Unit 11: Atmospheric Pollution // Section 8: Mercury Deposition
Mercury (Hg) is a toxic pollutant whose input to ecosystems has greatly increased over the past century due to anthropogenic emissions to the atmosphere and subsequent deposition. Mercury is ubiquitous in the environment and is unique among metals in that it is highly volatile. When materials containing mercury are burned, as in coal combustion or waste incineration, mercury is released to the atmosphere as a gas either in elemental form, Hg(0) or oxidized divalent form, Hg2+. The oxidized form is present as water-soluble compounds such as HgCl2 that are readily deposited in the region of their emission. By contrast, Hg(0) is not water-soluble and must be oxidized to Hg2+ in order to be deposited. This oxidation takes place in the atmosphere on a time scale of one year, sufficiently long that mercury can be readily transported around the world by atmospheric circulation. Mercury thus is a global pollution problem.
Deposition of anthropogenically emitted mercury to land and ocean has considerably raised mercury levels in the biosphere. This accumulation is evident from sediment cores that provide historical records of mercury deposition for the past several centuries (Fig. 13). Ice core samples from Antarctica, Greenland, and the western United States indicate that pre-industrial atmospheric mercury concentrations ranged from about 1 to 4 nanograms per liter, but that concentrations over the past 150 years have reached as high as 20 ng/L (footnote 3).
Figure 13. Mercury in sediment profiles from straits south of Norway
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Source: Courtesy United Nations Environment Programme (adapted from Geological Survey of Norway).
Once deposited, oxidized mercury can be converted back to the elemental form Hg(0) and re-emitted to the atmosphere. This repeated re-emission is called the "grasshopper effect," and can extend the environmental legacy of mercury emissions to several decades. The efficacy of re-emission increases with increasing temperature, which makes Hg(0) more volatile. As a result mercury tends to accumulate to particularly high levels in cold regions such as the Arctic where re-emission is slow.
Divalent mercury deposited to ecosystems can be converted by bacteria to organic methylmercury, which is absorbed easily during digestion and accumulates in living tissues. It also enters fishes' bodies directly through their skin and gills (Fig. 14). U.S. federal agencies and a number of states have issued warnings against consuming significant quantities of large predatory fish species such as shark and swordfish, especially for sensitive groups such as young children and women of childbearing age (footnote 4).
Figure 14. Conceptual biogeochemical mercury cycle
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Mercury interferes with the brain and central nervous system. The expression "mad as a hatter" and the Mad Hatter character in Lewis Carroll's Alice in Wonderland are based on symptoms common among 19th-century English hat makers, who inhaled mercury vapors when they used a mercurous nitrate solution to cure furs. Many hatters developed severe muscle tremors, distorted speech, and hallucinations as a result. Sixty-eight people died and hundreds were made ill or born with neurological defects in Minamata, Japan in the 1950s and 1960s after a chemical company dumped mercury into Minamata Bay and families ate fish from they bay. Recently, doctors have reported symptoms including dizziness and blurred vision in healthy patients who ate significant quantities of high-mercury fish such as tuna (footnote 5).
Developed countries in North America and Europe are largely responsible for the global build-up of mercury in the environment over the past century. They have begun to decrease their emissions over the past two decades in response to the recognized environmental threat. However, emissions in Asia have been rapidly increasing and it is unclear how the global burden of mercury will evolve over the coming decades. Because mercury is transported on a global scale, its control requires a global perspective. In addition, the legacy of past emissions through re-emission and mercury accumulation in ecosystems must be recognized.