Seasonal variations, speciation and possible sources of mercury in the snowpack of Zhadang glacier, Mt. Nyainqêntanglha, southern Tibetan Plateau

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Seasonal variations, speciation and possible sources of mercury in the snowpack ofZhadang glacier, Mt. Nyainqêntanglha, southern Tibetan Plateau

Jie Huang a, Shichang Kang a,b,⁎, Junming Guo a,c, Qianggong Zhang a, Jianzhong Xu b, Matt G. Jenkins d,Guoshuai Zhang a,c, Kang Wang a,c

a Key Laboratory of Tibetan Environment Changes and Land Surface Processes, Institute of Tibetan Plateau Research, Chinese Academy of Sciences, Beijing, 100101, Chinab State Key Laboratory of Cryospheric Science, Chinese Academy of Sciences, Lanzhou, 730000, Chinac Graduate University of the Chinese Academy of Sciences, Beijing, 100049, Chinad Department of Geological Sciences, Central Washington University, 400 E. University Way, Ellensburg, WA, 98021, USA

a b s t r a c ta r t i c l e i n f o

Article history:Received 2 October 2011Received in revised form 17 April 2012Accepted 17 April 2012Available online 15 May 2012

Keywords:MercurySeasonal variationsSpeciationSourcesSnowpitTibetan Plateau

Ten snowpits were sampled at the Zhadang glacier during 2008 and 2011 to investigate the seasonal varia-tions, speciation, and sources of mercury (Hg) in the southern Tibetan Plateau. In the 2008 snowpit, totalHg (HgT), particulate matter, most of major ions were found in higher concentrations during the non-monsoon season than in the monsoon season. Analysis of Hg speciation indicated that HgT in the 2011snowpits was dominated by particulate-bound Hg (HgP). Most of particulate matter in the 2008 snowpitwas dominated by fine particulates, indicating that the influx of particulate matter and HgP was probably oc-curring by long-range transportation via general atmospheric circulation. Analysis of dominant ion Ca2+ andalkaline pH values has suggested that the long-range transported HgP, originating from dust storm activities,may be the most important source for Hg in the Zhadang glacier snowpit during the non-monsoon season.Backward-trajectory analysis indicates the majority of the air masses arriving at the Zhandang glacier origi-nated from the arid regions of northwestern India (e.g., Thar Desert), confirming that arid regions in centraland southern Asia are likely the main sources of Hg being deposited in the Zhadang glacier snowpit. Thisstudy also suggests that ice core records from the Tibetan Plateau may be useful tools for interpretinglong-term historical records of atmospheric Hg deposition, and reconstructing Hg biogeochemical cycling.

© 2012 Elsevier B.V. All rights reserved.

1. Introduction

The Tibetan Plateau, known as “The Third Pole”, is one of the mostimposing topographic features on the surface of the earth, with anarea of about 2,500,000 km2 and a mean elevation of more than4000 m a.s.l. Due to its sparse human population and limited industrialactivities, the atmosphere over the plateau is thought to be minimallyimpacted by human activities in the Asian continent. However, long-range transport of contaminants via atmospheric circulation is an im-portant pathway for the accumulation of environmental pollutants inremote regions (Fitzgerald et al., 1998; Jackson, 1997). Several studieshave suggested that pollutants originating from outside the TibetanPlateau are transported through the atmosphere and deposited onthe regions glaciers (Hou et al., 2003; Kang et al., 2009; Lee et al.,2008; Wang et al., 2008; Xiao et al., 2002). This becomes significantin light of future global warming, when the accumulated contaminantsreleased from the melting snow and ice of the Tibetan Plateau may

endanger the ecosystems and human health of the downstream re-gions. Of particular interest to this study is Hg, which is commonlytransported long-range and is among the most highly bioaccumulatedtoxic trace metals to ecosystems and human health (Harris et al., 2007;Wolfe et al., 1998). Research regarding the accumulation of Hg in Tibet-an Plateau snow and ice is imperative to better understanding its po-tential downstream impact.

Limited studies exist investigating seasonal variations and sources ofHg in snowpit and ice core records from the Tibetan Plateau (Loewen etal., 2007;Wang et al., 2008) and currently no speciation measurementsof Hg exist in this region. For instance, a pioneer study indentified thatHgT concentrations are higher in the non-monsoon season than in themonsoon season in three snowpits along a southwest-northeast tran-sect across the central Tibetan Plateau, and suggested that atmosphericHg deposition here may be primarily associated with particulate matterover the Tibetan Plateau (Loewen et al., 2007). Additionally, similar sea-sonal variations of HgT were observed in a shallow firn core from theDasuopu glacier in the Himalayas (Wang et al., 2008). Although bothstudies agree regarding the seasonal variations of HgT in snow andice, they do not agree on the potential sources of Hg. Wang et al.(2008) suggest the primary source of Hg in the Dasuopu ice core tobe related to industrial energy consumption in India, whereas Loewen

Science of the Total Environment 429 (2012) 223–230

⁎ Corresponding author at: Institute of Tibetan Plateau Research, Chinese Academyof Sciences, Jia 4 Datun Road, Chaoyang District, Beijing, China 100101. Tel./fax: +8610 84097092.

E-mail address: [email protected] (S. Kang).

0048-9697/$ – see front matter © 2012 Elsevier B.V. All rights reserved.doi:10.1016/j.scitotenv.2012.04.045

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et al. (2007) suggest the Hg in their snowpits is associated with partic-ulate matter locally from the Himalayas and/or from central Asia. Fur-ther research is required in order to delineate the sources of Hg andto investigate its speciation in snow and ice on the Tibetan Plateau.

In this paper, samples from ten snowpits, retrieved from the passof Zhadang (ZD) glacier on Mt. Nyainqêntanglha in 2008 and 2011,were analyzed in order to further understand the seasonal variationof HgT, to investigate the speciation of Hg, and to further delineatethe sources of Hg in the ZD glacier snowpit. This research will notonly expand the snow and ice Hg database for the high-elevation gla-ciers of the Tibetan Plateau, but also enhance our understanding of at-mospheric deposition and transport of Hg in the region. In particular,this study will provide some motivation for interpreting historical re-cords of atmospheric Hg deposition, and reconstructing Hg biogeo-chemical cycling through ice cores from the Tibetan Plateau.

2. Experimental

2.1. Study area and sampling site

Mt. Nyainqêntanglha in the Nyainqêntanglha Range is approximate-ly 380 km northeast of Mt. Everest, on the south shore of Lake Nam Co(4718 m a.s.l.), the second largest lake on the southern Tibetan Plateau(Fig. 1). TheMt. Nyainqêntanglha is regarded as a pertinent location fordescribing and understanding changes in the chemistry of the atmo-sphere over the Tibetan Plateau (Kang et al., 2007a). The low pressureover the Tibetan Plateau induces warm air masses from the IndianOceans transport to the hinterland of the plateau in monsoon period.While in other time, the large scale atmospheric circulation patternsover the Tibetan Plateau were mainly dominated by westerlies (Yanaiand Wu, 2005). In this study, a total of ten snowpits were excavated

from the pass of ZD glacier (30°28′N, 90°39′E, 5800 m a.s.l.) (Fig. 1)onMt. Nyainqêntanglha in 2008 (one snowpit in 14th September, here-after, 2008 snowpit) and 2011 (nine snowpits from 16th August to 9thSeptember, hereafter, 2011 snowpits).

2.2. Snowpit sampling and analysis

The snowpits were dug into the snowpack of ZD glacier and snowsamples were collected at a vertical resolution of 10 cm on the wall ofthe snowpits. The two-person “clean hands, dirty hands” Hg samplingprotocol was followed strictly during the sampling (Fitzgerald, 1999).For the 2008 snowpit, the snow samples for HgT analysis were storedin new 50-mL polypropylene BD Falcon® (VWR) tubes spiked with100 μL of BV-III grade (CMOS) HCl (Beihua Chemical, China), and thesamples for analysis of δ18O, particulate matter and major ions werestored in new 20-mL high-density polyethylene tubes. For the 2011snowpits, nine snowpits were analyzed for Hg speciation to quantifyHgP and its proportion in HgT (i.e., two samples were collected in eachlayer, one for HgT determination and the other for dissolved Hg (HgD)analysis after being filtered through a 0.45 μm membrane (Durapore®,Millipore). HgP was calculated by the difference between HgT and HgD.All the samples were stored at 4 °C until analysis.

The snow samples from the 2008 snowpit were analyzed for HgT,δ18O, particulate matter, major ions and pH values, while the snowsamples from the 2011 snowpits were only analyzed for speciatedHg (i.e., HgT, HgP, HgD). All samples were analyzed at the laboratoryof the Institute of Tibetan Plateau Research. The details for the mea-surement of Hg, δ18O, particulate matter and major ions in meltedsnow samples are described in Loewen et al. (2007). The pH valueswere measured using a HANNA® HI98129 Tester. Field blanks werecollected using vials pre-filled with ultrapure water in the laboratory

Nam Co station

sampling site

Fig. 1. Map showing the location of the ZD glacier snowpit in the Tibetan Plateau.

224 J. Huang et al. / Science of the Total Environment 429 (2012) 223–230


and then opened during sample collection and handled as samples.All speciated Hg concentrations in the field blanks were less thanthe 0.2 ng L−1 method detection limit, indicating that contaminationduring sampling, transport, and treatment was negligible.

3. Results and discussion

3.1. Seasonal variations of HgT, major ions and particulate matter in the2008 snowpit

δ18O values in the 2008 snowpit were less negative (i.e. higher) inthe non-monsoon season (spring and winter) and more negative values(i.e. lower) in the monsoon season (summer and autumn), with season-al changes in δ18O values ranging from −22.3‰ to−8.8‰ (Fig. 2). Thesnowpit was determined to represent a near complete 1-year accumula-tion based on snowpit dating by δ18O in the Tibetan Plateau region(Kang et al., 2007a, 2007b, 2008). HgT concentrations in the 2008snowpit ranged from b1 ng L−1 to 38.2 ng L−1 with an average of8.1 ng L−1; the majority of snow samples have HgT concentrationslower than 10 ng L−1 (Fig. 2). HgT concentrations were as much asseventeen-fold higher during the non-monsoon season than duringthe monsoon season (Fig. 2). Previous studies have suggested that thescavenging effect of precipitation has a strong influence on atmosphericcontaminants (e.g., black carbon) over the Tibetan Plateau during themonsoon season (Kang et al., 2009; Lee et al., 2008; Xu et al., 2009b).Therefore, the lower HgT concentrations during the monsoon seasonmay be explained by increased precipitation effectively scavenging Hgfrom the atmosphere, resulting in low concentrations in the 2008snowpit. This is supported by the major ion record and the particulatematter record, which both display lower values during the monsoonseason compared to the non-monsoon season (Fig. 3). The concentra-tions of HgT and particulate matter in this study were of the sameorder of magnitude of that in an earlier study (Loewen et al., 2007).Also, seasonal variations of HgT in the 2008 snowpit were similar tothat of two ZD glacier snowpits sampled in June and October 2006(Loewen et al., 2007).

3.2. Concentration levels and speciation of Hg in the 2011 snowpits

HgT concentrations ranged from b1 ng L−1 to 20.8 ng L−1 with anaverage of 4.3 ng L−1 (Fig. 4), which are comparable to the concen-trations reported for surface snow samples in remote regions of theArctic (Berg et al., 2003; Douglas and Sturm, 2004; Lahoutifard et

al., 2005) and prior studies in the Tibetan Plateau (Huang et al.,2012). Average concentrations of HgT in the snowpits sampled inAugust (2.0 ng L−1 to 3.4 ng L−1) were found to be lower than thosein September (4.6 ng L−1 to 6.9 ng L−1) (Fig. 4). The average concen-tration of HgT for all snow samples in the 2011 (4.3 ng L−1, n=43)was lower than that in the 2008 (8.1 ng L−1, n=20), which is likelydue to snowmelt occurring in this high-elevation region during August(Kang et al., 2008). HgP concentrations in the 2011 snowpits varied be-tween b1 ng L−1 and 19.9 ng L−1 with an average of 3.8 ng L−1; HgTand HgP followed a pattern similar to that of particulate matter insnow samples, with peak concentrations occurring in the dust layers(Fig. 4). Analysis of Hg speciation demonstrated that the HgP constitut-ed up to 99.2% (avg.=76.6%) of the ZD glacier snowpit Hg. A notablecorrelation (R2=0.99, n=43, pb0.05) was found between HgT andHgP (Fig. 5), confirming that the atmospheric deposition of Hg was oc-curring via particulate matter over the Tibetan Plateau and HgP is thedominant factor influencing the levels and distribution patterns of Hgin glacier snow.

3.3. Possible sources of Hg in the ZD glacier snowpit

3.3.1. Relationships among Hg, particulate matter, major ions and pHvalues

The influx of particulate matter to the ZD glacier is important forassessing the sources of Hg due to the fact that atmospheric deposi-tion of Hg over the Tibetan Plateau is occurring via particulate matter,and HgP dominated the snow Hg in the ZD glacier. A previous studyhas pointed out that glaciers on the northern and western side ofthe Tibetan Plateau lie adjacent to vast arid regions of westernChina, and can provide coarse particulate matter deposition recordsof local significance. Glaciers in the southern Tibetan Plateau, on theother hand, record the influx of fine particulate matter generated inthe arid regions to the west and transported long distances with thewesterly jet stream (Wake et al., 1994). Several studies confirm this,indicating that fine atmospheric particulate matter (b5 μm) deposit-ed onto the high elevation glaciers over the southern Tibetan Plateauare subjected to long-range transport in the atmosphere (Wu et al.,2009, 2010, 2006). This study reveals that the mean diameter of partic-ulate matter is dominated by fine particulates (D=6.3 μm) in the 2008snowpit (Fig. 6), which is comparable to the size distribution of partic-ulate matter in glaciers of the southern Tibetan Plateau. Furthermore,much of the particulate-phase Hg present in the atmosphere is foundin the fine micron particle diameter size range (Rowland et al., 2010)

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Fig. 2. Vertical profiles of HgT, δ18O, particulate matter in the 2008 snowpit (light grey shading represents dust layers).

225J. Huang et al. / Science of the Total Environment 429 (2012) 223–230


and thus the fine HgP is capable of undergoing long-range transport inthe atmosphere (Lee et al., 2001; Sheu and Mason, 2001). In this study,the sampling site was above the atmospheric boundary layer at an ele-vation of 5800 m a.s.l., and therefore exposed to the long-range trans-ported fine particulate matter and HgP. Analysis of Hg speciation hassuggested that most of Hg in the ZD glacier snowpit was contributedby HgP. As a result, it would be possible to deduce that the dominantinflux of fine particulate matter and HgP are most likely derived fromthe same sources, and likely have been transported long-range via gen-eral atmospheric circulation.

Numerous studies have suggested that both Ca2+ and HCO3− dom-

inate the ion concentrations of snow and ice samples, and Ca2+ iscommonly referred to as the ideal indicator for the transport of atmo-spheric particulate matter to the Tibetan Plateau (Kang et al., 2002,2003, 2004; Yao et al., 2004). Analysis of major ions in the 2008snowpit has suggested that among all species Ca2+ was the dominantcation and HCO3

− was the dominant anion (Table 1). A previous study

has suggested that dust storm activity may be the largest source of Hgdeposition on the plateau that is primarily associated with atmo-spheric particulate matter, although it is still difficult to quantitativelydifferentiate between the Hg that is long-distance transported withdust particulates and the Hg that is partitioning onto particulate mat-ter near the deposition site (Loewen, 2008). Vast arid zones in centraland south Asia (e.g., Taklamakan Desert, Thar Desert) are key dustsources in the Northern Hemisphere (Wu et al., 2010, 2006, 2008),and dust storms are a frequent phenomenon in winter and especiallyin spring on the Tibetan Plateau (Wang et al., 2005, 2004). For in-stance, a recent work from the East Rongbuk glacier in Mt. Everesthas suggested that sources of particulate matter in snow and ice aremainly influenced by the external sources of the northwestern Indianarid regions in south Asia (Xu et al., 2009a). The high content of Ca2+

and HgT during the non-monsoon season indicates that snow Hg inthe ZD glacier snowpit is largely influenced by the dust storm activi-ties originating from these arid regions, and most of HgP associated

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Fig. 3. Vertical profiles of major ions and HgT in the 2008 snowpit (light grey shading represents dust layers).



with fine particulate matter were long-range atmospherically trans-ported to the southern Tibetan Plateau by the westerly jet stream.

pH values measured from the 2008 snowpit ranged from 7.24 to8.39 with a mean value of 7.82 (Table 1), which indicates that snowsamples in the ZD glacier snowpit were constantly alkaline. Previous

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Average HgT concentration:

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HgP%: 13.0%-99.2%

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HgP%: 43.8%-92.2%

2011-8-27n=6

Average HgT concentration:4.6 ±5 .6ng/LHgP%: 67.1%-96.9%

2011-9-05n=4

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Fig. 4. Vertical profiles of HgT and HgP in the 2011 snowpits (light grey shading represents dust layers; zero represent the position of dust layers in the snowpits).

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Fig. 5. HgT vs. HgP concentrations for all the snow samples in the 2011 snowpits.

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Fig. 6. Size distribution of dust particulates in the 2008 snowpit.



studies have suggested that desertification is a dominant process ofnatural land surface change in arid regions and produces abundantsalts (e.g., CaCO3) that can form dust in the atmosphere, and causesalkaline precipitation being recorded in many arid zones around theworld (e.g., India, Israel, Africa) (Goni et al., 2001; Khemani et al.,1985; Mamane et al., 1987). The snow samples were found to behighly mineralized (Ca2+ + HCO3

−: 83%) along with being constantlyalkaline (pH value: 7.82) in this study. Considering that the southernTibetan Plateau is adjacent to the largest dust source regions in cen-tral and southern Asia, the alkaline particulate matter may buffer nor-mally acidic precipitation and deposit onto the glaciers of the TibetanPlateau, leading to higher-than-7.0 average pH values in the ZD gla-cier snowpit. The westerlies may also bring considerable HgP fromthese arid regions to the hinterland of the Tibetan Plateau by generalatmospheric circulation.

3.3.2. Backward trajectory analysisIn order to analyze the sources and transport pathways of Hg arriv-

ing at the ZD glacier, 5-day backward trajectories were computed dailyfor the period of measurement using the HYSPLIT 4.8 model, developedby the National Oceanic and Atmospheric Administration (NOAA)'s AirResources Laboratory (ARL) (Draxler and Hess, 1998). The trajectorieswere generated using GBL (global latitude-longitude projection) mete-orological archive maintained by ARL (available online at ftp://arlftp.arlhq.noaa.gov/archives/reanalysis). For the HYSPLIT model, the arrivalheight of the air mass should bewithin or just above the local boundarylayer (Jaffe et al., 1999). The atmospheric boundary layer (ABL) can ex-tend to heights of almost 3 km above the ground surface in the TibetanPlateau (Yang et al., 2004), although the height of the ABL at Nam CoLake region as well as the ZD glacier is about 1.75 km (Lu et al.,2008). Therefore, 1 km was selected as the arrival height in thisstudy, which is within the local ABL height at the ZD glacier. The trajec-tories are calculated at an altitude of 1 km above ground level fromwinter in 2007 to fall in 2008 as indicated by the seasonal variationsof δ18O (Fig. 2). The analysis revealed that three different types of airmasses arrived at the sampling site (Fig. 7). Most of the air masses(59%) came from the southwestern arid regions in central and south-ern Asia; while a relatively small percentage (32%) of air masses arerapidly moving and originate from the southern Tibetan Plateau. Aneven smaller fraction (9%) of air traveled long-distance originating inthe west. That themajority of air masses (59%) originated in the south-west indicates that dominant fine particulate matter and HgP in the ZDglacier snowpit most likely have been transported long-range fromthese arid regions in central and southern Asia.

Previous studies have indicated that air masses may bring consid-erable contaminants from industrialized and urbanized regions of theIndian continent to the hinterland of the Tibetan Plateau when mon-soon circulation dominates the general atmospheric circulation pat-tern (Cong et al., 2007, 2010), and thus it is also a possible sourcefor Hg in the ZD glacier snowpit. However, during the monsoon season

lower wind speeds result in less particulate matter entrained in the air,which when combined with increased precipitation enhancing particu-late matter and Hg wash-out from the atmosphere, likely results in asmall contribution to Hg in snow and ice over the Tibetan Plateau.Therefore, it is less likely that the Hg in the ZD glacier snowpits werea product of the heavily polluted industrial areas in India, and morelikely resulting from long-range transported HgP originating as duststorm activities from the arid regions in central and southern Asia dur-ing the non-monsoon season.

3.4. Implications for ice core Hg records over the Tibetan Plateau

Ice cores from polar regions and low/mid-latitudes glaciers havebeen recognized as valuable tools for reconstructing paleoclimaticand paleoenvironmental records (Jitaru et al., 2009; Schuster et al.,2002; Thompson et al., 1998; Witherow and Lyons 2008). In particu-lar, ice cores from the Tibetan Plateau provide a unique environmen-tal media for describing and understanding changes in the chemistryof the atmosphere (Kang et al., 2007a, 2007b; Thompson et al., 1989,1997). Thus, future work investigating ice core Hg records over the Ti-betan Plateau will be helpful in understanding the biogeochemical Hgcycle on a regional/global scale. In this study, analysis of Hg speciationhas indicated that snow Hg from a high elevation glacier in the Tibet-an Plateau was dominated by the HgP, which has been shown to bemore stable than reactive Hg (e.g., RGM) in redox reactions and lessprone to post-depositional changes in the Tibetan Plateau (Huang etal., 2012). Therefore, Hg records from Tibetan ice cores may be betterpreserved than their polar counterparts, and these cores could pro-vide a high-resolution record of atmospheric Hg deposition.

A previous long-term ice core record (>100 Ka BP) has indicatedthat Ca2+ was an excellent proxy for historical changes in particulatematter over the Tibetan Plateau (Yao et al., 2004). Our findings in thisstudy have confirmed that particulate matter is the dominant factorinfluencing the concentration levels and seasonal distribution patternsof Hg in glacier snow, and atmospheric Hg deposition is primarily asso-ciated with particulate matter over the Tibetan Plateau. As a result, ithas been suggested that Ca2+ and particulate matter are potentiallyuseful tools for interpreting long-term historical records of atmospher-ic Hg deposition, and hence reconstructing Hg biogeochemical cyclingon a regional/global scale through ice cores from the Tibetan Plateau.

4. Summary and conclusions

In this study, seasonal variations of HgT, major ions and particulatematter were presented in the 2008 snowpit. A total of nine sequentialsnowpits sampled in August and September, 2011, were analyzed forthe speciation of Hg. Possible sources for Hg in the ZD glacier snowpitwere investigated by analyzing the relationships among size distributionof particulate matter, major ions, pH values and backward trajectories.

Analysis of HgT, particulate matter and most of major ions in the2008 snowpit has revealed a striking seasonal difference: concentra-tions were significantly higher during the non-monsoon season thanduring the monsoon season. Analysis of Hg speciation in the 2011snowpits has suggested most of Hg was accounted for HgP, confirmingthat atmospheric deposition of Hg over the Tibetan Plateau was pri-marily associatedwith particulate matter. Mean diameter of particulatematter in the 2008 snowpit was dominated by fine particulates, indi-cating that the dominant influx of particulate matter and HgP to theZD glacier snowpit was likely from long-range transportation via gen-eral atmospheric circulation. Analysis of the dominant ion Ca2+ and al-kaline pH values in snow samples have indicated that sources for thelong-range transported Hg in the ZD glacier are probably derivedfrom dust source regions in central and southern Asia (e.g., TaklamakanDesert, Thar Desert). Backward trajectory analysis has further indicatedthat most of the air masses (59%) originated in the arid regions ofnorthwestern India (e.g., Thar Desert) to the ZD glacier. Long-range

Table 1Statistical results of snow chemistry in the 2008 snowpit.

Range Mean S.D.

pH values 7.24–8.39 7.82 0.31PM (⁎106 mL−1) 0.02–1.40 0.28 0.36Na+(μeq L−1) 0.17–8.01 2.88 2.63NH4

+(μeq L−1) 0.49–6.21 2.95 1.62K+(μeq L−1) 0.08–0.92 0.39 0.25Mg2+(μeq L−1) 0–10.71 2.62 3.07Ca2+(μeq L−1) 0–162.13 33.74 46.27Cl− (μeq L−1) 0.20–3.50 1.37 1.01SO4

2− (μeq L−1) 0.21–5.87 1.85 1.68NO3

− (μeq L−1) 0.87–4.96 2.43 1.17HCO3

− (μeq L−1) 1.65–172.29 36.96 48.06

PM: particulate matter.



transported HgP originating from dust storm activities in the arid re-gions in central and southern Asia might be the most important sourcefor the Hg in glaciers over the Tibetan Plateau. Further studies (e.g., Hgstable isotope) are further required to quantitatively explore the sourcesof Hg in the glaciers over the Tibetan Plateau beyond this scope of thiswork.

Acknowledgments

The authors appreciate the arduous sampling work of Mr. Wangzhong and Mr. Ciren duo jie. Great help was offered from staff of NamCo Monitoring and Research station for Multisphere Interactions, Insti-tute of Tibetan Research, Chinese Academy of Sciences. Many thanksare given to all of them. The authors also gratefully acknowledge theNOAA Air Resources Laboratory (ARL) for the provision of the HYSPLITmodel used in this publication. This work was supported by the NationalNatural Science Foundation of China (Grant Nos. 40830743), the GlobalChange Research Program of China (Grant Nos. 2010CB951401), thePostdoctoral Science Foundation of China (Grant Nos. 2011M500404),and the Foundation of State Key Laboratory of Cryospheric Science,Chinese Academy of Sciences (Grant No. SKLCS-ZZ-2008-01 and SKLCS-09-06).

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Seasonal variations, speciation and possible sources of mercury in the snowpack of Zhadang glacier, Mt. Nyainqêntanglha, southern Tibetan Plateau