Abstract

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 influenced 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 30 March to 3 September 2019, to study the influence of the Indian summer monsoon (ISM) on the origin, transport, and behavior of Hg. The GEM and PBM during the preceding Indian summer monsoon (PISM) period (1.20±0.35 ng m−3 and 11.4±4.8 pg m−3 for GEM and PBM, respectively) were significantly higher than those during the ISM period (0.95±0.21 ng m−3, and 8.8±6.0 pg m−3); the GOM during the PISM period (13.5±7.3 pg m−3) was almost at the same level as that during the ISM period (12.7±14.3 pg m−3). The average GEM concentration in the Nyingchi region, obtained using passive sampler, was 1.12±0.28 ng m−3 (from 4 April 2019 to 31 March 2020). The GEM concentration showed that the sampling area was very clean compared to other high-altitude sites. The GEM has several patterns of diurnal variation during different periods. Stable high GEM concentrations occur at night and low concentrations occur in the afternoon during PISM, which may be related to the nocturnal boundary layer structure. High values occurring in the late afternoon during the ISM may be related to long-range transport. Low concentrations of GEM observed during the morning in the ISM may originate from vegetation effects. The results of the trajectory model demonstrate that the sources of pollutants at Nyingchi are different with different circulation patterns. During westerly circulation in the PISM period, pollutants mainly originate from central India, northeastern India, and central Tibet. During the ISM period, the pollutants mainly originate from the southern part of the SET site. The strong precipitation and vegetation effects on Hg species 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. Long-distance transport factor dominates during PISM and ISM3, while local emissions is the major contributor between PISM and ISM3. Our results reveal the Hg species distribution and possible sources of the most important water vapor channel in the Tibetan Plateau and could serve as a basis for further transboundary transport flux calculations.

Highlights

  • Mercury (Hg) is classified as a hazardous pollutant because it is bio-accumulative and toxic (Mason et al, 1994, 1995)

  • During the preceding Indian summer monsoon (PISM) period, the gaseous elemental mercury (GEM) concentrations decreased continuously as the Indian monsoon developed and intensified (Fig. 2), which may indicate a change in the local GEM source as the wind field changes from westerly to Indian monsoon

  • GEM concentrations remained relatively stable during ISM1 and ISM2 (0.92 ± 0.23 to 0.92 ± 0.18 ng m−3), which may indicate that the source of GEM was relatively stable during this period

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Summary

Introduction

Mercury (Hg) is classified as a hazardous pollutant because it is bio-accumulative and toxic (Mason et al, 1994, 1995). GOM and PBM are removed from the atmosphere through chemical reaction and deposition because of their chemical activity and water solubility and could have significant impacts on the local environment (Lindberg and Stratton, 1998; Seigneur et al, 2006). Both GOM and PBM have complex fundamental physicochemical properties and may have complicated relationships with other regional pollutants (Gustin et al, 2015). There exists some gaps in understanding the sources and transport of atmospheric Hg in some remote areas, especially in harsh environmental areas where performing monitoring is difficult

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Conclusion

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