Elsevier

Atmospheric Environment

Volume 62, December 2012, Pages 540-550
Atmospheric Environment

Wet deposition of mercury at a remote site in the Tibetan Plateau: Concentrations, speciation, and fluxes

https://doi.org/10.1016/j.atmosenv.2012.09.003Get rights and content

Abstract

Precipitation samples collected at a remote high elevation site (i.e., Nam Co Station, 4730 m a.s.l.) in the southern Tibetan Plateau were analyzed for total mercury (HgT) between July 2009 and 2011, particulate-bound mercury (HgP) between July 2010 and 2011 and methylmercury (MeHg) from July through August of 2009. The volume-weighted mean (VWM) concentrations and wet deposition fluxes of HgT and MeHg in precipitation were 4.8 ng L−1 and 1.75 μg m−2 yr−1, 0.031 ng L−1 and 0.01 μg m−2 yr−1, respectively. VWM HgT concentration was approximately two times higher during the non-monsoon season than during the monsoon season, while 83% of the HgT wet deposition fluxes occurred during the monsoon season. The HgT and MeHg concentrations are comparable to the reported data for some of the most remote alpine and polar regions worldwide (e.g., Churchill), but the wet deposition fluxes of HgT and MeHg were among the lowest in the world. Analysis of Hg speciation has presented that HgP and MeHg concentrations are high, making up 71.2% and 1.82% of the HgT on average (VWM), respectively. The high HgP%, as well as a significantly positive between HgT and HgP (R2 = 0.91; n = 44; p < 0.001), confirmed that atmospheric deposition of Hg in the Tibetan Plateau was occurring in the form of HgP. A decreasing trend in HgT concentrations with increasing amount of precipitation (R2 = 0.08; N = 101; p < 0.005) was found at Nam Co Station, indicative that scavenging of HgP from the atmosphere was an important mechanism contributing Hg to precipitation. The precipitation amount, rather than HgT concentration, was found to be the governing factor affecting HgT wet deposition flux. Moreover, a comparison between measured wet deposition flux of Hg at Nam Co Station and the estimates from environmental records indicated that both snowpits and lake sediments appear to be reliable archives for estimating historical Hg accumulation rates over the Tibetan Plateau.

Highlights

► We present a 2-year measured data of wet Hg deposition at a remote site in Tibet. ► HgP dominated the concentration and flux of HgT in the wet-deposition samples. ► The precipitation amount was the governing factor affecting HgT wet deposition flux. ► Measured annual HgT flux was comparable to the estimates from environmental records.

Introduction

Mercury (Hg), a highly toxic trace metal, is of increasing interest lately due to its impact on environmental and human health, and the potential bioaccumulation of methyl Hg (MeHg) (Selin, 2009). Due to Hg's propensity to be transported airborne from polluted sources, atmospheric deposition is an important source of regional and/or long-range transported Hg to aquatic and terrestrial environments (Landis and Keeler, 2002; Rolfhus et al., 2003; Wiener et al., 2003), particularly in the remote areas (Jackson, 1997; Fitzgerald et al., 1998; Loewen et al., 2007). Moreover, it has been shown that atmospheric deposited Hg may be more biologically available to aquatic and terrestrial ecosystems than in-situ Hg, or Hg that is transported to surface waters via runoff or groundwater (Mason et al., 1999). Once deposited into the environment, inorganic Hg becomes available for methylation producing MeHg, which is the bioaccumulative and neurotoxic form of Hg. As a result, MeHg may pose a potential serious threat to human health and wildlife after being bioaccumulated and biomagnified through food webs (Wolfe et al., 1998; Krabbenhoft et al., 2002; Wiener et al., 2003).

Hg mainly exists in the atmosphere in three operationally defined forms: gaseous elemental mercury (GEM, Hg0), reactive gaseous mercury (RGM, comprised of Hg2+ compounds), and particulate-bound mercury (HgP). Globally, atmospheric Hg is dominated by GEM; RGM and HgP are thought to be rapidly removed by dry deposition and effectively wet scavenged by precipitation (Schroeder and Munthe, 1998). Dry deposition is Hg flux in the absence of precipitation and is believed to include all three Hg phases, whereas wet deposition is defined as the air-to-surface flux in precipitation (occurring as rain, snow), which scavenges mainly RGM and HgP from the atmosphere due to their higher surface reactivity and water solubility (Lindberg and Stratton, 1998; Schroeder and Munthe, 1998). Speciation could therefore play an important role in the atmospheric cycling of Hg by affecting its susceptibility to wet and dry removal, and subsequent deposition to the environment. Moreover, although Hg in precipitation (rain and snow) is mainly composed of scavenged RGM and HgP, MeHg has been found in precipitation from numerous studies (Mason et al., 1997a, 1997b, 2000; Guo et al., 2008; Fu et al., 2009), indicating direct atmospheric deposition of MeHg is also possible (Hammerschmidt et al., 2007).

In order to investigate Hg's impact on aquatic and terrestrial ecosystems, it is necessary to better understand the biogeochemical cycling of Hg by measuring the rate of atmospheric Hg deposition. In recent decades, environmental records (e.g., snow/ice, lake sediments) have been widely obtained in attempts to estimate atmospheric Hg deposition rates on regional scales (Schuster et al., 2002; Loewen et al., 2007; Wang et al., 2008; Jitaru et al., 2009; Wang et al., 2010; Yang et al., 2010). However, networks of properly chosen monitoring stations can provide more accurate estimates of regional-scale wet deposition than Hg environmental records since wet deposition is collected directly from the precipitation chemistry data. Additionally, since wet deposition typically accounts for 50–90% of the atmospheric Hg entering the environment (Lamborg et al., 1995; Mason et al., 1997a; Landis and Keeler, 2002), monitoring Hg in precipitation is the most direct way of assessing atmospheric Hg deposition fluxes to aquatic and terrestrial ecosystems (Prestbo and Gay, 2009). As a result, major monitoring stations have been established in many parts of the world (Sakata and Marumoto, 2005; Wängberg et al., 2007; Prestbo and Gay, 2009; Sanei et al., 2010) to monitor Hg concentrations and fluxes in precipitation at a high temporal resolution (typically weekly to bi-weekly). Such data provide important insights into the sensitivity of aquatic and terrestrial ecosystems to atmospheric Hg deposition and environmental Hg risk assessments, and for establishing the effectiveness of Hg emission reductions (Lindberg et al., 2007; Sanei et al., 2010). Of particular notice is the Mercury Deposition Network (MDN, 2010) in the US and Canada which has been operating since 1996 and currently includes more than 100 stations (Prestbo and Gay, 2009).

The Tibetan Plateau is one of the most imposing topographic features on the surface of the earth with an immense area of 2.5 million km2 and an average elevation of more than 4000 m a.s.l. It has been regarded as a sensitive region to global contaminant impact due to its unique landform, fragile ecosystem and special monsoon circulation (Qiu, 2008). Although there have been several studies estimating atmospheric Hg total deposition rates through environmental records (e.g., snow/ice, lake sediments) in the Tibetan Plateau (Loewen et al., 2007; Wang et al., 2008, 2010; Yang et al., 2010), accurate measurements of atmospheric Hg deposition are still limited. Moreover, atmospheric Hg deposition has been investigated at multiple sites all over the world e.g., East Asia (Sakata and Marumoto, 2005; Guo et al., 2008; Fu et al., 2009; Wan et al., 2009); North America (Prestbo and Gay, 2009; Sanei et al., 2010), yet long-term Hg monitoring sites remain deficient, and little data are available for concentration and deposition flux of Hg in high altitude regions (e.g., the Tibetan Plateau). Nam Co basin is a pristine region located in the southern Tibetan Plateau. Here the landscape mainly consists of high mountains, glaciers, lakes, and grassland, which are representative of most areas of the Tibetan Plateau. In this paper, as an important part of the mass balance study for Nam Co region, concentrations and wet deposition fluxes of speciated Hg were measured in precipitation from July 2009 to 2011 at Nam Co Station. These data are compared with the reported data for some of the most remote alpine and polar regions worldwide, and seasonal variations of concentration and wet flux of HgT are discussed. Additionally, the measured HgT wet deposition flux data are presented for evaluating the Hg deposition fluxes estimated from the environmental records in the Nam Co region. Here we present the first data of Hg wet deposition at a remote high elevation site in the Tibetan Plateau with the objective not only to provide important information on the status of the atmospheric Hg pollution, but also to shed light on the biogeochemical cycling of Hg in the highest plateau in the world.

Section snippets

Study area and sampling site

Known as the “Third Pole”, the Tibetan Plateau is one of the most remote and cold regions in the world. The Nam Co (30°30′–56′N, 90°16′–91°03′E; 4730 m a.s.l.) is the second largest saline lake on the Tibetan Plateau and the highest lake in the world. Nam Co lies at the foot of the Nyainqêntanglha Mountain (Mt. Nyainqêntanglha, 7102 m a.s.l.) in southern Tibet and covers an area of 1960 km2 at an elevation of 4718 m. The Nam Co Monitoring and Research Station for Multisphere Interactions

HgT

Precipitation data, HgT concentrations measured in precipitation, and daily fluxes of HgT during the study period are presented in Fig. 4. In total, 101 samples were collected over 25 months from July 2009 to 2011 at Nam Co Station. Because precipitation samples were immediately preserved by acid and remained in the dark once inside the collection tubes (thereby preventing reduction of Hg(II) to volatile Hg(0)), these data represent gross wet deposition concentrations and fluxes of HgT. The HgT

Summary and conclusions

Concentrations and wet deposition fluxes of speciated Hg were measured from July 2009 to 2011 at Nam Co Station, a remote high elevation site in the southern Tibetan Plateau. The HgT and MeHg concentrations are comparable to the reported data for some remote alpine and polar regions worldwide, further suggesting Nam Co Station is an ideal background site for monitoring long-term changes of the global environment. VWM HgT concentration in the non-monsoon season was approximately two times higher

Acknowledgments

We are grateful to the arduous precipitation sampling work of Mr. Wang Zhong and Ci Ren Duo Jie. Great help was offered from the staff of the Nam Co Monitoring and Research Station for Multisphere Interactions, Institute of Tibetan Plateau Research, Chinese Academy of Sciences. Many thanks are given to all of them. The authors also greatly appreciate three anonymous referees for their constructive comments and thoughtful suggestions. This work was supported by the National Natural Science

References (63)

  • N. Lahoutifard et al.

    Total and methyl mercury patterns in Arctic snow during springtime at Resolute, Nunavut, Canada

    Atmospheric Environment

    (2005)
  • C. Li et al.

    Major ionic composition of precipitation in the Nam Co region, Central Tibetan Plateau

    Atmospheric Research

    (2007)
  • R.P. Mason et al.

    Mercury in the Chesapeake Bay

    Marine Chemistry

    (1999)
  • R.P. Mason et al.

    Annual and seasonal trends in mercury deposition in Maryland

    Atmospheric Environment

    (2000)
  • R.P. Mason et al.

    Atmospheric deposition to the Chesapeake Bay watershed-regional and local sources

    Atmospheric Environment

    (1997)
  • R.P. Mason et al.

    The concentration, speciation and sources of mercury in Chesapeake Bay precipitation

    Atmospheric Environment

    (1997)
  • A.J. Poulain et al.

    Redox transformations of mercury in an Arctic snowpack at springtime

    Atmospheric Environment

    (2004)
  • E.M. Prestbo et al.

    Wet deposition of mercury in the US and Canada, 1996–2005: results and analysis of the NADP mercury deposition network (MDN)

    Atmospheric Environment

    (2009)
  • M. Sakata et al.

    Wet and dry deposition fluxes of mercury in Japan

    Atmospheric Environment

    (2005)
  • H. Sanei et al.

    Wet deposition mercury fluxes in the Canadian sub-Arctic and southern Alberta, measured using an automated precipitation collector adapted to cold regions

    Atmospheric Environment

    (2010)
  • W.H. Schroeder et al.

    Atmospheric mercury-an overview

    Atmospheric Environment

    (1998)
  • E.M. Sunderland et al.

    Speciation and bioavailability of mercury in well-mixed estuarine sediments

    Marine Chemistry

    (2004)
  • I. Wängberg et al.

    Trends in air concentration and deposition of mercury in the coastal environment of the North Sea Area

    Atmospheric Environment

    (2007)
  • Q. Wan et al.

    Atmospheric mercury in Changbai Mountain area, northeastern China II. The distribution of reactive gaseous mercury and particulate mercury and mercury deposition fluxes

    Environmental Research

    (2009)
  • X. Wang et al.

    One century sedimentary records of polycyclic aromatic hydrocarbons, mercury and trace elements in the Qinghai Lake, Tibetan Plateau

    Environmental Pollution

    (2010)
  • X. Wang et al.

    The recent deposition of persistent organic pollutants and mercury to the Dasuopu glacier, Mt. Xixiabangma, central Himalayas

    Science of the Total Environment

    (2008)
  • M.C. Ahn et al.

    Mercury wet deposition in rural Korea: concentrations and fluxes

    Journal of Environmental Monitoring

    (2011)
  • P.A. Ariya et al.

    The Arctic: a sink for mercury

    Tellus B

    (2004)
  • H. Biester et al.

    Modeling the past atmospheric deposition of mercury using natural archives

    Environmental Science and Technology

    (2007)
  • F.J. Black et al.

    Stability of dimethyl mercury in seawater and its conversion to monomethyl mercury

    Environmental Science and Technology

    (2009)
  • Z. Cong et al.

    Individual particle analysis of atmospheric aerosols at Nam Co, Tibetan Plateau

    Aerosol and Air Quality Research

    (2009)
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