Rocky Mountains (Daly et al., 2007a), Andes (Estellano et al., 2008; Tremolada et al., 2008), Mount Meru in Tanzania (Parolini et al., 2013), the Himalayas (Wang et al., 2006, 2009; Yang et al., 2008), and Balang Mountain in China (Chen et al., 2008; Liu et al., 2010). A positive correlation between the levels of POPs and altitude may relate to the temperature dependence of precipitation scavenging (Chen et al., 2008; Daly et al., 2007a; Liu et al., 2010; Tremolada et al., 2008). However, when the mountains are close to the emission sources, higher concentrations of POPs occur in the lowlands (Parolini et al., 2013). The relative compositions of POPs also vary with altitude. The more volatile POPs (e.g., HCB, HCHs) are prone to transport to high altitudes, whereas the less volatile POPs (e.g., DDTs) are more likely to accumulate in the lowlands (Chen et al., 2008; Wania and Westgate, 2008). Therefore, the relative compositions of more volatile POPs in air increase with increasing altitude, whereas those of less volatile POPs decrease with increasing altitude. In addition, mountain forests play a role in enhancing the POP deposition on mountains (Nizzetto et al., 2006; Wenzel et al., 2002), which is

P. Gong et al. / Environmental Pollution 192 (2014) 44e51

reflected by the higher concentrations of POPs detected in soil under a canopy (Daly et al., 2007a). The Himalayan range, with an altitude of <100e8844 m, is the highest mountain range in the world. The regions to the south of the Himalayas form the South Asian subcontinent, which has a high population density and high emissions of pollutants (Gupta, 2004; Ramanathan et al., 2005). Organochlorine pesticides are used in agriculture (Abhilash and Singh, 2009), and for the control of malaria and black vectors (Mittal et al., 2004). Additionally, PCBs may be emitted from electronic waste, ship breaking activities, and dumped solid waste in South Asia (Chakraborty et al., 2013). Many studies have reported that POP usage in South Asia is marked by increased concentrations of OCPs and PCBs in the air and soil (Chakraborty et al., 2010, 2013; Devi et al., 2011, 2013; Kumar et al., 2011, 2013; Kumari et al., 2008; Mishra et al., 2012; Syed et al., 2013; Zhang et al., 2008). The climate of South Asia is controlled by the Indian monsoon in the summer (Yao et al., 2012). The Indian monsoon undergoes an orographic uplift (Yao et al., 2012), and affects the transport of POPs to the high-elevation regions of the Tibetan Plateau (Sheng et al., 2013; Wang et al., 2010). For example, OCPs and PCBs can be detected in the high-altitude regions of the Himalayas (Guzzella et al., 2011; Wang et al., 2006). However, there is still no systematic method for observing the transport and accumulation of POPs in Nepal. This study investigates the spatial distribution of POPs in the air, leaves, and soil along a transect on a southern slope of the Himalayas in Nepal with an altitude of 135e5100 m. The objective of this study was to investigate the atmospheric transport characteristics of POPs from the lowlands of Nepal to the heights of the Himalayas, and to discuss the accumulation of POPs in the soil and the vegetation of the Himalayas. 2. Materials and methods

45

mass spectrometer (MS) (Finnigan Trace GC/PolarisQ). The details on the GC/MS analysis are described in Text S2. All samples were analyzed for the following target compounds: PCB 28, 52, 101, 138, 153 and 180; HCB; o,p0 -DDE, p,p0 -DDE, o,p0 -DDT, and p,p0 -DDT. The carbon contents of the soil (total carbon, TC, and total organic carbon, TOC) were measured using a TOC analyzer (Shimadzu 5000-A, Japan). The lipid content of the leaves was measured gravimetrically for each sample (Schrlau et al., 2011). 2.3. Quality assurance/quality control (QA/QC) All analytical procedures were monitored using strict QA/QC measures. Procedural blanks and field blanks were extracted and analyzed in the same way as the samples. For the air sample, the trace target compounds were detected in the field blanks (Table S3). The method detection limits (MDLs) were derived as three times the standard deviation of the field blanks, and the MDLs were 0.01e1.71 ng/sample (Table S3). The recoveries were between 74% and 101% for PCB-30, and between 63% and 78% for Mirex. For the soil and the leaf samples, procedural blanks (extraction of a thimble filled with Na2SO4) were treated in exactly the same manner as the samples. The MDLs were derived as three times the standard deviation of the blanks. The MDLs based on a 25 g soil sample and 5 g leaf sample were 0.01e7.44 pg/g and 0.05e37.2 pg/g, respectively (Table S4). Recoveries were between 55% and 89% for PCB-30 and 53e99% for Mirex. All of the reported values were field-blank-corrected (mean blank concentrations were subtracted) but not corrected for the recovery rates. 2.4. Statistical method The hierarchical clustering analysis (HCA) method was used to classify the sampling sites into groups based on the variable relative compositions of POPs in sites (Gong et al., 2013; Liu and Wania, 2014), which were then used to discuss the sources of the atmospheric POPs in Nepal in this study. The relative compositions were defined as the ratio of an individual component concentration to the total concentration of POPs at each site. Then, this ratio was chosen as a variable when conducting the HCA analysis (SPSS 16.0). Ward's method and the Euclidean distance were applied.

3. Results and discussion 3.1. Levels of organochlorine compounds in air

2.1. Site description and sample collection The observed northesouth transect spanned the whole of Nepal from the region near the IndiaeNepal border (altitude: 135 m, Fig. S1 in the supplementary material) to the terminal point of a glacier in the high Himalayas (NepaleChina border, altitude: 5100 m, Fig. S1). The ecosystem and morphology varied with the wide altitude range, and the steep slope of the Himalayas. The timber line was at approximately 3000 m (Negi, 1994). The sampling sites included Simara (an agricultural region), Kathmandu (an urban region), Syabru Besi (a forest region), Richhe (a forest region), Kyanjin Gumba (a grassland), and Yala Peak (a glacier region). Detailed information about the sampling sites is listed in Table S1. A passive air sampler (PAS) based on XAD-2 resin was used to collect air-phase POPs in this study. The PAS units were deployed at six sites, which are listed in Table S1 in detail. Prior to deployment, the XAD resin was Soxhlet extracted using the solvents methanol, acetonitrile, and dichloromethane (DCM). The XAD resin (60 mL of wet XAD in methanol) was transferred to a pre-cleaned stainless steel mesh cylinder and dried in a clean desiccator (Wania et al., 2003). Dry cylinders were sealed in airtight stainless steel tubes with Teflon lids. Three blank resin columns were carried to the sites, which served as the field blanks. The air samples were collected from May to November in 2012 (Table S1). The harvested XAD cylinders were stored in sealed and solvent rinsed glass jars, and sent to the laboratory of the Institute of Tibetan Plateau Research in Beijing. The samplers were stored at
20  C until extraction. The soil and leaf samples were collected at the same time that the PAS units were deployed (Table S2). In the regions around the PAS sites within a radius of 200 m, five surface soil samples (0e5 cm) were conducted randomly with a cleaned stainless-steel hand trowel, and the samples were mixed as one sample. Similar to the soil sampling, mixed leaf samples were collected near the PAS sites. The dominant tree species were determined at each site, and the samples were taken at heights ~2 m above the ground. The soil and the leaf samples were then wrapped in clean aluminum foil and stored in plastic bags, and immediately mailed to the laboratory. Prior to analysis, the soil samples were freeze-dried and manually ground. The soil samples were then sieved through a 0.2 mm mesh to remove large pieces of debris, roots and other rubble. 2.2. Sample processing and analysis The XAD, soil, and leaf samples were extracted and cleaned as described by Wang et al. (2010, 2012). A description of these methods is given in Text S1. All samples and blanks were analyzed on a gas chromatograph (GC) with an ion-trap

The POPs amounts in PAS (ng/sample or pg/sample/day) were often reported (Liu et al., 2009; Zhang et al., 2008) if the sampling rates of PAS (R) fluctuated slightly. However, in this study, the temperature varied from
6.4 to 28.4  C, and the air pressure from 629 to 1145 hPa (Table S5). The variable meteorological parameters caused R to fluctuate by more than two times (Table S5). Therefore, the atmospheric concentrations in this study were calibrated using the R values. The details about R are discussed in Text S3. By using the obtained R values, the volumetric concentrations of atmospheric POPs were estimated (Table 1). The volumetric air concentrations (pg/m3) were compared with the data from active samplers (Table S6), and the amounts in PAS (pg/sample/day) were also reported for comparing with the other data based on XAD-PAS (Table S7). The levels of HCB were 128e416 pg/m3 (Table 1), which is one order of magnitude higher than those in remote regions, such as the Bay of Bengal and the Indian Ocean (Gioia et al., 2012) (Table S6). The concentrations of the total DDTs (DDE þ DDD þ DDT) were 71.1e1613.7 pg/m3. The average concentrations of o,p0 -DDT, p,p0 DDT, and p,p0 -DDE were 159.7, 125.5, and 154.8 pg/m3. The compounds o,p0 -DDT, p,p0 -DDT, and p,p0 -DDE accounted for 39.7 ± 6.7%, 26.8 ± 3.0%, and 31.3 ± 6.2% of the DDTs. The concentrations of DDTs in Nepal were compared to those in urban India (Zhang et al., 2008, P Table S6). The average concentration of 6PCB was 26.9 ± 25.6 pg/ 3 m , which is less than those reported for urban India (Zhang et al., 2008, Table S6). The concentrations of atmospheric POPs (pg/sample/day, Table S7) in Nepal were also compared with those in the Asian cities where the XAD-PAS were deployed. It was shown that the levels of HCB, DDTs, and PCBs in Nepal were higher than or of the same order of magnitude as those in the Asian cities (Baek et al., 2013; Nasir

9.3 ± 11.7 61.6 ± 114.7 7.5 ± 6.1 0.1 0.2 111.6 ± 129.6

et al., 2014; Shunthirasingham et al., 2010). These results imply that local emissions may be the sources of POPs in Nepal.

55.3 3.0 0.0

159.8 ± 152.4 485.4 ± 310.4 1264.3 ± 456.7 504.5 24.6 635.5 ± 196.3

0.1 ± 9.8 ± 3.4 ± BDLa BDLa 6.5 ±

0.0 9.8 4.3

0.8 ± 1.4 ± 1.6 ± BDLa 0.4 3.0 ±

0.3 2.6 1.1

13.9 ± 46.4 ± 19.3 ± BDLa BDLa 68.5 ±

0.5 56.6 3.2

13.7 ± 16.7 62.9 ± 109.7 22.8 ± 18.9 0.1 0.1 108.9 ± 125.0

22.4 ± 28.7 104.9 ± 191.9 31.9 ± 27.5 0.1 0.2 182.4 ± 221.3

3.2. Sources

± 24.2

BDL: below the detection limits. a

74.3 ± 54.8 50.7 ± 48.4 160.1 ± 5.6 31.1 17.8 56.6 ± 58.7 BDLa 12.0 ± 0.0 0.9 ± 0.0 BDLa BDLa 12.0 ± 0.0

Air (pg/m ) Simara Syapru Bensi Richhe Kyanjing Gumba Yala Peak Kathmandu Leaves (pg/g dw) Simara Syapru Bensi Richhe Kyanjing Gumba Kathmandu Soil (pg/g dw) Simara Syapru Bensi Richhe Kyanjing Gumba Yala Peak Kathmandu

18.7 17.4 21.2 8.9 9.5 20.4

± 4.2 ± 19.2 ± 7.0

241.2 ± 17.3 110.8 ± 68.9 117.5 82.4 208.3 39.3 ± 5.6 19.5 ± 8.7 18.9 19.7 36.1 140.5 ± 19.3 101.5 ± 30.7 74.3 71.2 166.6

33.6 ± 8.1 54.5 ± 55.9 109.8 ± 78.5 25.2 24.9 54.7 ± 67.7

101.1 ± 40.6 56.1 ± 48.1 51.3 66.7 313.5 70.8 ± 21.9 46.0 ± 38.2 62.4 48.0 224.4 297.6 ± 192.3 149.3 ± 137.1 59.8 231.5 769.1 7.4 ± 2.4 5.6 ± 4.1 5.4 4.4 41.6 73.4 ± 61.2 126.8 ± 83.8 11.6 113.7 41.6 1548.9 ± 464.9 890.8 ± 892.6 512.9 625.6 1420.0

4.1 3.4 1.4 0.8 0.3 3.2 3.8 2.4 1.8 0.8 0.2 3.7 10.0 5.7 4.3 3.3 1.5 18.0 13.4 1.1 2.3 2.9 BDLa 47.1 455.6 68.3 60.1 34.1 21.4 83.5 509.3 92.3 77.4 73.3 33.2 130.4 597.4 103.4 54.3 36.0 16.5 133.4 40.9 7.3 3.2 2.7 BDLa 6.1 416.3 202.8 150.9 279.1 127.7 261.0

677.5 ± 128.8 378.9 ± 314.8 244.2 478.7 560.3

0.6 1.2 0.6 0.1 0.3 1.3 6.1 6.0 2.3 1.1 0.9 5.2

PCB-138 PCB-153 PCB-101 PCB-52 PCB-28 p,p0 -DDT o,p0 -DDT p,p0 -DDE o,p0 -DDE HCB 3

Location

Table 1 Concentrations of target compounds in air, leaves, and soil of the Nepali transect.

19.7 ± 8.1 10.3 ± 10.3 11.8 17.7 99.7

P. Gong et al. / Environmental Pollution 192 (2014) 44e51

PCB-180

46

HCB air concentrations are lower and uniform in the Arctic (Su et al., 2006a), with the ratio of the highest concentration to the lowest concentration (H/L values) being ~2. In the present study, the HCB levels are much higher than those in other mountain regions, and the H/L values for HCB were ~4. The higher levels of HCB and the larger H/L values indicated that the local emission of HCB likely occurred in Nepal. Bailey (2001) and Barber et al. (2005) reviewed industrial and agricultural uses, which, together with emissions from incomplete combustion, are the main sources of HCB. The open burning of wood and straw is very common in Nepal (Sapkota et al., 2012, 2013), and this may lead to higher levels of HCB in the Nepali air. The relative compositions of DDTs in the low-altitude sites (e.g., Simara, Syabru Besi, and Kathmandu) of this study are higher than 50%. This high proportion of DDTs may be related to the historical/ fresh usage of DDTs. The isomer ratios of o,p0 -DDT/p,p0 -DDT were 1.1e2.1 (Table S8), much less than the ratio of dicofol usage (6.5). This indicates that the usage of dicofol may not be the primary source. The isomer ratios of p,p0 -DDE/p,p0 -DDT ranged from 0.6 to 1.3, with an average value of 0.9 ± 0.2 (Table S8). Because the p,p0 DDE/p,p0 -DDT is approximately 1, it can be inferred that both fresh and historical DDT sources exist in Nepal, which is possibly due to extensive use of technical DDT for malaria and black-fever-vector control (Mittal et al., 2004). The proportion of PCBs was almost 20% in Kathmandu (Fig. 1a), and the concentrations of PCB-28 and PCB-52 in Kathmandu reached 47 and 18 pg/m3, respectively, which is 2e3 times higher than those observed at other sites (Table 1). PCBs are used in electrical equipment (Safe, 1994), and open burning also emits PCBs (Lohmann et al., 2000). Therefore, high concentrations of PCBs often occur in urban areas with frequent human activity. Kathmandu is the capital and the most populous city in Nepal, with a population of more than 1.7 million. Local PCB emission, with the use of electrical equipment and domestic burning, may be the primary reason for the relatively high levels of PCBs in Kathmandu. The hierarchical cluster analysis (HCA) methods were used to classify the sampling sites into groups based on the relative compositions of POPs at each site, with sites in the same group likely to have similar sources. India also has extensive POP emission, and it may have similar emissions characteristics to those of neighboring Nepal. Due to the limited HCB data in India (Zhang et al., 2008), only the data on DDTs and PCBs in India and Nepal could be applied to a HCA to test whether the two countries displayed similar congener signatures. The result is presented in Fig. 1b and Table S9. The sites in Nepal and India were divided into three groups (Fig. 1b). Group 1 included the sites in the low altitudes of Nepal (Kathmandu, Simara, and Syapru Besi) and the Indian urban sites (Fig. 1b), which all showed a relatively higher composition of p,p0 -DDE (Fig. 1a and Table S8). This indicates that these sites are likely to have similar sources of DDT. The long-term DDT usage for controlling malaria has been reported for both India and Nepal, and this may be the reason that urban sites in these two countries shared similar DDTisomer patterns. The high-altitude sites with elevations above 3000 m in this study (e.g., Richhe, Kyanjin Gumba, and Yala Peak) were classified into Group 2 (Fig. 1b), and o,p0 -DDT was the major component (Table S9). The relative enrichment of o,p0 -DDT at the high-elevation sites may be attributed to the higher vapor pressure of o,p0 -DDT than those of other DDT congeners (Liu et al., 2009). The rural/background sites of India were classified into Group 3, which was characterized by relatively abundant PCBs (Table S8).

P. Gong et al. / Environmental Pollution 192 (2014) 44e51

47

Fig. 1. (a) Proportion of POP amounts in PAS and (b) dendrograph of cluster analysis for atmospheric POPs in Nepal and India.

The relative compositions of POPs can be used to probe the potential transportation pathway of POPs. Lavin and Hageman (Lavin and Hageman, 2013) suggested that the mountain sites at the highest altitudes were primarily influenced by long-range synoptic-scale winds. When the relative compositions at higher sites on the mountain show similar patterns to the distant source regions, it may indicate the influence of LRAT (Lavin and Hageman, 2013); in contrast, similar compositions at higher sites to those of lower sites may be considered to be the influence of local emissions from the lowland (mountain valley) areas (Lavin and Hageman, 2013). From Fig. 1b, Group 1 and Group 2 had a close relationship (Fig. 1b) marked by similar congener profiles (Fig. 1a). The close relationship between Group 1 and Group 2 may indicate that the higher-elevation sites can accept the upward transport of POPs from low-elevation sites. 3.3. Elevation distribution and re-proportioning of POPs The altitude distribution of atmospheric POPs concentration showed a clear decreasing pattern with increasing elevation (Fig. S1). From 135 m to 5100 m, the concentrations of HCB, DDTs, and PCBs varied from 416, 1614, and 38 pg/m3 to 128, 71, and 3.2 pg/ m3, respectively, which decreased three times and by two orders of magnitude. The low-elevation sites displayed higher POP concentrations, suggesting that the atmospheric POPs at low-elevation sites may be strongly affected by the nearby contaminant sources. Global fractionation is the environmental process by which certain chemicals, most notably volatile pollutants, are transported from warmer to colder regions of the Earth (Wania and Mackay, 1993). If this process occurs along a mountain slope, it is called mountain re-proportioning. In this study, the proportion of change in HCB, DDTs, and PCBs along the mountain slope is depicted in Fig. 2a. Although the HCB concentration showed less change among different site elevations, the proportion of HCB increased from 20% in 135 m to more than 50% in 5100 m. Due to the higher volatility of HCB (Bailey, 2001), a remarkable increase in the HCB proportion (global fractionation) was observed in high-latitude Arctic regions (Hung et al., 2010). Similar to this global fractionation, the high HCB fraction at high-elevation sites suggested the compositional fractionation of POPs occurred during the transport from low-elevation (warm) to high-elevation (cold) regions. In contrast to HCB, p,p0 DDT and DDEs have lower long-range atmospheric transport tendencies (Liu et al., 2009), and thus, their proportions decreased with increasing elevation. As discussed above, high-elevation sites in this study were mainly influenced by the upward transport of POPs from lowland source regions. On the basis of the prevailing wind patterns (Fig. 2b) and the elevated proportion of volatile POPs observed at high-elevation sites, it may be inferred that more

volatile compounds are prone to transport to higher-elevation sites, whereas less volatile chemicals, once deposited from the air, are more likely to be stored or cycled in the soil-water-plant system, rather than ‘hopping’ to higher altitude. 3.4. Levels of POPs in leaves and soil The levels of HCB, DDTs, and PCBs in the collected leaves were 0.05e0.16, 0.11e3.27, and 0.11e1.49 ng/g dw, respectively. The levels in the Nepali leaves are slightly lower than those in other urban or remote sites (Table S10). The lipids on the surfaces of the leaves were one of the important factors influencing the levels of POPs, explaining 40.9%e62.8% of the variances (p < 0.01, Fig. S3). The lipid-normalized concentrations were 0.92e1.63, 15.9e29.1, and 4.18e6.24 ng/(g lipid) for HCB, DDTs, and PCBs, respectively. The concentrations of HCB in soil were 8.9e18.7 pg/g dw, which is comparable to the measured concentrations in the Mt. Everest region, Eastern Nepal (Guzzella et al., 2011). However, the level of HCB in Nepal is 1e3 orders of magnitude lower than those in other South Asian countries (Table S11). The concentrations of DDTs in the soil samples of this study were 67.3e1151.6 pg/g dw, higher than those measured in remote regions (Mt. Everest, Nepal side). This may be due to the local emission of DDTs in the region measured by this study. However, the level of DDTs in Nepal is significantly lower than those in agricultural fields in India (Kumar et al., 2011; Kumari et al., 2008) and Pakistan (Syed et al., 2013). The PCB levels in this study are higher than those in Mt. Everest region (Nepal side) (Guzzella et al., 2011), and are slightly lower than the concentrations measured in a source region of Nepal (Aichner et al., 2007). 3.5. Altitudinal distribution and accumulation of POPs in leaves and soil Forest canopies can provide an extensive organic surface for the partitioning of POPs in the atmosphere and can increase the net atmospheric deposition of POPs. The altitudinal pattern in leaves and soil reflects the deposition characteristics of POPs during their transport along mountain slopes (Daly and Wania, 2005). Corresponding to the altitudinal distribution of POPs in air, the concentrations of most of POPs (DDTs, HCB, PCB-52, -153, -138, and -180) in leaves decreased with increasing altitude (Fig. S4). The ratios of the concentrations in forest leaves to the concentrations in the air (Cleaves/Cair, m3/g lipid) can be used to describe the efficiency of the POPs loading from air to the forest canopy. In this study, the Cleaves/ Cair ratios of most compounds (except HCB and PCB-52) have a positive trend with altitude (Fig. 3), signifying the stronger tendencies of POPs to deposit into leaves at higher altitudes.

48

P. Gong et al. / Environmental Pollution 192 (2014) 44e51

Fig. 2. (a) The compositional shift for atmospheric POPs along the southern slope of the Himalayas; (b) schematic diagram of the atmospheric circulation along the Himalayas and over the Tibetan Plateau (Yao et al., 2013).

Fig. 3. Ratios of Cleaves/Cair of DDTs along the altitudinal gradient of the southern slope of the Himalayas.

P. Gong et al. / Environmental Pollution 192 (2014) 44e51

Litterfall can transport the pollutants from the atmosphere to the forest soil (Horstmann and McLachlan, 1996; McLachlan and Horstmann, 1998; Moeckel et al., 2008; Su et al., 2006b), and this process is known as the “forest filter effect (FFE)” (Daly et al., 2007a; Guazzoni et al., 2013). In this study, soil samples at elevations of 1475 m and 2485 m were sampled under forest canopy (Table S2), while all other soil samples were collected in non-forested regions. High concentrations of all the target compounds in soil were observed at forest sites (Figs. S5eS7), indicating that the forest may be an important factor influencing the POPs deposition. Of the target compounds, HCB, DDTs and PCB-52, PCB-138, and PCB-153 had peak levels at the altitude of 2485 m (Figs. S5eS7). A survey reported that both the total net primary productivities and the biomass in the forest of the central Himalayas reached a peak at approximately 2500 m in elevation (Singh et al.,1994). Litterfall supplies organic matter for soil. Soil organic matter is considered to have a strong ability to absorb chemicals (Moeckel et al., 2009). Due to their high content of organic matter, forest soils are regarded as a final sink/strong reservoir of POPs. In this study, the highest content of total organic carbon (TOC) in the soil was at approximately 2500 m (Fig. S8), which is coincident with the pattern of soil POPs (Figs. S5eS7). Therefore, the high content of soil organic matter may be the reason for the higher concentrations of POPs in the soil at approximately 2500 m in elevation. In addition, many studies (Burbank et al., 2003; Kansakar et al., 2004; Putkonen, 2004) have reported that the highest precipitation on the southern slopes of the Himalayas occurs at an altitude of 2500e3000 m. Westgate and Wania (2013) modeled the accumulation of POPs in high mountains and suggested that the rain on the hillside of the Himalayas washes out the air-phase and particulate POPs, which then partly contribute to the higher levels of POPs at approximately 2500 m. For the regions above 4000 m, precipitation decreases (Burbank et al., 2003; Kansakar et al., 2004; Putkonen, 2004), and the landscape is dominated by grasses, leading to a low TOC content in the soil. As a result, the POP levels in this study showed a sharp decrease (Figs. S5eS7). After normalization by the TOC content of each of the soil samples, the altitude distribution of the POPs (Fig. S9) showed different patterns than in Figs. S5eS7. The altitude pattern of the POP (HCB, DDTs, and PCBs) concentrations can be broadly broken down into two stages (Fig. S9). A decrease was observed below 3000 m, followed by a sharp increase above 3000 m. The decreasing trend in the low-altitude regions can be explained by the proximity to emission sources. In the regions above 4000 m, both the precipitation and the TOC of the soil are at a minimum. Thus, the gas deposition of POPs from the air to the surface soil may significantly contribute to the higher concentration of POPs (ng/g OC) at highaltitude sites (Wania and Westgate, 2008).

fs ¼ Csoil rs =0:4114om Koa ;

(2)

fa ¼ Cair RT;

(3)

where Csoil is the soil concentration (ng/g dw), rs is the density of the soil solids (g/m3), 4om is the fraction of the organic matter in the soil, Cair is the gaseous POP concentration (ng/m3), Koa is the octanol-air partition coefficient of the compound, R is the gas constant equal to 8.314 Pa m3/mol/K, and T is the average air temperature (K). It was assumed that the organic fraction is 1.8 times the soil organic carbon fraction (TOC). The detailed information and the results of the calculation are shown in Text S4. The ff values from 0.15 to 0.85 represent equilibrium status; an ff value larger than 0.85 implies net volatilization from soil; and a value below 0.15 suggests net deposition (Daly et al., 2007b; Liu et al., 2010). In this study, the ff values were 0.261 ± 0.194, 0.008 ± 0.006, 0.010 ± 0.006, 0.055 ± 0.050, 0.183 ± 0.259, 0.074 ± 0.102, 0.210 ± 0.235, 0.053 ± 0.082, 0.053 ± 0.086, and 0.040 ± 0.070 for HCB, p,p0 -DDE, o,p0 -DDT, p,p0 -DDT, PCB-28, PCB-52, PCB-101, PCB153, PCB-138, and PCB-180, respectively. Only the average ff values of HCB, PCB-28, and PCB-101 were larger than 0.15, showing that the airesoil exchange of these compounds nearly reached equilibrium status. Other compounds showed the trend of net deposition. The fugacity fractions at each site were also shown in Fig. 4. It was shown that the higher ff values occurred in the low-altitude regions (Simara and Syapru Besi), which suggested that the pollutants at low altitude were more prone to reach equilibrium than those at high-altitude sites (Richhe, Kyanjin Gumba and Yala Peak, Fig. 4). The higher TOC at the forest site (<3000 m) suggested the larger accumulation capacity of POPs in forest soil and that the forest soil may play the role of a ‘sink’ for POPs. For the sites above the timber line (Kyanjin Gumba and Yala Peak), the fugacity fractions showed relatively low values, indicating a net deposition of POPs from the air to the soil. This means that the higher altitude regions of the south Himalayas are a continuous receptor of POPs. The continuous emission of POPs in Nepal and the sustained accumulation of POPs by the forest/ grassland/soil, followed by emissionedeposition events, may cause a potential threat to the Himalayan forest environment. 4. Conclusions Due to the frequency of human activity, local emission of POPs occurs in the lowlands of Nepal. During the transport from the

3.6. Airesoil exchange To understand gas deposition, it is helpful to compare the fugacities of the chemicals in the soil and the ambient air, and to infer the direction of the airesoil exchange. The fugacity of soil is due mainly to the organic fraction (Mackay, 2001), and the fugacity of air is mainly related to the air concentrations (Mackay, 2001). In this study, the air concentrations used for calculating the air fugacity were the concentrations from PAS, and the soil fugacity is calculated according to the organic content. The airesoil exchange was assessed by the fugacity fraction (ff):

ff ¼ fs =ðfs þ fa Þ;

(1)

where fs and fa are the fugacities of the POPs in the soil and in the air, respectively. The equations of the fugacity were derived from Mackay (2001):

49

Fig. 4. Airesoil fugacity fractions in Nepal.

50

P. Gong et al. / Environmental Pollution 192 (2014) 44e51

lowlands to high altitudes, the lighter POPs (e.g. HCB) tended to be transported to the higher mountains, and heavier POPs (e.g. DDTs) were found to have a greater tendency to deposit. The presence of forests and precipitation enhanced the deposition of atmospheric POPs, and the deposited POPs accumulated in the leaves and finally in the soil. The soil, especially at a higher altitude, may act as the final ‘sink’ for heavier PCBs and DDTs. Considering the strong local emissions and the high levels of POPs in South Asia, the Himalayas may delay the global diffusion of POPs. The transport processes, deposition fluxes, residence time, and storage of POPs should be monitored further. Transport modeling may be a better method to evaluate the transport dynamics under atmospheric circulation, and calculate the transported quantities along the southern slope of the Himalayas. Acknowledgments We would like to thank the staff of the Third Pole Environment (TPE) program and the International Centre for Integrated Mountain Development (ICIMOD) for helping with sample collection. This study was supported by the National Natural Science Foundation of China (40871233, 41222010, and 41371083) and the China Postdoctoral Science Foundation (2013T60180). Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.envpol.2014.05.015. References Abhilash, P.C., Singh, N., 2009. Pesticide use and application: an Indian scenario. J. Hazard. Mater. 165, 1e12. Aichner, B., Glaser, B., Zech, W., 2007. Polycyclic aromatic hydrocarbons and polychlorinated biphenyls in urban soils from Kathmandu, Nepal. Org. Geochem. 38, 700e715. Baek, S.-Y., Jurng, J., Chang, Y.-S., 2013. Spatial distribution of polychlorinated biphenyls, organochlorine pesticides, and dechlorane plus in Northeast Asia. Atmos. Environ. 64, 40e46. Bailey, R.E., 2001. Global hexachlorobenzene emissions. Chemosphere 43, 167e182. Barber, J.L., Sweetman, A.J., van Wijk, D., Jones, K.C., 2005. Hexachlorobenzene in the global environment: emissions, levels, distribution, trends and processes. Sci. Total Environ. 349, 1e44. Burbank, D.W., Blythe, A.E., Putkonen, J., Pratt-Sitaula, B., Gabet, E., Oskin, M., Barros, A., Ojha, T.P., 2003. Decoupling of erosion and precipitation in the Himalayas. Nature 426, 652e655. Chakraborty, P., Zhang, G., Li, J., Xu, Y., Liu, X., Tanabe, S., Jones, K.C., 2010. Selected organochlorine pesticides in the atmosphere of major Indian cities: levels, regional versus local variations, and sources. Environ. Sci. Technol. 44, 8038e8043. Chakraborty, P., Zhang, G., Eckhardt, S., Li, J., Breivik, K., Lam, P.K.S., Tanabe, S., Jones, K.C., 2013. Atmospheric polychlorinated biphenyls in Indian cities: levels, emission sources and toxicity equivalents. Environ. Pollut. 182, 283e290. Chen, D., Liu, W., Liu, X., Westgate, J.N., Wania, F., 2008. Cold-trapping of persistent organic pollutants in the mountain moils of Western Sichuan, China. Environ. Sci. Technol. 42, 9086e9091. Daly, G.L., Wania, F., 2005. Organic contaminants in mountains. Environ. Sci. Technol. 39, 385e398. Daly, G.L., Lei, Y.D., Teixeira, C., Muir, D.C.G., Wania, F., 2007a. Pesticides in western Canadian mountain air and soil. Environ. Sci. Technol. 41, 6020e6025. Daly, G.L., Lei, Y.D., Teixeira, C., Muir, D.C.G., Castillo, L.E., Jantunen, L.M.M., Wania, F., 2007b. Organochlorine pesticides in the soil and atmosphere of Costa Rica. Environ. Sci. Technol. 41, 1124e1130. Devi, N.L., Qi, S.H., Chakraborty, P., Zhang, G., Yadav, I.C., 2011. Passive air sampling of organochlorine pesticides in a northeastern state of India, Manipur. J. Environ. Sci. China 23, 808e815. Devi, N.L., Chakraborty, P., Qi, S.H., Zhang, G., 2013. Selected organochlorine pesticides (OCPs) in surface soils from three major states from the northeastern part of India. Environ. Monit. Assess. 185, 6667e6676. Estellano, V.H., Pozo, K., Harner, T., Franken, M., Zaballa, M., 2008. Altitudinal and seasonal variations of persistent organic pollutants in the Bolivian Andes Mountains. Environ. Sci. Technol. 42, 2528e2534. Gioia, R., Li, J., Schuster, J., Zhang, Y.L., Zhang, G., Li, X.D., Spiro, B., Bhatia, R.S., Dachs, J., Jones, K.C., 2012. Factors affecting the occurrence and transport of

atmospheric organochlorines in the China Sea and the Northern Indian and South East Atlantic Oceans. Environ. Sci. Technol. 46, 10012e10021. Gong, P., Wang, X., Sheng, J., Xu, B., 2013. Sources and atmospheric transport of POPs in the Tibetan Plateau using relative composition probe. Res. Environ. Sci. 26, 350e356 (in Chinese). Guazzoni, N., Comolli, R., Binelli, A., Tremolada, P., 2013. Environmental variables affecting the distribution of POPs on Mt.Meru, Tanzania. Environ. Sci. Process. Impacts 15, 1573e1581. Gupta, P.K., 2004. Pesticide exposureeIndian scene. Toxicology 198, 83e90. Guzzella, L., Poma, G., De Paolis, A., Roscioli, C., Viviano, G., 2011. Organic persistent toxic substances in soils, waters and sediments along an altitudinal gradient at Mt. Sagarmatha, Himalayas, Nepal. Environ. Pollut. 159, 2552e2564. Horstmann, M., McLachlan, M.S., 1996. Evidence of a novel mechanism of semivolatile organic compound deposition in coniferous forests. Environ. Sci. Technol. 30, 1794e1796. €m-Lunde n, E., Olafsdottir, K., Hung, H., Kallenborn, R., Breivik, K., Su, Y., Brorstro Thorlacius, J.M., Lepp€ anen, S., Bossi, R., Skov, H., Manø, S., Patton, G.W., Stern, G., Sverko, E., Fellin, P., 2010. Atmospheric monitoring of organic pollutants in the Arctic under the Arctic Monitoring and Assessment Programme (AMAP): 19932006. Sci. Total Environ. 408, 2854e2873. Kansakar, S.R., Hannah, D.M., Gerrard, J., Rees, G., 2004. Spatial pattern in the precipitation regime of Nepal. Int. J. Climatol. 24, 1645e1659. Kumar, B., Kumar, S., Gaur, R., Goel, G., Mishra, M., Singh, S.K., Prakash, D., Sharma, C.S., 2011. Persistent organochlorine pesticides and polychlorinated biphenyls in intensive agricultural soils from North India. Soil Water Res. UZEI 6, 190e197. Kumar, B., Verma, V.K., Kumar, S., Sharma, C.S., 2013. Probabilistic health risk assessment of polycyclic aromatic hydrocarbons and polychlorinated biphenyls in urban soils from a tropical city of India. J. Environ. Sci. Health Part A 48, 1253e1263. Kumari, B., Madan, V.K., Kathpal, T.S., 2008. Status of insecticide contamination of soil and water in Haryana, India. Environ. Monit. Assess. 136, 239e244. Lallas, P.L., 2001. The Stockholm Convention on persistent organic pollutants. Am. J. Int. Law 95, 692e708. Lavin, K.S., Hageman, K.J., 2013. Contributions of long-range and regional atmospheric transport on pesticide concentrations along a transect crossing a mountain divide. Environ. Sci. Technol. 47, 1390e1398. Liu, W., Chen, D., Liu, X., Zheng, X., Yang, W., Westgate, J.N., Wania, F., 2010. Transport of semivolatile organic compounds to the Tibetan Plateau: spatial and temporal variation in air concentrations in mountainous Western Sichuan, China. Environ. Sci. Technol. 44, 1559e1565. Liu, X.D., Wania, F., 2014. Cluster analysis of passive air sampling data based on the relative composition of persistent organic pollutatns. Environ. Sci. Process. Impacts 16, 453e463. Liu, X., Zhang, G., Li, J., Yu, L.L., Xu, Y., Li, X.D., Kobara, Y., Jones, K.C., 2009. Seasonal patterns and current sources of DDTs, chlordanes, hexachlorobenzene, and endosulfan in the atmosphere of 37 Chinese cities. Environ. Sci. Technol. 43, 1316e1321. Lohmann, R., Northcott, G.L., Jones, K.C., 2000. Assessing the contribution of diffuse domestic burning as a source of PCDD/Fs, PCBs, and PAHs to the U.K. atmosphere. Environ. Sci. Technol. 34, 2892e2899. Mackay, D., 2001. Multimedia Environmental Models: the Fugacity Approach. Lewis/CRC, Boca Raton, FL. McLachlan, M.S., Horstmann, M., 1998. Forests as filters of airborne organic pollutants: a model. Environ. Sci. Technol. 32, 413e420. Mishra, K., Sharma, R.C., Kumar, S., 2012. Contamination levels and spatial distribution of organochlorine pesticides in soils from India. Ecotoxicol. Environ. Saf. 76, 215e225. Mittal, P.K., Wijeyaratne, P., Pandey, S., 2004. Status of Insecticide Resistance of Malaria, Kala-azar and Japanese Encephalitis Vectors in Bangladesh, Bhutan, India and Nepal (BBIN). Environmental Health Project Activity Report 129, pp. 44e48. Moeckel, C., Nizzetto, L., Guardo, A.D., Steinnes, E., Freppaz, M., Filippa, G., Camporini, P., Benner, J., Jones, K.C., 2008. Persistent organic pollutants in boreal and montane soil profiles: distribution, evidence of processes and implications for global cycling. Environ. Sci. Technol. 42, 8374e8380. Moeckel, C., Nizzetto, L., Strandberg, B., Lindroth, A., Jones, K.C., 2009. Air-boreal forest transfer and processing of polychlorinated biphenyls. Environ. Sci. Technol. 43, 5282e5289. Nasir, J., Wang, X.P., Xu, B.Q., Wang, C.F., Joswiak, D.R., Rehman, S., Lodhi, A., Shafiq, S., Jilani, R., 2014. Selected organochlorine pesticides and polychlorinated biphenyls in urban atmosphere of Pakistan: concentration, spatial variation and sources. Environ. Sci. Technol. 48, 2610e2618. Negi, S.S., 1994. Forests and Forestry in Nepal. Ashish Publishing House, New Delhi. Nizzetto, L., Cassani, C., Di Guardo, A., 2006. Deposition of PCBs in mountains: the forest filter effect of different forest ecosystem types. Ecotoxicol. Environ. Saf. 63, 75e83. Parolini, M., Guazzoni, N., Comolli, R., Binelli, A., Tremolada, P., 2013. Background levels of polybrominated diphenyl ethers (PBDEs) in soils from Mount Meru area, Arusha district (Tanzania). Sci. Total Environ. 452e453, 253e261. Putkonen, J., 2004. Continuous snow and rain data at 500 to 4400 m altitude near Annapurna, Nepal, 1999-2001. Arct. Antarct. Alp. Res. 36, 244e248. Ramanathan, V., Chung, C., Kim, D., Bettge, T., Buja, L., Kiehl, J.T., Washington, W.M., Fu, Q., Sikka, D.R., Wild, M., 2005. Atmospheric brown clouds: impacts on South Asian climate and hydrological cycle. Proc. Natl. Acad. Sci. U. S. A. 102, 5326e5333.

P. Gong et al. / Environmental Pollution 192 (2014) 44e51 Safe, S.H., 1994. Polychlorinated biphenyls (PCBs): environmental impact, biochemical and toxic responses, and implications for risk assessment. Crit. Rev. Toxicol. 24, 87e149. Sapkota, A., Yang, H., Wang, J., 2012. Securing rural livelihoods and climate change through sustainable use of biogas and improved cooking stoves in rural households in Nepal. Environ. Sci. Technol. 47, 330e331. Sapkota, A., Yang, H., Wang, J., Lu, Z., 2013. Role of renewable energy technologies for rural electrification in achieving the Millennium Development Goals (MDGs) in Nepal. Environ. Sci. Technol. 47, 1184e1185. Schrlau, J.E., Geiser, L., Hageman, K.J., Landers, D.H., Simonich, S.M., 2011. Comparison of lichen, conifer needles, passive air sampling devices, and snowpack as passive sampling media to measure semi-volatile organic compounds in remote atmospheres. Environ. Sci. Technol. 45, 10354e10361. Shen, H., Henkelmann, B., Levy, W., Zsolnay, A., Weiss, P., Jakobi, G., Kirchner, M., Moche, W., Braun, K., Schramm, K.W., 2009. Altitudinal and chiral signature of persistent organochlorine pesticides in air, soil, and spruce needles (Picea abies) of the Alps. Environ. Sci. Technol. 43, 2450e2455. Sheng, J., Wang, X., Gong, P., Joswiak, D.R., Tian, L., Yao, T., Jones, K.C., 2013. Monsoon-driven transport of organochlorine pesticides and polychlorinated biphenyls to the Tibetan Plateau: three year atmospheric monitoring study. Environ. Sci. Technol. 47, 3199e3208. Shunthirasingham, C., Oyiliagu, C.E., Cao, X.S., Gouin, T., Wania, F., Lee, S.C., Pozo, K., Harner, T., Muir, D.C.G., 2010. Spatial and temporal pattern of pesticides in the global atmosphere. J. Environ. Monit. 12, 1650e1657. Singh, S.P., Adhikari, B.S., Zobel, D.B., 1994. Biomass, productivity, leaf longevity, and forest structure in the central Himalaya. Ecol. Monogr. 64, 401e421. Su, Y., Hung, H., Blanchard, P., Patton, G.W., Kallenborn, R., Konoplev, A., Fellin, P., Li, H., Geen, C., Stern, G., Rosenberg, B., Barrie, L.A., 2006a. Spatial and seasonal variations of hexachlorocyclohexanes (HCHs) and hexachlorobenzene (HCB) in the Arctic atmosphere. Environ. Sci. Technol. 40, 6601e6607. Su, Y., Wania, F., Harner, T., Lei, Y.D., 2006b. Deposition of polybrominated diphenyl ethers, polychlorinated biphenyls, and polycyclic aromatic hydrocarbons to a boreal deciduous forest. Environ. Sci. Technol. 41, 534e540. Syed, J.H., Malik, R.N., Liu, D., Xu, Y., Wang, Y., Li, J., Zhang, G., Jones, K.C., 2013. Organochlorine pesticides in air and soil and estimated airesoil exchange in Punjab, Pakistan. Sci. Total Environ. 444, 491e497. Tremolada, P., Villa, S., Bazzarin, P., Bizzotto, E., Comolli, R., Vighi, M., 2008. POPs in mountain soils from the Alps and Andes: suggestions for a ‘precipitation effect’ on altitudinal gradients. Water Air Soil Pollut. 188, 93e109. van Drooge, B.L., Grimalt, J.O., Camarero, L., Catalan, J., Stuchlík, E., Torres García, C.J., 2004. Atmospheric semivolatile organochlorine compounds in European highmountain areas (Central Pyrenees and High Tatras). Environ. Sci. Technol. 38, 3525e3532.

51

Vighi, M., 2006. The role of high mountains in the global transport of persistent organic pollutants. Ecotoxicol. Environ. Saf. 63, 108e112. Wang, P., Zhang, Q., Wang, Y., Wang, T., Li, X., Li, Y., Ding, L., Jiang, G., 2009. Altitude dependence of polychlorinated biphenyls (PCBs) and polybrominated diphenyl ethers (PBDEs) in surface soil from Tibetan Plateau, China. Chemosphere 76, 1498e1504. Wang, X.P., Yao, T.D., Cong, Z.Y., Yan, X.L., Kang, S.C., Zhang, Y., 2006. Gradient distribution of persistent organic contaminants along northern slope of centralHimalayas, China. Sci. Total Environ. 372, 193e202. Wang, X.P., Gong, P., Yao, T.D., Jones, K.C., 2010. Passive air sampling of organochlorine pesticides, polychlorinated biphenyls, and polybrominated diphenyl ethers across the Tibetan Plateau. Environ. Sci. Technol. 44, 2988e2993. Wang, X.P., Sheng, J.J., Gong, P., Xue, Y.G., Yao, T.D., Jones, K.C., 2012. Persistent organic pollutants in the Tibetan surface soil: spatial distribution, air-soil exchange and implications for global cycling. Environ. Pollut. 170, 145e151. Wania, F., Mackay, D., 1993. Global fractionation and cold condensation of low volatility organochlorine compounds in Polar regions. Ambio 22, 10e18. Wania, F., Shen, L., Lei, Y.D., Teixeira, C., Muir, D.C.G., 2003. Development and calibration of a resin-based passive sampling system for monitoring persistent organic pollutants in the atmosphere. Environ. Sci. Technol. 37, 1352e1359. Wania, F., Westgate, J.N., 2008. On the mechanism of mountain cold-trapping of organic chemicals. Environ. Sci. Technol. 42, 9092e9098. Wenzel, K.D., Manz, M., Hubert, A., Schuurmann, G., 2002. Fate of POPs (DDX, HCHs, PCBs) in upper soil layers of pine forests. Sci. Total Environ. 286, 143e154. Westgate, J.N., Wania, F., 2013. Model-based exploration of the drivers of mountain cold-trapping in soil. Environ. Sci. Process. Impacts 15, 2220e2232. Yang, R., Yao, T., Xu, B., Jiang, G., Zheng, X., 2008. Distribution of organochlorine pesticides (OCPs) in conifer needles in the southeast Tibetan Plateau. Environ. Pollut. 153, 92e100. Yao, T.D., Thompson, L., Yang, W., Yu, W.S., Gao, Y., Guo, X.J., Yang, X.X., Duan, K.Q., Zhao, H.B., Xu, B.Q., Pu, J.C., Lu, A.X., Xiang, Y., Kattel, D.B., Joswiak, D., 2012. Different glacier status with atmospheric circulations in Tibetan Plateau and surroundings. Nat. Clim. Change 2, 663e667. Yao, T.D., Masson-Delmotte, V., Gao, J., Yu, W.S., Yang, X.X., Risi, C., Sturm, C., Werner, M., Zhao, H.B., He, Y., Ren, W., Tian, L.D., Shi, C.M., Hou, S.G., 2013. A review of climatic controls on d18O in precipitation over the Tibetan Plateau: observations and simulations. Rev. Geophys. 51 http://dx.doi.org/10.1002/ rog.20023. Zhang, G., Chakraborty, P., Li, J., Sampathkumar, P., Balasubramanian, T., Kathiresan, K., Takahashi, S., Subramanian, A., Tanabe, S., Jones, K.C., 2008. Passive atmospheric sampling of organochlorine pesticides, polychlorinated biphenyls, and polybrominated diphenyl ethers in urban, rural, and wetland sites along the coastal length of India. Environ. Sci. Technol. 42, 8218e8223.