2018 年 52 巻 1 号 p. 1-12
Re-emission of mercury (Hg), previously deposited in soils and oceans, plays an important role in the global Hg cycle. Mercury evasion from surrounding sea surfaces might be one of the main emission sources of atmospheric Hg in Japan. To obtain information on Hg cycling around the Japanese Islands, dissolved gaseous mercury (DGM) and Hg evasion fluxes from surface seawaters of the Tsushima Strait and East China Sea were investigated using a two-layer gas exchange model. The mean DGM concentrations in the Tsushima Strait are 22 ± 7 pg L–1 in October 2014 and 27 ± 6 pg L–1 in August 2015, suggesting slightly higher values in summer (P < 0.05, t-test). The DGM concentrations in the Kuroshio Current are lower than in the Tsushima Strait and Pacific Ocean. However, the DGM concentrations are supersaturated at all sites, except at the site with the highest atmospheric total gaseous Hg (TGM). This suggests that Hg is emitted from these sea surfaces into the atmosphere. The TGM concentrations are sometimes increased due to the long-range transport from the Asian continent and volcanic activities on the southern Kyushu Island. The DGM concentrations and %DGM of the total Hg in the Tsushima Strait are positively correlated with the solar radiation, indicating that the DGM is partly produced by photochemical formation from Hg(II) in the surface seawater. No positive relationships between the DGM and sunlight were observed in the Kuroshio Current. Given that most of the observations for the Kuroshio Current were made far from the land, at water depths >200 m, the photochemical production of DGM is likely related to the distance from the coast and water depth, more specifically the inflow of terrestrial substances such as Fe and humic substances. The factors controlling the DGM variations differ at various sites, depending on the sea area and seasonal conditions. Based on the literature and results of this study, the Hg emissions of the East China Sea, including the Yellow Sea, are estimated to be 49 ± 17 t yr–1. This value is greater than the annual anthropogenic Hg emission from the Japanese Islands (18–34 t yr–1) but might be low compared with the outflow of Hg from the Asian continent.