Humans pump thousands of tons of vapor from the metallic element mercury into the atmosphere each year, and it can remain suspended for long periods before being changed into a form that is easily removed from the atmosphere.
New research shows that the upper troposphere and lower stratosphere work to transform elemental mercury into oxidized mercury, which can easily be deposited into lakes and oceans and ultimately enter the food chain. Mercury-filled fish are not safe to eat.
Biologist Seth Lyman says, "The upper atmosphere is acting as a chemical reactor to make the mercury more able to be deposited to ecosystems." This may have to do with the warming of the troposphere and stratosphere due to global warming.
Exactly how the oxidation takes place is not known with certainty but, once the transformation takes place, the oxidized mercury is quickly removed from the atmosphere, mostly through precipitation (rain) or air moving to the surface. After it settles to the surface, the oxidized mercury is transformed by bacteria into methyl mercury, a form that can be taken into the food chain and eventually can result in mercury-contaminated fish.
Some areas, such as the Southwest United States, appear to have specific climate conditions that allow them to receive more oxidized mercury from the upper atmosphere than other areas, and where the mercury settles to the surface can be thousands of miles from where it was emitted. For example, mercury from coal burning in Asia could rise into the atmosphere and circle the globe several times before it is oxidized, then could come to the surface anywhere.
Lyman says, "Much of emitted mercury is deposited far from its original sources. Mercury emitted on the other side of the globe could be deposited right at our back door, depending on where and how it is transported, chemically transformed and deposited."
In the January 24th edition of the New York Times, Anthony DePalma quotes ecologist Timothy H. Tear as saying, "You don’t see birds falling off tree limbs because they have too much mercury, but they’re not doing the job they used to."
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