Volcanic breath analysis
High on the slopes of Mount Norikura, a 3,026-meter-tall active stratovolcano in the Japanese Alps, scientists are analyzing the air. This volcano, like others worldwide, naturally releases gases from its interior, including a steady stream of mercury. Volcanoes are a primary natural source of atmospheric mercury, a toxic heavy metal that can be distributed globally. The mercury is emitted mainly as Gaseous Elemental Mercury (GEM), a form that can remain in the atmosphere for long periods.
This is difficult for atmospheric chemists. Human activities, particularly the burning of coal, also release large quantities of mercury into the atmosphere. Distinguishing the natural volcanic mercury from anthropogenic (human-caused) pollution is important for accurately modeling the global mercury cycle and understanding its environmental impact. To solve this, researchers at high-altitude facilities like the Norikura Observatory (2,770 m) use a sophisticated technique: stable isotope analysis. This method is a unique chemical fingerprint for the mercury, revealing its origin.
A chemical fingerprint
Mercury has seven stable isotopes, atoms with the same number of protons but different numbers of neutrons. Physical and chemical processes in the environment can slightly alter the ratios of these isotopes, a phenomenon known as fractionation. Scientists measure two types of fractionation: mass-dependent fractionation (MDF) and mass-independent fractionation (MIF). These measurements provide a signature, or fingerprint, for mercury from different sources.
Studies of volcanic gases show they often have near-zero MIF values (expressed as ∆¹⁹⁹Hg). This signature helps distinguish them from mercury that has undergone extensive photochemical reactions in the atmosphere or terrestrial ecosystems, which can produce significant positive or negative MIF values. By capturing volcanic gases and analyzing them with instruments like a multi-collector inductively coupled plasma mass spectrometer, scientists can define the specific isotopic composition of Norikura's emissions.
This volcanic fingerprint is then compared to the background atmospheric mercury and to the known signatures of industrial sources. Mercury from coal combustion, for example, has its own range of isotopic values based on the geochemistry of the coal deposit. By identifying the unique isotopic signature of Norikura's mercury plume, researchers can trace its movement and contribution to the regional and global atmosphere, separating the volcano's natural breath from the noise of industrial pollution.