New insights into trace elements deposition in the snow packs at remote alpine glaciers in the northern Tibetan Plateau, China

Science of the Total Environment 529 (2015) 101–113

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

New insights into trace elements deposition in the snow packs at remote alpine glaciers in the northern Tibetan Plateau, China Zhiwen Dong a,⁎, Shichang Kang a,b,⁎, Xiang Qin a,c, Xiaofei Li a, Dahe Qin a, Jiawen Ren a a

State Key Laboratory of Cryospheric Sciences, Cold and Arid Regions Environmental and Engineering Research Institute, Chinese Academy of Sciences, Lanzhou 730000, China CAS Center for Excellence in Tibetan Plateau Earth Sciences, Beijing 100101, China Qilian Mountain Glacier and Ecological Environment Research Station, Cold and Arid Regions Environmental and Engineering Research Institute, Chinese Academy of Sciences, Lanzhou 730000, China

b c

H I G H L I G H T S • • • •

We present new data of trace elements deposition in snowpacks at LHG and TGL Increased elemental concentrations and EFs are observed in the TGL glacier basin Heavy metals (e.g. Cu, Zn, Mo, Sb, and Pb) were from anthropogenic sources Higher elemental concentration and EFs were observed in the surface snow

a r t i c l e

i n f o

Article history: Received 26 February 2015 Received in revised form 15 April 2015 Accepted 16 May 2015 Available online 23 May 2015 Editor: Xuexi Tie Keywords: Trace element Concentration Source Glacier snowpack Northern Tibetan Plateau

a b s t r a c t Trace element pollution resulting from anthropogenic emissions is evident throughout most of the atmosphere and has the potential to create environmental and health risks. In this study we investigated trace element deposition in the snowpacks at two different locations in the northern Tibetan Plateau, including the Laohugou (LHG) and the Tanggula (TGL) glacier basins, and its related atmospheric pollution information in these glacier areas, mainly focusing on 18 trace elements (Li, Be, V, Cr, Co, Ni, Cu, Zn, Ga, Rb, Nb, Mo, Cd, Sb, Cs, Ba, Tl, and Pb). The results clearly demonstrate that pronounced increases of both concentrations and crustal enrichment factors (EFs) are observed in the snowpack at the TGL glacier basin compared to that of the LHG glacier basin, with the highest EFs for Sb and Zn in the TGL basin, whereas with the highest EFs for Sb and Cd in the LHG basin. Compared with other studies in the Tibetan Plateau and surrounding regions, trace element concentration showed gradually decreasing trend from Himalayan regions (southern Tibetan Plateau) to the TGL basin (central Tibetan Plateau), and to the LHG basin (northern Tibetan Plateau), which probably implied the significant influence of atmospheric trace element transport from south Asia to the central Tibetan Plateau. Moreover, EF calculations at two sites showed that most of the heavy metals (e.g., Cu, Zn, Mo, Cd, Sb, and Pb) were from anthropogenic sources and some other elements (e.g., Li, Rb, and Ba) were mainly originated from crustal sources. MODIS atmospheric optical depth (AOD) fields derived using the Deep Blue algorithm and CALIOP/CALIPSO transect showed significant influence of atmospheric pollutant transport from south Asia to the Tibetan Plateau, which probably caused the increased concentrations and EFs of trace element deposition in the snowpack on the TGL glacier basin. © 2015 Elsevier B.V. All rights reserved.

1. Introduction Trace element pollution resulting from anthropogenic emissions is evident throughout most of the atmosphere and has the potential to create environmental and health risks (Pacyna and Pacyna, 2001; ⁎ Corresponding authors at: State Key Laboratory of Cryospheric Sciences, Cold and Arid Regions Environmental and Engineering Research Institute, Chinese Academy of Sciences, Lanzhou 730000, China. E-mail addresses: [email protected] (Z. Dong), [email protected] (S. Kang).

http://dx.doi.org/10.1016/j.scitotenv.2015.05.065 0048-9697/© 2015 Elsevier B.V. All rights reserved.

Kang et al., 2007). Ice cores from the remote polar ice sheets and highaltitude glaciers receive trace elements exclusively from the atmosphere and can therefore be used to precisely assess the possible large-scale impact of anthropogenic activities through time. Research carried out on trace elements in snow and ice from Greenland, Antarctica, and high-altitude Andes, Himalayas, and European Alps has provided distinct evidence that atmospheric pollution of various trace elements exists on regional, hemispheric, and global scales, as a result of long-range transport and dispersion (Liu et al., 2011; Boutron et al., 1995; Barbante et al., 2004; Hong et al., 2004, 2009; Kaspari

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Fig. 1. Map showing two remote alpine sites of LHG and TGL in the northern Tibetan Plateau.

et al., 2009). Many of these studies have also documented increasing concentrations for many trace elements in the 20th century but decreasing concentrations for certain trace elements (e.g., Pb, Cd, and Zn) during recent decades, due to the control of emissions in Europe and North America (Boutron et al., 1995; de Velde and Boutron, 2000). As the differences in the sources, transport pathways, and residence time of trace elements in the atmosphere, the composition and concentration of trace elements may vary greatly from region to region. Moreover, trace elements can also be long-range transported through the atmosphere and deposited in remote alpine regions (Kyllonen et al., 2009). Atmospheric pollution information can be stored in snow and ice in high altitude regions. However, the data of trace element deposition in snow at the various remote sites of the Tibetan Plateau is very limited as the environmental

condition is very difficult for field work sampling due to high altitude and remoteness. Recently, some work has reported the trace element deposition at the remote sites in the southern Tibetan Plateau, and some work indicated great environmental pollution from south Asia (Ramanathan et al., 2007; Tripathee et al., 2014). However, the transport process and potential influence of trace element pollution to the northern Tibetan Plateau remains unclear. Furthermore, little research has been carried out on atmospheric trace element deposition to the glaciers in the northern Tibetan Plateau regions. Therefore, different locations on the northern Tibetan Plateau including the Laohugou (LHG) and Tanggula (TGL) remote sites with high altitudes were selected for evaluating atmospheric trace element deposition on the glaciers. Thus this study can provide a first valuable data set on trace elements and new evidence on atmospheric

Fig. 2. Photos showing the snowpit and surroundings in the sampling glaciers, (a) snowpit in the LHG glacier basin, and (b) sampling in the TGL snowpack.

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Table 1 Trace element concentration (μg/L) in the snowpack of the TGL and LHG glacier basins. Elements

Li Be V Cr Co Ni Cu Zn Ga Rb Nb Mo Cd Sb Cs Ba Tl Pb Total

LHG snowpits

LHG surface snow

TGL snowpit

TGL surface snow

AM

SD

Max

Min

AM

SD

Max

Min

AM

SD

Max

Min

AM

SD

Max

Min

0.218 0.012 0.048 0.187 0.017 1.207 0.619 2.495 0.011 0.175 0.004 0.044 0.020 0.072 0.005 5.346 0.009 0.073 10.565

0.077 0.003 0.018 0.096 0.007 0.525 1.589 4.533 0.006 0.158 0.001 0.045 0.012 0.027 0.002 4.121 0.005 0.031

0.36 0.019 0.076 0.391 0.031 3.19 6.94 19.7 0.025 0.671 0.006 0.209 0.042 0.119 0.009 14.3 0.019 0.132

0.135 0.01 0.017 0.072 0.007 0.715 0.078 0.13 0.003 0.038 0.003 0.009 0.006 0.026 0.003 1.06 0.004 0.039

0.417 0.020 0.073 0.654 0.054 1.232 0.479 0.712 0.011 0.474 0.004 0.049 0.024 0.075 0.006 13.510 0.008 0.099 17.904

0.470 0.011 0.044 0.475 0.047 0.826 0.525 0.672 0.006 0.412 0.002 0.037 0.017 0.045 0.004 12.875 0.006 0.146

2.41 0.042 0.18 1.68 0.182 3.49 2.31 2.76 0.022 1.39 0.007 0.133 0.059 0.19 0.019 53.7 0.023 0.779

0.062 0.01 0.021 0.133 0.01 0.291 0.089 0.101 0.003 0.046 0.003 0.005 0.006 0.013 0.003 7.665 0.003 0.03

0.326 0.016 0.016 0.311 0.036 0.590 0.527 18.479 0.009 0.281 0.003 0.088 0.044 1.423 0.007 4.810 0.011 0.169 23.146

0.515 0.008 0.011 0.435 0.052 0.294 0.638 5.647 0.007 0.304 0.001 0.223 0.077 4.748 0.004 10.612 0.012 0.377

2.32 0.032 0.054 2.16 0.256 1.59 3.01 67 0.027 1.16 0.005 1.04 0.345 22.6 0.015 51.1 0.029 1.88

0.037 0.011 0.007 0.103 0.007 0.173 0.12 0.827 0.007 0.01 0.002 0.006 0.007 0.014 0.003 0.649 0.002 0.018

0.538 0.044 0.026 0.210 0.018 0.672 0.412 16.531 0.005 0.333 0.002 0.042 0.022 0.115 0.022 4.466 0.08 0.078 23.544

0.609 0.026 0.025 0.087 0.009 0.203 0.341 41.488 0.002 0.450 0.002 0.032 0.012 0.112 0.021 5.931 0.003 0.052

1.58 0.071 0.096 0.358 0.033 1.08 1.25 145 0.007 1.51 0.002 0.09 0.049 0.421 0.055 16.9 0.011 0.217

0.006 0.011 0.004 0.107 0.004 0.377 0.047 0.164 0.003 0.01 0.002 0.006 0.008 0.013 0.002 0.393 0.004 0.016

environment pollution information in two different locations of the northern Tibetan Plateau. The main objectives of this study are to measure the concentrations of trace element deposition in the snowpack at two remote alpine glacier regions and compare the data with other regions, and investigate their possible sources of trace element at these two remote sites, and finally evaluate the influence of atmospheric pollution of trace elements in south Asia to the Tibetan Plateau region. 2. Sampling and lab analysis We collected snowpit and surface snow samples at different altitudes in two remote alpine glacier areas in the northern Tibetan Plateau,

including the LHG and the TGL Glacier basins (Fig. 1). The Laohugou Glacier basin (LHG, 39°20′N, 96°34′E, with the altitude of 5010 m a.s.l.) is located in the northeastern Tibetan Plateau, at the northern slope of western Qilian Mountains with typical continental climatic conditions (Dong et al., 2014a, 2014b). The Laohugou Glacier No.12 is the most typical glacier at the basin, with a length of 10 km and an area of 20 km2, and was divided into two branches at the altitude of 4560 m a.s.l. However, The Tanggula (TGL) Dongkemadi glacier is located in the central region (hinterland) of the Tibetan Plateau (33°04′N, 92°04′E, 5729 m a.s.l.), with an area of 1.705 km2, and glacial elevation varied between 5420–5919 m a.s.l., and the annual precipitation of TGL Dongkemadi glacier is 680 mm, and annual air temperature in the ELA

Fig. 3. Comparison of trace element concentration in the snowpacks at two remote alpine sites, (a) LHG snowpit, (b) TGL snowpit, (c) LHG surface snow, and (d) TGL surface snow.

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is −8.6 °C. Compared to the LHG glacier basin, the TGL glacier has a relatively clean local surrounding environment because it is far away from human industrial activity, whereas there are many cities surrounding the LHG glacier region, such as Yumen city and Jiuquan city. Both of these two glaciers have nearly flat glacier surface, leading to uniform snow deposition in the accumulation zone (Fig. 2). During June to August in 2014, we collected snowpits and surface snow samples at Laohugou Glacier No.12 and TGL Dongkemadi glacier (Fig. 1). A total of 138 snow samples were acquired, including 36 snow samples in TGL Dongkemadi glacier with one snowpit (120 cm, at the elevation of 5729 m a.s.l.) and 12 surface snow samples, and 102 samples in LHG glacier No.12 with two snowpits (95 and 90 cm, respectively, at the elevation of 5010 m a.s.l.) and 58 surface snow samples. Snowpit samples were collected in triplicate at a vertical resolution of 5 cm on the wall of the snowpit using a pre-cleaned stainlesssteel shovel and polyethylene gloves, and surface snow samples were collected at different elevations along the glaciers. Pre-cleaned Lowdensity polyethylene (LDPE) bottle (Thermo scientific) was used for the sample collection. All samples were acidified with ultra-pure nitric acid (0.5% V/V) to dissolve the trace elements associated with atmospheric particles and to prevent their adsorption onto the walls of the bottles and were kept frozen until they were transported to the lab for analysis. All snow samples were melted at room temperature and the concentrations of trace elements were determined directly by inductively coupled plasma-mass spectrometry (ICP-MS, Thermo Scientific Element/XR) at the Analytical Laboratory of Beijing Research Institute of uranium Geology. Finally, 18 trace elements (Li, Be, V, Cr, Co, Ni, Cu, Zn, Ga, Rb, Nb, Mo, Cd, Sb, Cs, Ba, Tl, and Pb) were measured totally and analyzed for their environmental significance.

Quality assurance and quality control (QA/QC) of element analysis was carried out to avoid contamination of samples in both field and laboratory, and to ensure the reliability of our results. Non-powder vinyl clean room gloves and masks were worn to avoid possible contamination during the sampling and laboratory analysis. All samples were kept frozen until the analysis in the laboratory. Field blank analyses showed that there was negligible contamination (b5%), during the sampling, storage and transportation. Elemental concentrations were quantified using external calibration standards. An analytical standard was analyzed after the initial calibration after every 10 samples. The method detection limits (MDLs), which is defined as three times the standard deviation of replicated blank measurements were: Li 0.01 μg/L; Be 0.002 μg/L; V 0.002 μg/L; Cr 0.01 μg/L; Co 0.0018 μg/L; Ni 0.032 μg/L; Cu 0.002 μg/L; Zn 0.002 μg/L; Ga 0.003 μg/L; Rb 1.632 ng/L; Nb 0.022 μg/L; Mo 0.002 μg/L; Cd 6.254 ng/L; Sb 0.002 μg/L; Cs 0.002 μg/L; Ba 0.002 μg/L; Tl 0.019 μg/L; and Pb 0.002 μg/L. The accuracy of the analytical protocol was ascertained based on repeated measurement of an externally certified reference solution (AccuTrace™ Reference Standard). The accuracy of the analytical protocol with the recoveries ranged from 85% for Cr to 105% for Ni. For analytical precision, the corresponding RSD values of all element concentrations measured in the reference material were found to be less than 5%. Enrichment factor (EF) calculations were performed to identify the potential sources of trace elements in the regions. The EF value of elements in snow and ice relative to the crustal material (e.g., Al, Sc) is often used to evaluate the degree of anthropogenic influence (Kyllonen et al., 2009; Cong et al., 2010; Huang et al., 2013). In order to identify the relative effects of natural crustal and anthropogenic sources on concentrations of each element, EFs were calculated by

Fig. 4. Vertical profiles of chemical constituents and dust mass in a typical snowpit at the LHG glacier No.12 derived in August 2014.

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structure as it depends on aerosol concentration and its optical properties (Bou Karam et al., 2010). Examination of the NCEP/NCAR reanalysis (Kalnay et al., 1996) and local meteorological data together with the Moderate Resolution Imaging Spectroradiometer (MODIS) atmospheric optical depth (AOD) (Hsu et al., 2006) data enabled dating aerosol events to a precision of specific days and establishing broad source regions of trace element deposition at remote alpine glaciers. 3. Results and discussion 3.1. Trace elements concentration and tempo-spatial distribution in the snowpack We calculated the concentration of trace elements deposition in the snowpack at two remote sites in the northern Tibetan Plateau (Table 1), including that of snowpits and surface snow distribution at different elevations. The snowpits' result reflect a relatively long term deposition of trace elements, while the surface snow collected in summer could reveal the information of element deposition in summer and their spatial variation along the glacier surface. Based on the dust layers and seasonal variation of chemical constituents, and the observed net accumulation of about 500 mm/a (Sun et al., 2012; Cui et al., 2011), the snowpits on the LHG Glacier No.12 (95 cm in depth) and the TGL Dongkemadi glacier (120 cm in depth) reflect a nearly two-year deposition of snow in 2013–2014, which thus could reflect almost the same time period for atmospheric trace element deposition between two sites in the TGL and the LHG glacier basins. We find that, the elements' mean concentration in the snowpit of TGL (with total of 23.146 μg/L) is obviously higher

Fig. 5. Trace element concentration variations with the snow depth at the LHG basin (a), and the TGL basin (b).

comparing the elemental ratio found between elements in snow to a similar ratio for a reference crustal material. The EF is defined as the concentration ratio of a given element to that of a conservative crustal element, normalized to the same concentration ratio characteristics of the upper continental crust. Using Sc, as a crustal reference element, for example, the calculation of EF for Pb is as follows: EFðPbÞ ¼ ðPb=ScÞsnow =ðPb=ScÞCrust : The uncertainty in EF calculation is primarily attributed to the differences between elemental compositions of local soil and reference crustal compositions (Hong et al., 2009). Therefore, EFx values ranging from 1 to 10 are considered to be not enriched due to the differences between chemical compositions of local soil and reference crustal composition. EFx values between 10 and 100 are considered moderately enriched, indicating greater concentrations of a particular element in glacier snow than that would be expected from the crustal material. Moreover, EFx values greater than 100 show highly enriched conditions, indicating a severe contamination due to anthropogenic activities (Al-Momani, 2003). Moreover, data from the Cloud Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) mission (http://www-calipso.larc.nasa. gov) were used to characterize vertical distribution of atmospheric pollution transportation around the Tibetan Plateau. Attenuated backscatter (reflectivity) profiles at 532 nm are provided by the Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) installed on CALIPSO (Kutuzov et al., 2013; Winker et al., 2003) with vertical and horizontal resolutions of 60 m and 12 km, respectively. The CALIOP-derived reflectivity is used as a proxy to describe the aerosol (with pollutions) layer

Fig. 6. Spatial change of various trace elements in the surface snow with the elevation rise at the LHG basin (a), and the TGL basin (b).

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Table 2 Comparison with results of other similar studies in the Tibetan Plateau and surrounding regions. Element

Li Be V Cr Co Ni Cu Zn Ga Rb Nb Mo Cd Sb Cs Ba Tl Pb Total a b c d e

LHG snowpit

LHG surface snow

TGL snowpit

TGL surface snow

Kathmandu-a

Dhunche-a

Dimsa-a

Gosainkunda-a

Namco, Tibet-b

Singapore-c

Nakanoto, Japan-d

Chuncheon Korea-e

AM

AM

AM

AM

AM

AM

AM

AM

AM

VWM

VWM

VWM

0.218 0.012 0.048 0.187 0.017 1.207 0.619 2.495 0.011 0.175 0.004 0.044 0.020 0.072 0.005 5.346 0.009 0.073 10.56

0.417 0.020 0.073 0.654 0.054 1.232 0.479 0.712 0.011 0.474 0.004 0.049 0.024 0.075 0.006 13.51 0.008 0.099 17.91

0.326 0.016 0.016 0.311 0.036 0.590 0.527 18.479 0.009 0.281 0.003 0.088 0.044 1.423 0.007 4.810 0.011 0.169 23.146

0.538 0.044 0.026 0.210 0.018 0.672 0.412 16.531 0.005 0.333 0.002 0.042 0.022 0.115 0.022 4.466 0.008 0.078 23.544

1.11 0.69 0.49 1.35 16.91

0.20 0.38 1.02 0.87 9.78

1.06 1.18 1.03 0.92 8.40

0.95 0.79 0.47 0.45 13.15

0.093 0.39 0.08 0.32 0.76 7.91

1.62 0.57 3.86 5.58 7.23

0.65 0.81 12

0.52 1.73 9.90

0.071

0.061

0.018

0.01

0.005

0.33

0.14

0.07

0.981

0.908

0.589

0.35

0.16

3.37

4.6

1.51

0.36 0.18

Tripathee et al. (2014). Cong et al. (2010). Hu and Balasubramanian (2003). Sakata and Asakura (2009). Kim et al. (2012).

than that of LHG (17.904 μg/L), which may reflect the difference of atmospheric element transportation between two sites. Moreover, the element concentration of surface snow is larger than that of snowpit (average mean) at both two sites (Table 1), reflecting the significant deposition of trace element in summer. Previous research in the southern Tibetan Plateau has indicated that, the atmospheric trace element

concentration is often high in winter and springtime from anthropogenic pollutants and dust inputs (e.g., Huang et al., 2013). However, this study showed that large amount of trace element deposition during summer was probably also remarkable with atmospheric transport of anthropogenic pollutant to the alpine glacier basins in the northern Tibetan Plateau.

Fig. 7. Crustal enrichment factors for various trace elements. (a) LHG snowpit, (b) LHG surface snow, (c) TGL snowpit, and (d) TGL surface snow.

Z. Dong et al. / Science of the Total Environment 529 (2015) 101–113

Fig. 3 showed the comparison of trace element concentration between two remote sites, and Fig. 3a–b is the comparison of snowpit, while Fig. 3c–d is that of the surface snow. We find that, the total concentration in TGL is obviously higher in both snowpit and surface snow, and the concentration varied largely between various elements. The Zn and Ba both showed high concentrations at two alpine sites. Moreover, the Sb concentration is also very high in TGL snowpit, but Ni concentration is very high in LHG snowpits. The comparison between element concentrations of the surface snow at two sites showed similar result with that of the snowpits (Fig. 3b), and Ba concentration is very high in summer surface snow in the TGL basin. As one of the earth's main crustal elements, the high concentration of Ba may be caused by local crustal dust inputs. Moreover, the Zn showed significantly high concentration in TGL surface snow, whereas the Cd and Sb showed high concentration in LHG surface snow. Fig. 4 showed the vertical profile of chemical constituents and dust mass in a typical snowpit at the LHG glacier No.12, in which we find clear seasonal variation of chemical constituents (e.g., Ca2 +, pH, and EC) and dust deposition with four typical dust layers, and there exists relatively good correlation between dust and snow chemistry in the

107

profile. Fig. 5 showed the vertical distribution of various trace elements with the snow depth change in the snowpits of the LHG and the TGL glaciers. Because of the high elevations (above 5000 m a.s.l.) and low air temperature during sampling, the snowpit deposition information stored very well in the snowpack at two glaciers, and thus it could reflect the temporal change of various elements during 2013–2014. Result showed obvious temporal variation of trace element deposition in the snowpit (especially for Li, Co, Ni, Cu, Cd, and Pb). Moreover, the higher concentration of trace elements corresponded well to the dust layers in the snowpit profiles at two sites (by comparing the profiles of dust and trace elements, Fig. 4), which may imply the important effects of mineral dust particles to trace element transportation and deposition in the snowpack. Fig. 6 indicated the spatial distribution of various trace elements in the glacier surface snow with the altitude change at two glaciers. Most of the trace element concentrations showed a severe decreasing trend with the elevation rise in the LHG glacier (especially for Co, Cr, Zn, Ba, Pb), though V concentration showed a slightly increased trend with the elevation rise (Fig. 6a). Similarly, in the TGL Dongkemadi glacier, many trace elements showed a decrease trend with the elevation rise,

Fig. 8. Plots showing relationship between the crustal enrichment factors of elements versus Sc concentrations (log EF versus log Sc) with regression line and correlation coefficient for snow at two alpine sites.

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surface snow. Results of the snowpit in LHG shown in Fig. 7a indicate that the EFs for Zn, Cd, and Sb are with values greater than 10 times to crustal compositions, indicating that a major contributor to these elements, probably including Ni and Mo, is of anthropogenic pollutants origin (Fig. 7a). Moreover, results in the surface snow of the LHG basin showed that the EFs for Mo, Cd, and Sb are greater than 10 times, which are considered as anthropogenic emission source of regional atmospheric environment during summer period (Fig. 7b). Similarly, in the TGL glacier basin, the results of snowpit indicate that the EFs for Zn, Mo, Cd, Sb, and Pb are much greater than 100 times to crustal compositions, and the values for Zn and Sb even reached to 1000 times (Fig. 7c), clearly implying that these elements are originated from anthropogenic emissions. However, in surface snow of the TGL glacier, Ni, Cu, Zn, Mo, Cd, Sb and Pb are all enriched obviously, with EFs for Zn, Cd and Sb larger than 200 times (Fig. 7d), also indicating anthropogenic pollutants deposition during summer in the TGL glacier basin. The increased EFs for various trace elements at two glacier basins showed obviously regional anthropogenic pollutant deposition at remote alpine sites in the northern Tibetan Plateau. Moreover, the large EF value in surface snow revealed the pollutants' inputs on the glacier during the summer sampling period (Fig. 7). Since westerly air masses prevail in the northern Tibetan Plateau during the non-monsoon seasons in winter and springtime, the long range transport of anthropogenic pollutants from distant western areas to the LHG and TGL glacier basins is very possible. The westerly could bring large amount of anthropogenic pollutants from cities in central Asia. Some countries in central Asia, such as Russia, Uzbekistan, and Kazakhstan could be important source countries, with their large anthropogenic emission to atmosphere (e.g., Kakareka et al., 2004). Moreover, The Xinjiang Province of China, located in the upwind region to our study areas, also has many large industries with trace element emission. However, during the monsoon seasons in summer and autumn, the southern air masses from south Asia could probably bring plenty of anthropogenic pollutants to the Tibetan Plateau, as the south Asian region is one of the world's most populous and fast-developing regions, with highly diverse living habits and fast growing industrial and automobile sectors, diverse fuel use for domestic and industrial purposes, which make this region a large cauldron of emissions and atmospheric processes. Here we also discuss the possible anthropogenic sources for related trace elements with high EFs. The worldwide potential anthropogenic sources of Cd emissions to the atmosphere include various human activities such as metal production, combustion of fossil fuels, and other industrial processes (Pacyna and Pacyna, 2001). Some countries in central Asia

e.g., Co, Ba, Sb, Rb, and Pb. However, Zn and Ni concentrations in TGL were obviously increasing with the elevation rise (Fig. 6b). We can infer that, such a distribution of trace element with elevation change in the glacier surface of TGL and LHG is probably caused by atmospheric loading conditions of elements in different elevation. However, the glacier surface snow melting will also cause the accumulation of trace element concentration in the ablation zone. In this work, as we collected the surface snow samples in the LHG glacier after several days of snow fall, thus the surface snow was not fresh snow, and the melting and accumulation process may have caused the increased element concentration in the ablation zone. While in the TGL glacier, the surface snow was fresh snow (with snowfall before sampling), thus the trace element concentration could reflect the atmospheric loading distribution in different elevations. The different distribution of V, Zn and Ni with others may be caused by snow melting on the glacier surface and different element fates in the snowpack. Furthermore, we also compared our results with other similar studies on atmospheric trace elements in the Tibetan Plateau and surrounding regions (Table 2), to show the regional representative of element deposition at these two remote sites. Previous work has showed the trace elements concentrations in wet deposition at four sites (including urban and remote sites) over the central Himalayas (Tripathee et al., 2014). We also collected the data of other previous studies at remote sites in adjacent Asian regions, such as Namco in the southern Tibetan Plateau, Nakanoto of Japan, and Chuncheon of Korea (Table 2). The comparison showed that trace element concentration is relatively high at both two sites of this work, and reflected gradually decreasing trend from Himalayan regions (southern Tibetan Plateau) to TGL (central Tibetan Plateau), and to LHG (northern Tibetan Plateau), which may imply the significant influence of atmospheric trace element pollutant transport from south Asia to the central Tibetan Plateau, though there exists many large cities in the LHG surrounding regions. Compared to the LHG basin, the TGL glacier basin is located in a more remote site (in the hinterland of Tibetan Plateau), far away from urban regions without human activities in surrounding areas. 3.2. Estimation of natural and anthropogenic contributions of trace elements The crustal enrichment factor (EF) was calculated to evaluate the degree to which the temporal and spatial changes in concentrations of trace elements are linked to crustal dust inputs, and thus to identify the potential sources of trace elements at two glacier basins. Fig. 7 shows the EFs of various trace elements in the snowpits and glacier

Table 3 Principal component analysis (PCA) of trace element concentrations in the snowpacks at two sites. LHG snowpits

Li V Cr Co Ni Cu Zn Ga Rb Nb Mo Cd Sb Cs Ba Tl Pb % variance

LHG surface snow

TGL snowpit

TGL surface snow

1

2

3

1

2

3

1

2

3

1

2

3

0.461 0.93 0.85 0.828 −0.697 −0.51 −0.013 0.734 0.612 −0.001 0.021 0.069 0.491 0.658 0.918 0.066 −0.271 48.39

−0.143 0.119 0.22 0.423 0.614 0.706 0.88 0.427 −0.073 −0.187 0.744 0.921 0.701 0.263 0.26 −0.256 0.233 33.21

0.691 0.002 −0.015 0.025 0.103 0.222 0.73 −0.147 0.476 −0.112 0.368 −0.133 0.256 −0.146 0.065 0.277 0.596 20.27

0.859 0.853 0.908 0.935 0.774 0.326 −0.056 0.568 0.779 0.122 0.153 0.331 0.079 0.377 0.936 0.254 −0.072 35.83

0.172 −0.139 −0.151 −0.019 0.239 0.473 0.452 −0.306 −0.194 −0.382 0.797 0.678 0.692 −0.542 0.055 0.572 0.262 23.45

−0.118 −0.115 −0.014 0.003 0.39 0.698 0.03 −0.018 −0.156 0.11 −0.274 0.497 −0.335 0.193 −0.137 −0.297 −0.07 15.63

0.499 0.843 0.964 0.961 0.799 0.884 0.952 0.706 0.268 −0.162 0.135 0.143 0.071 0.639 0.958 0.830 0.164 52.53

−0.268 −0.173 −0.038 0.136 0.537 0.759 0.624 0.500 0.749 −0.444 0.968 0.947 0.979 0.510 −0.098 0.089 0.967 36.07

−0.544 −0.190 0.196 0.014 −0.259 −0.091 0.152 0.094 −0.306 0.563 0.124 −0.002 0.118 −0.112 −0.061 0.349 0.062 19.86

0.926 0.931 0.239 0.796 0.3 0.21 −0.169 0.509 0.951 0.412 0.125 0.083 0.214 0.928 0.922 −0.645 −0.044 46.66

−0.174 −0.086 −0.449 0.434 0.685 0.706 0.861 0.572 −0.184 0.322 0.672 0.848 0.842 −0.231 −0.11 −0.031 0.948 29.32

−0.089 0.033 −0.666 0.045 0.178 0.496 −0.452 0.081 −0.02 0.009 0.067 −0.008 −0.438 −0.091 −0.079 0.282 −0.101 12.45

Factors were extracted by principal components analysis, with varimax rotation significant at p b 0.05.

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Fig. 9. The MODIS atmospheric optical depth (AOD) fields derived using the Deep Blue algorithm (Fig. 8a–d) in the Tibetan Plateau and surrounding regions, and air mass backward trajectory to the study sites in typical days (Fig. 8e–h).

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could be important source countries for Cd emissions (Kakareka et al., 2004), and south Asia also has many large industries with Cd emissions. Zn may be derived from similar sources, or traffic-related activities (Adachi and Tainosho, 2004). While potential anthropogenic sources of Sb emission, to the atmosphere, include coal combustion, lead and copper smelting, refuse burning, and various end uses (Pacyna and Pacyna, 2001; Krachler et al., 2008). The anthropogenic sources for Mo are mainly from steel industrial processes and coal combustion, and Ni is generally regarded as an indicator of emissions from fuel burning and traffic sources. The major sources of Cu in atmospheric particles are from the combustion of fossil fuels, industrial metallurgical process and waste incineration (Nriagu, 1996). Since the 1980s, leaded gasoline has been phased out gradually, Pb still exists in some areas due to coal burning and re-suspension of contaminated soil particles (McConnell and Edwards, 2008; Ozsoy and Ornektekin, 2009). Because of little emissions of industrial pollutants from local sources in the Tibetan Plateau, trace elements from anthropogenic source in the alpine glacier snow is probably long-range transported into the remote glacier regions by atmospheric circulation (especially for the TGL glacier area), and

deposited in the high altitude areas, e.g., the LHG and TGL basins (above 5000 m a.s.l.). Moreover, comparing the EFs of various trace elements at two remote sites in this work, we find large difference between them, as the element of the TGL snowpack shows much larger EFs than that of LHG (as several elements reached 1000 times in the TGL basin). In the above discussion we have mentioned that, the TGL glacier basin is located in the hinterland of Tibetan Plateau, far away from urban regions, whereas the LHG glacier basin is near the surrounding cities, e.g., Yumen city, Jiuquan city, and also some cities in Xinjiang Province. There are large industries with trace element emissions in these cities. Thus we can infer that, atmospheric pollutants in the LHG basin are mainly from these surrounding regions with westerly circulation. However, the greatly increased concentration and EFs for trace element from anthropogenic sources (with high EFs value) in the TGL basin is probably originated from other regions, besides that from westerly circulation. In Fig. 8 log transformed EFs versus log Sc are presented for each trace element. Purely crustal elements should present constant EF

Fig. 10. CALIOP/CALIPSO transact showing attenuated backscatter coefficient (km−1 sr−1) profiles at 532 nm with 60 m vertical and 12 km horizontal resolution, indicating the aerosol transportation over the Tibetan Plateau from south Asia.

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values at varying Sc concentrations, but anthropogenically derived elements would decrease with increasing Sc concentrations (Hong et al., 2009; Tripathee et al., 2014; Al-Momani, 2003). Fig. 8 clearly shows that Li, Rb and Ba do not have good correlations, whereas the remaining elements have a more defined inverse relationship, indicating a source of anthropogenic other than crustal dust. 3.3. Sources of trace elements deposition at two remote sites In order to further identify the possible sources of trace elements in the snowpack, principal component analysis (PCA) was also performed in this work. PCA is a multivariate statistical method frequently used to simplify large and complex data sets to identify correlated variables (potential pollution sources). Varimax rotated PCA was performed using SPSS 16.0 software for this study. Table 3 shows the principal component analysis (PCA) of element concentrations in the glacier snowpack at two alpine sites, including both snowpits and surface snow results. Because of low concentrations in most samples, Be concentrations were not listed. Factor loadings considered as significant are marked bold in Table 3. In the LHG glacier snowpits, the first factor was largely associated with Li, Rb, Ba, Ga (crustal) as well as Cr, Co and Cs (also crustal). It may indicate crustal dust sources, especially regional dust inputs in central Asia. In the snowpit profile, concentrations of these elements also corresponded well to dust layers. The second factor was loaded with Cu, Zn and Cd (anthropogenic) as well as Mo, Cd, and Sb (anthropogenic), representing the anthropogenic input from fossil fuel combustion, traffic emission and metal smelting (Al-Momani, 2003). Finally, the third factor was loaded with Li and Pb, for which the source is mainly from both crustal and anthropogenic. Similarly, at the TGL basin, the first factors were highly loaded with Cr, Co, Ba, Cs and Ga, which are clearly from the crustal sources; and

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Zn, Cu and Ni might have deposited at the same time from crustal sources (e.g., Tripathee et al., 2014). Additionally, the remaining factors were loaded with (Ni, Cu, Zn, Mo, Cd, Sb, and Pb) which are anthropogenic emissions from the combustion of fossil fuels and industrial sources. Moreover, these elements are derived from the nonferrous metal production and fossil fuel combustion (Hong et al., 2009; Tripathee et al., 2014; Al-Momani, 2003). However, compared to that of the snowpits, trace element of the surface snow shows very different PCA composition at two remote sites, which may indicate the influence of simply incidents of anthropogenic or crustal dust to the trace element deposition in the glacier surface (Table 3). Fig. 9 showed the MODIS atmospheric optical depth (AOD) fields derived using the Deep Blue algorithm (Fig. 9a–d) in Tibetan Plateau and surrounding regions, and air mass backward trajectory to the study sites (Fig. 9e–h). MODIS AOD data during each season in 2013–2014 indicated that the aerosol depth in south Asia is much stronger than other regions in Asia, probably with plenty of atmospheric pollutants (Ramanathan et al., 2007). Backward trajectory from Hysplit model showed the obvious difference of air mass transport during winter and summer. The air mass to the LHG glacier basin is mainly originated from the westerly (Fig. 9e and f), which could bring pollutants from central Asia and Xinjiang province. However, the air mass to the TGL glacier basin (hinterland) is often originated from the southern region of Tibetan Plateau and south Asia (Fig. 9g and h), probably bringing large amount of anthropogenic pollutants from India. Therefore, we can infer that, the large difference of both concentrations and crustal enrichment factors (EFs) of trace element between the LHG and TGL glacier basins is caused by different source regions of element transport under different atmospheric circulations. Moreover, CALIOP/CALIPSO transact (Fig. 10) also clearly indicated the great influence of atmospheric pollutant transportation from south Asia to central Tibetan

Fig. 11. Backward dispersion model showing the different origin and backward transport of atmospheric mass concentration and particles to the study areas in the northern Tibetan Plateau, (a) mass concentration to TGL, (b) particles dispersion to TGL, (c) mass concentration to LHG, and (d) particles dispersion to LHG.

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Plateau (e.g., the TGL glacier basin), which probably caused the pronounced increases of both concentrations and crustal enrichment factors (EFs) observed in the snowpack at the TGL glacier region compared to the LHG glacier basin. NOAA Hysplit backward dispersion model (http://ready.arl.noaa.gov/HYSPLIT_disp.php) also demonstrated that there exist clearly differences of pollutant transport origin between two remote sites (Fig. 11), as the backward pollutants' dispersion source of TGL basin is mainly from the southern regions of the Tibetan Plateau (probably from south Asia after several days transport), whereas the pollutants dispersion to LHG basin is mainly from the surrounding cities in northwestern China. 4. Conclusions Based on 18 trace elements (Li,Be, V, Cr, Co, Ni, Cu, Zn, Ga, Rb, Nb, Mo, Cd, Sb, Cs, Ba, Tl, and Pb) investigation, this work discussed atmospheric trace element deposition concentration in the snowpacks at two remote alpine sites in the northern Tibetan Plateau, including the Laohugou glacier basin (LHG) and the Tanggula glacier basin (TGL), and its related atmospheric pollution information in these glacier regions. The results clearly demonstrate that pronounced increases of both concentrations and crustal enrichment factors (EFs) are observed in the snow at TGL glacier region compared to LHG glacier basin, with the highest enrichment factors for Sb and Zn in the TGL basin, while with the highest for Sb and Cd in the LHG basin. Such increases of trace element in the TGL glacier basin are attributed to anthropogenic emissions of these elements, largely from metal production, combustion of fossil fuels, and other industrial processes. Moreover, the higher concentration and EFs of trace elements in the surface snow at both two sites indicated great deposition of anthropogenic element inputs in summer. With the elevation change, trace element in the glacier surface snow showed decreasing from lower elevation to higher elevation (especially for Ni, Zn and Pb). Compared with other results of Tibetan Plateau and surrounding regions, trace element concentration showed gradually decreasing trend from Himalayan regions (southern Tibetan Plateau) to the TGL basin (central) and the LHG basin (northern), which may imply the significant influence of atmospheric trace element pollutant transport from south Asia to the central Tibetan Plateau, though there exists many large cities in the LHG surrounding regions. EF calculations at two sites showed that most of the heavy metals (e.g., Cu, Zn, Mo, Cd, Sb, and Pb) were from anthropogenic sources and some elements (e.g., Li, Rb, Ba) were originated from crustal sources. Principal component analysis (PCA) also indicated that the snowpack chemistry was mostly influenced by crustal and anthropogenic sources. MODIS atmospheric optical depth (AOD) fields derived using the Deep Blue algorithm and CALIOP/CALIPSO transect showed significant influence of atmospheric pollutant transportation from south Asia to the Tibetan Plateau, which may have caused the increases of concentrations and crustal enrichment factors of trace elements in snowpack at the TGL glacier basin. Acknowledgments This work was funded by the National Natural Science Foundation of China (41301065), the Chinese Academy of Sciences (KJZD-EW-G0304), the State Key Laboratory of Cryospheric Sciences, the West Light Program for Talent Cultivation of Chinese Academy of Sciences, and the Youth Innovation Promotion Association, CAS (2015). The authors would like to thank the field work team in the TGL Dongkemadi glacier in August 2014 for their hard field work. Many thanks to the Qilian Mountain Glacier and Ecological Environment Research Station, and to Dr. Wang X.J. for the help on drawing the location map. The authors also thank two anonymous reviewers and the scientific Editor, Dr. Tie Xuexi, whose comments and suggestions were very helpful in improving the quality of this paper.

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