Abstract
The Tibetan Plateau is generally considered to be a significantly clean area owing to its high altitude; however, the transport of atmospheric pollutants from the Indian subcontinent to the Tibetan Plateau has infected the Tibetan environments. Nyingchi is located at the end of an important water vapor channel. In this study, continuous monitoring of gaseous elemental mercury (GEM), gaseous oxidized mercury (GOM), and particle-bound mercury (PBM) was conducted in Nyingchi from March 30 to September 3, 2019, to study the influence of the Indian summer monsoon (ISM) on the origin, transport and behavior of mercury. The atmospheric Hg concentrations during the preceding Indian summer monsoon (PISM) period (1.20 ± 0.35 ng m−3, 13.5 ± 7.3 pg m−3, and 11.4 ± 4.8 pg m−3 for GEM, GOM, and PBM, respectively) were relatively higher than those during the ISM period (0.95 ± 0.21 ng m−3, 12.7 ± 14.3 pg m−3 and 8.8 ± 6.0 pg m−3). The average annual total gaseous mercury concentration in the Nyingchi region was obtained using a passive sampler as 1.12 ± 0.28 ng m−3. The GEM concentration showed that the sampling area was very clean. The GEM has several patterns of daily variation during different periods. Stable high GEM concentrations occur at night during PISM, which may be related to the nocturnal boundary layer. High values occurring in the late afternoon during the ISM may be related to long-range transport. The results of the trajectory model demonstrate that the sources of pollutants at Nyingchi are different under the control of different airflow fields. During westerly circulation, pollutants mainly originate from northeast India or Nepal. During the ISM period, the pollutants mainly originate from northeast India, or the Bay of Bengal, and the Indian Ocean. The strong precipitation and vegetation effects on Hg during the ISM resulted in low Hg concentrations transmitted to Nyingchi during this period. Further, principal component analysis showed that long-distance transport, local emissions, meteorological factors, and snowmelt factors are the main factors affecting the local Hg concentration in Nyingchi.
Highlights
Mercury (Hg) is classified as a hazardous pollutant because it is bio-accumulative and toxic.Generally, atmospheric Hg can be categorized into three major types: gaseous elemental mercury (GEM), gaseous oxidized mercury (GOM), and particle-bound mercury (PBM) (Selin, 2009)
To further discuss the patterns of Hg concentrations, the entire monitoring period was divided into the PISM period
During the PISM period, the locally monitored GEM concentrations decreased continuously as the Indian monsoon developed and intensified (Figure 2), which may indicate a change in the local Hg source as the decisive wind field changes from westerly to Indian monsoon
Summary
Mercury (Hg) is classified as a hazardous pollutant because it is bio-accumulative and toxic.Generally, atmospheric Hg can be categorized into three major types: gaseous elemental mercury (GEM), gaseous oxidized mercury (GOM), and particle-bound mercury (PBM) (Selin, 2009). The stable chemical properties of GEM coupled with its long atmospheric lifetime (approximately 0.3 to 1 year) makes GEM an important global pollutant (Selin, 2009;Travnikov et al, 2017). GOM and PBM are removed from the atmosphere through wet and dry deposition because of their chemical activity, and have significant impacts on the local environment (Lindberg and Stratton, 1998;Seigneur et al, 2006). Both GOM and PBM have complex fundamental physiochemical properties and may have complicated relationships with other regional pollutants (Gustin et al, 2015). Since 2013, the Minamata Convention was established to control the global mercury pollution (UNEP, 2013a).
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