Unexpectedly high concentrations of atmospheric mercury species in Lhasa, the largest city on the Tibetan Plateau

Abstract. Lhasa City is located in the central Tibetan Plateau and is the most densely populated area. As the first continuous monitoring of atmospheric mercury (Hg) species in a city on the Tibetan Plateau, our monitoring in Lhasa showed that the concentrations of gaseous elemental Hg (GEM), gaseous oxidized Hg (GOM), and particle-bound Hg (PBM) during subsequent of the Indian Summer Monsoon (S-ISM) period were 2.73 ± 1.48 ng m-3, 38.4 ± 62.7 pg m-3, and 59.1 ± 181.0 pg m-3, respectively. During the Westerly Circulation (WEC) period, the GEM, GOM and PBM concentrations were 2.11 ± 2.09 ng m-3, 35.8 ± 43.3 pg m-3, and 52.9 ± 90.1 pg m-3, respectively. The atmospheric Hg species concentrations were higher than those of previous monitoring on the Tibetan Plateau and other provincial capitals in China. Typical high-value occurrence processes were studied to investigate random events with high atmospheric Hg concentrations in Lhasa. Combustion event nearby or further away may be the main contributor of the high-concentration events. The lowest GEM concentrations occurred in the afternoon and persistently high concentrations were observed at night. The changes in GEM concentrations were consistent with the trends of other pollutant concentrations and contradictory to those of the wind speed. The high GEM concentrations at night can be attributed to the lower boundary layer height and lower wind speed. For both GOM and PBM, higher GOM concentrations occurred during the day and PBM during the night. The results of the principal component analysis indicated that local sources and wind speed are important factors influencing atmospheric Hg concentrations in Lhasa. The trajectory simulation showed that the source of the GEM in Lhasa gradually shifted from the south to the west of Lhasa from the S-ISM to the WEC periods, while both the southern and western sources were important in the late WEC period. The concentrations and change patterns of Hg species in Lhasa were significantly different than those at other monitoring sites on the Tibetan Plateau. Monitoring Hg species in Lhasa shows the possible maximum anthropogenic influences on the Tibetan Plateau and demonstrates the dramatic effect of wind on changes in urban atmospheric Hg concentrations.