Continuous observation of atmospheric speciated mercury at a site near the crater of Mt. Aso volcano

Volcanic and geothermal activities are one of the major emission sources of mercury (Hg), emitting 500 tons y− 1 into the atmosphere, and it is known that there are three main forms of Hg released to the atmosphere from these activities: gaseous elemental mercury (GEM), gaseous oxidized mercury (GOM), and particle-bound mercury (PBM). In this study, the concentrations of GEM, GOM and PBM2.5 (PBM in particles less than 2.5 μm) were continuously observed for 2 years at Sensuikyo Station, located 2.5 km northeast from Nakadake No. 1 Crater of Mt. Aso. The observed mean concentrations of GEM, GOM, and PBM2.5 in the entire period were 1.64, 0.021 and 0.021 ng m− 3, while maximum concentrations were 9.04, 2.45 and 3.20 ng m− 3, respectively. The concentrations increased significantly when the volcanic activity was high and when the wind blew from the crater. Under high volcanic activity, the mean concentrations of GEM, GOM and PBM2.5 were 1.3, 4.1, and 15.3 times higher than those during calm conditions. Changes in GOM and PBM2.5 concentrations between active and calm conditions were significantly higher than those of GEM. The amplitude of volcanic tremors is usually used as a reference indicator of the volcanic activity. GOM and PBM2.5 concentrations became higher before the volcanic tremor amplitude increased. However, the reliability of atmospheric GOM measurements in volcanic regions with high levels of acidic gases is still debatable. Therefore, PBM2.5 concentrations can be used as one of the indicators to assess the activity of Mount Aso volcano.