ISOTOPIC COMPOSITION OF GASEOUS ELEMENTAL MERCURY (Hg0) AT VARIOUS SITES IN JAPAN

Abstract:

Mercury (Hg) is a toxic heavy metal, which exists in various chemical forms in the environmental system. In the atmosphere, Hg exists in three forms (Hg⁰_(g), Hg⁺²_(g), and Hg_(p)). Hg⁰_(g) is the dominant species of atmospheric Hg, accounting for >95% of the total Hg in the atmosphere. Because Hg⁰_(g) is highly volatile and has limited solubility in water, it cannot be easily removed by wet or dry deposition processes. Therefore, the residence time of Hg⁰_(g) in the atmosphere is relatively long (1 to 2 years), allowing long-range transport from mercury emission source(s). Conversely, Hg⁺²_(g) and Hg_(p) are effectively removed from the atmosphere through wet and dry depositions. The determination of mercury source attribution using quantitative data is challenging because Hg⁰_(g) may be deposited on an area upon oxidation to Hg⁺²_(g) and associated with aerosols and particulates to form Hg_(p) while the global cycling of Hg⁰_(g).

Over the last decade, the development of analytical methods of highly precise Hg isotopic measurements demonstrated mass-dependent fractionation (MDF) and mass-independent fractionation (MIF) of Hg isotopes in environmental samples. For instance, MDF of Hg isotopes is thought to occur during various natural and industrial Hg transformations. MIF of Hg isotopes is observed during abiotic reduction, photochemical and non-photochemical, and physical and chemical processes. Such processes lead to differences in the Hg isotopic composition of different emission sources, both natural and anthropogenic, and atmospheric processes (i.e., transportation, oxidation/reduction, deposition, and reemission). Therefore, Hg isotopic compositions could be used to trace the sources and processes of atmospheric Hg.

For securing the reliability and accuracy of atmospheric Hg isotope data, the methods of collection, pretreatment, and isotopic measurement for Hg⁰_(g) were developed to obtain high recovery yield of samples with no Hg isotopic fractionation during each analytical procedures. Using the optimum setting, Hg⁰ samples were collected in the rural, urban, and coastal environments for isotopic measurements.