Monsoon-driven transport of atmospheric mercury to the South China Sea from the Chinese mainland and Southeast Asia—Observation of gaseous elemental mercury at a background station in South China

Concentrations of gaseous elemental mercury (GEM) were continuously monitored from May 2011 to May 2012 at the Wuzhishan State Atmosphere Background Monitoring Station (109°29′30.2″ E, 18°50′11.0″ N) located in Hainan Island. This station is an ideal site for monitoring long-range transport of atmospheric pollutants from mainland China and Southeast Asia to South China Sea. Annual average GEM concentration was 1.58 ± 0.71 ng m −3 during the monitoring period, which was close to background values in the Northern Hemisphere. GEM concentrations showed a clear seasonal variation with relatively higher levels in autumn (1.86 ± 0.55 ng m −3 ) and winter (1.80 ± 0.62 ng m −3 ) and lower levels in spring (1.16 ± 0.45 ng m −3 ) and summer (1.43 ± 0.46 ng m −3 ). Long-range atmospheric transport dominated by monsoons was a dominant factor influencing the seasonal variations of GEM. The GEM diel trends were related to the wind speed and long-range atmospheric mercury transport. We observed 30 pollution episodes throughout the monitoring period. The analysis of wind direction and backward trajectory suggested that elevated GEM concentrations at the monitoring site were primarily related to the outflows of atmospheric Hg from mainland China and the Indochina peninsula. The △GEM/△CO values also suggested that GEM was significantly affected by the long-range transport from the anthropogenic sources and biomass burning in Asia and Indochina peninsula.