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Recent decline of atmospheric mercury recorded by Androsace tapete on the Tibetan Plateau Yindong Tong, Xiufeng Yin, Huiming Lin, Bu Duo, Dan Zeng, Huanhuan Wang, Chunyan Deng, Long Chen, Jinling Li, Wei Zhang, James Jay Schauer, Shichang Kang, Guoshuai Zhang, Xiaoge Bu, Xuejun Wang, and Qianggong ZHANG Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.6b04632 • Publication Date (Web): 14 Nov 2016 Downloaded from http://pubs.acs.org on November 15, 2016
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Environmental Science & Technology
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Recent decline of atmospheric mercury recorded by Androsace tapete
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on the Tibetan Plateau
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Yindong Tonga,b, Xiufeng Yinc,d,e, Huiming Linf, Bu Duog, Dan Zengg, Huanhuan Wangf, Chunyan Dengf, Long Chenf, Jinling Lif, Wei Zhangh, James Jay Schaueri, Shichang Kangc, j, Guoshuai Zhangd , Xiaoge Bua,b, Xuejun Wangf*, Qianggong Zhangd,j**
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a. School of Environmental Science and Engineering, Tianjin University, Tianjin, P.R. China; b. Tianjin Key Laboratory of Indoor Air Environmental Quality Control, Tianjin University, Tianjin, P.R. China; c. State Key Laboratory of Cryospheric Sciences, Northwest Institute of Eco-Environment and Resources, Chinese Academy of Sciences, Lanzhou, P.R. China; d. Key Laboratory of Tibetan Environment Changes and Land Surface Processes, Institute of Tibetan Plateau Research, Chinese Academy of Sciences, Beijing, P.R. China; e. University of Chinese Academy of Sciences, Beijing, P.R. China; f. College of Urban and Environmental Sciences, Peking University, Beijing, P.R. China; g. Department of Chemistry & Environmental Science, Tibet University, Lhasa, P.R. China; h. School of Environment and Natural Resources, Renmin University of China, Beijing, P.R. China; i. Civil & Environmental Engineering, University of Wisconsin-Madison, WI, USA; j. CAS Center for Excellence in Tibetan Plateau Earth Sciences, Beijing, P.R. China; *Corresponding author:
Xuejun Wang Phone & Fax: +86-10-62756190 Email:
[email protected] Qianggong Zhang Phone & Fax: +86-10-84097043 Email:
[email protected] TOC/Graphical Abstract
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Abstract
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We combined the mercury passive sampling method and plant biomonitoring
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together to understand the long-term changes of atmospheric mercury concentrations
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on the Tibetan Plateau. Through the analysis of leaves of Androsace tapete that
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represent growing periods spanning the past decade, we explored the historical
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records of atmospheric mercury from 2006 to 2015. Mercury concentration was stable
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in the leaves of Androsace tapete during the period between 2006 and 2009, while a
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significant decrease occurred after the year of 2010. The decreasing trend is consistent
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with the monitoring data of the ground-based stations in other regions globally.
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Despite the potential uncertainties using passive sampling method and biomonitoring
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technique, we suggest that Androsace tapete is a potential biomarker that could
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provide
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concentrations, and offer new perspectives in monitoring mercury and perhaps other
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atmospheric pollutants in the regions where the long-term active monitoring was
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missing.
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Introduction
reliable and effective historical records of atmospheric mercury
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Mercury (Hg) is one of the most toxic metals and a global pollutant.1,2 Mercury is
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mainly emitted from the anthropogenic activities such as the coal combustion, mineral
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extraction and processing, industrial activities, and disposal of mercury containing
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products.3,4 Atmospheric mercury is largely present in two inorganic forms: gaseous
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elemental mercury (GEM, oxidation state=0) and reactive mercury that includes
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particulate mercury and reactive gaseous mercury (RGM, primarily Hg(II)).5,6 The
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majority of atmospheric mercury is present as GEM (>95%).6,7 During the past
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decades, significant decreases in GEM concentrations in South Africa, Europe and
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North America have been observed.8-12 Surface air mercury concentrations in the
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northern hemisphere were reported to decline by 30-40% between 1990 and
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2010,11,13-15 and similar decreasing trends have also been observed in mercury wet
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deposition across the North America and Western Europe.16,17 Such a decrease is
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consistent with the decrease in anthropogenic mercury emissions in Europe and North
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America.4,18,19 So far, most of the long-term monitoring of ground-based sites have
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been carried out in the developed countries. For the developing countries such as
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China, the preliminary atmospheric mercury monitoring network has only been
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established in the recent years, but there are no continuous long-term monitoring
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records that spanned over five years.18
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Vegetation accumulates persistent pollutants present in the atmosphere through the
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leaf surface absorption, and can be used for biomonitoring of persistent pollutants in
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the atmosphere.20-24 The biomonitoring of pollutants has been well applied in the
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volatile pollutants such as mercury and persistent organic pollutants (POPs). Lodenius
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et al.25 reported the use of vegetation for biomonitoring of GEM and showed that
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evaporation and leaching of adsorbed mercury in the plants’ leaves was negligible.25
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Greger et al.26 studied the transport of mercury from the root to the shoot in the
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terrestrial plant species, and found mercury translocation to the leaves of plants from
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the root was very small. A translocation barrier of mercury between the roots and
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stems of the plants was confirmed.26
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Alpine regions are generally considered to be vulnerable ecological environments
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because of their weak capabilities for self-recovery after physical disturbance.27
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Previous studies have shown that environments in alpine regions are critically
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sensitive to atmospheric mercury, especially the top soils and vegetations.27-29 For the
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application of biomonitoring in the POPs, Yang et al.,23 and Liu et al.,21 have used the
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moss, lichen, conifer needles and barks to trace the variations of atmospheric POPs
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concentrations along the elevation gradient of Mt. Gongga in the eastern edge of the
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Tibetan Plateau (TP). For being remote from the dense populations and industrial
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centers, and its high altitude, TP is considered as an ideal representative of regional
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and hemispheric atmospheric background.30-32 Given the location between South and
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East Asia, the levels of air pollutants on the TP are potentially affected by the two
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sources, China and India. China is the largest mercury emitter in the world, despite its
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continued effort in mercury emission reductions during the recent years4. India is the
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world’s fourth biggest coal reserve, and its mercury use and emission have been
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reported before.33-36 The anthropogenic mercury emission in Asia and its impacts on
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regional and global environmental background have been less understood partly due
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to the deficient long-term atmospheric mercury monitoring, which in turns hampers
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the introduction of targeted measures for future mercury controls.
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In this study, we combined the plant biomonitoring and mercury passive sampling
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method together to understand the long-term changes of atmospheric mercury
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concentrations on the TP. First, we applied the passive monitoring along the elevation
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gradient of the south-facing slope of Mt. Nyainqêntanglha to describe the variations of
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atmospheric mercury concentrations at different altitudes; then, the relationship of
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atmospheric mercury adsorption between passive sampling and an indigenous plant,
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Androsace tapete, was established. Through the mercury analysis of leaves of
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Androsace tapete that represent growing periods for the past decade, we explored the
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long-term records of atmospheric mercury on the TP.
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Materials and methods
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Study area
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The study was conducted along a south-facing slope of Mt. Nyainqêntanglha,
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starting at the foot of the mountain at 4,300 m a.s.l. and extending up to 5,300 m a.s.l.
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(Figure 1). This transect is largely covered by alpine meadow that leads to the higher
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elevations of the mountain with sparser vegetation. Exposed weathered rocks
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dominate the regions from 4800 to 5300 m a.s.l.. Higher elevations up to the summit
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(~5600 m a.s.l.) are typically above the snowline, where exiguous plants can be
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observed. In the study area, relatively high monthly wind speeds are found in spring
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and relatively low wind speeds are found in September, November and December.
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The monthly mean air temperature ranges from -7.8°C to 12.2°C, with the highest in
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July and the lowest in January.37
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Sample collection
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Passive sampling of atmospheric mercury
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A total of 30 passive samples were placed along the elevation gradient from 4300
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m a.s.l. to 5300 m a.s.l. (2 samplers were missing during field application). Triplicate
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passive samplers (set at the height of ~2 m above the ground) were deployed at each
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sampling elevation. Grinded sulfur-loaded activated carbon was used as the sorbents
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for GEM.38,39 Each triplicate set of samplers was deployed every 100 m to 150 m in
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altitude. The passive sampling measurements lasted for two months, from the end of
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July, 2015 to the early of September, 2015. When the sampling finished, all the
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samplers were sealed in the clean nylon bags and transported to the laboratory. The
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detailed information about locations of the passive samplers was provided in the
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Supporting Information (Table S1). This passive sampling method has been
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successfully applied in the monitoring of atmospheric mercury in the cities of North
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China (i.e., Beijing, Tianjin)39 and Nam Co Region on the TP.38
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Androsace tapete sampling
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A total of 29 Androsace tapete samples and 31 surface montane soils (at the depth
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of 0~5 cm) were collected along the elevation gradient in the September of 2015. All
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the plant samples were collected near the passive sampling site (with a distance of
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