Why the Mercury Falls
Heavy-metal rains may trace to oxidants, including smog
Janet Raloff
In the mid-1980s, some researchers in the northern Midwest,
Canada, and Scandinavia began reporting alarming concentrations
of mercury in freshwater fish. Curious about Florida's
largemouth bass and other finned delicacies, state scientists
there began assaying lake fish. Thomas Atkeson, then a Florida
state wildlife biologist, recalls that most of the fish he
examined fell just under the limit then recommended by the Food
and Drug Administration. "We were scratching our heads as to
whether this was a big deal," he recalls, until his team reached
the Everglades. In these wetlands, mercury contamination of fish
routinely averaged more than twice the concentrations seen
elsewhere in the state. Indeed, their mercury values were among
the highest ever reported for U.S. freshwater fish.
"There was no quibbling that these levels were high and a
potential health concern to humans and wildlife," Atkeson says.
Eating mercury-tainted fish can trigger a variety of problems,
ranging from hair loss and chronic fatigue in adults to nervous
system impairment of fetuses and children (See
http://www.sciencenews.org/20021221/food.asp).
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IN COLD LIGHT. The melting of Barrow's
annual polar ice and snow can release huge quantities of
pent-up halogens. In sunshine, these chemicals transform
mercury—found throughout the atmosphere—into a form that
readily falls onto land and water.
K. J. Scott
/ U. Manitoba |
When a study of water entering the Everglades showed that
feeder streams weren't responsible for the mercury excess, "we
realized, astonishingly, this was an air-pollution problem,"
says Atkeson, now the coordinator of mercury research for the
Florida Department of Environmental Protection in Tallahassee.
Subsequent data confirmed that 95 to 99 percent of the mercury
entering the Everglades each year comes from the air, so Florida
called in atmospheric scientists to determine why the Everglades
had become a mercury hot spot.
Efforts by those researchers are finally paying off in
explaining Florida's problem and, ironically, mercury pollution
as far away as the ice and water at Earth's poles.
Mercury taints the atmosphere worldwide, but there are large
variations in how much of it drops onto land or water at any
location. Recent experiments have begun identifying oxidizing
gases, such as ozone and molecules containing the halogens
bromine and chlorine, as triggers for that mercury fallout.
Which oxidants dominate that process appears to depend on the
environment, the season, the altitude of the airborne mercury,
and even the amount of daylight.
Only in the past 5 years have scientists seriously considered
that such gaseous oxidants might affect mercury fallout.
Previously, they knew that this metal was spewed largely from
smokestacks but were puzzled by why it fell out of the
atmosphere where it did. Although the magnitude of atmospheric
mercury oxidation and fallout is still hard to quantify, the
recent findings suggest its control could prove difficult and
politically thorny—because limiting mercury's fallout may hinge
on better controlling regional or even international emissions
of not just that metal but also sulfates, nitrates, and other
air pollutants.
For instance, mercury fallout in some areas may turn out to
depend on smog as much as on how much of the metal is being
released, says Douglas J. Steding, a geochemist who's now
studying law at the University of Washington in Seattle. Indeed,
the skies already hold so much mercury that even if industrial
emissions of the metal ended tomorrow, significant fallout of
the pollutant might persist for decades, he notes.
Quicksilver skies
Mercury enters the air easily. It's released when coal is
burned, gold is mined, some chlorine is manufactured, and even
when a fluorescent lightbulb breaks. Some 99 percent of the
airborne metal is elemental. Fairly insoluble and unreactive in
this form, it can circumnavigate the globe for up to 2 years.
What's contaminating the Everglades, therefore, may have
originated in Miami, India, or Siberia.
However, atmospheric chemists have discovered that when
elemental mercury encounters certain oxidants, it changes into
so-called reactive gaseous mercury. Unlike the element, this
form is both highly reactive and water soluble, so it remains
airborne only hours to days and falls—in rain or snow or
attached to dust—near where it's formed. In a lake or ocean,
bacteria transform it into methylmercury, the harmful form of
the metal that fish and, in turn, people and other predators
accumulate in their tissues.
When it comes to triggering the transformation of elemental
mercury, all oxidants are not equal. Anthony Hynes of the
University of Miami (Fla.) recently found that the hydroxyl
radical—abundant in the atmosphere and normally considered a
strong oxidant—is a poor oxidizer of mercury except perhaps in
extremely cold conditions, such as high in Earth's lower
atmosphere.
On the other hand, observes Steve Lindberg of Oak Ridge
(Tenn.) National Laboratory, certain halogen radicals—reactive
compounds containing bromine or chlorine—rapidly and efficiently
transform elemental mercury to the reactive gaseous form. It so
happens that sea spray and melting polar ice release especially
large quantities of these halogen radicals.
Working in Barrow, on the north-central coast of Alaska,
Lindberg and his colleagues correlated the buildup of these
halogens during the Arctic spring with a dramatic, localized
depletion of elemental mercury in the air. In the March 15, 2002
Environmental Science & Technology, they showed that the
missing elemental mercury had been oxidized; roughly 35 percent
remained airborne, and the rest fell onto the ground. In fact,
the surface snow proved so rich in oxidized mercury "that we
initially had a hard time believing the data," he recalls.
Peak readings of reactive gaseous mercury in the air at this
remote site ran to 1,000 picograms per cubic meter, says
Lindberg, "or up to 100 times what we typically find in
industrial areas of the eastern United States."
Polar extremes
Such findings demonstrate the natural vulnerability of polar
sites to mercury fallout. During the many weeks of total
darkness at Barrow, chemical precursors to the oxidants appear
to build up in the air, Lindberg says. Reactive gaseous mercury
remains undetectable until Arctic sunrise occurs in late
January. Then, he says, "Boom!"—the light triggers mercury
oxidation at a skyrocketing pace. Production of reactive gaseous
mercury "is just screaming as you go from January through May,"
when Barrow begins experiencing 24-hour sunlight, says Lindberg.
Measurements by other researchers at Arctic sites further
inland show less mercury pollution, indicating that the heavy
fallout may be restricted to the near-coastal environment and
parcels of open ocean where floes of annual ice melt. Halogen
impurities concentrate on the surface of ice crystals and
vaporize before the snow or ice begins to melt, Lindberg
explains.
Ralf Ebinghaus of the GKSS Research Center in Geesthacht,
Germany, and his colleagues have observed similar fallout of
reactive gaseous mercury at the Neumayer Research Station in
Antarctica. Again, it begins with the polar sunrise and
continues through the austral spring when generation of airborne
halogens is high.
In the Jan. 1 Environmental Science & Technology,
Ebinghaus' international team offers the first report of a
second, Antarctic-summertime peak in mercury fallout. Beginning
after much coastal sea ice has melted, this peak probably
results from mercury-oxidizing pollution drifting in from
industrial areas to the north, he says.
Even as a coastal phenomenon, Lindberg estimates that fallout
of oxidized mercury could still amount to "hundreds of tons per
year" in the Arctic and Antarctica. In large areas of the polar
seas, bacteria probably start the metal on its way up the food
chain, he says. Indeed, Lindberg notes, such events could
account for the high concentrations of methylmercury that
naturalists have measured in polar bears.
Temperate fallout
Recently, scientists collected oxidized mercury over the
temperate Atlantic Ocean. There, they encountered substantial
concentrations of reactive gaseous mercury—not predominantly at
low altitudes typical of polar regions, but mostly in the lower
atmosphere's upper reaches, at heights up to 3,000 meters,
report Robert K. Stevens, who works with Atkeson at the Florida
Department of Environmental Protection, and Matthew S. Landis of
the Environmental Protection Agency in Research Triangle Park,
N.C.
 |
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MERCURIAL RAINS. Downpours, like this one
crossing the Everglades, effectively scrub water-soluble
mercury from the air, thus contaminating the food chain.
D. Scheidt |
They have since turned to measuring elemental and oxidized
mercury at a ground station on Hawaii's Mauna Loa, where they
can sample air at 4,000 m above sea level. Over the past year,
they've seen wide swings in air concentrations of oxidized
mercury there. Much of it appears to be attached to dustlike
particles, which may foster the metal's fallout.
"The elemental mercury completely disappears for long
periods," Stevens finds. When that happens, he says, "reactions
are going on that are producing a water-soluble form of mercury
that can contaminate the oceans of the world.
In smog-chamber experiments, he and Landis showed that
sunlight activates certain airborne halogen compounds to convert
elemental mercury to reactive gaseous mercury. If the main
source of the halogens reacting with elemental mercury is sea
spray, Stevens says, this mechanism might increase
concentrations of the metal in the water of warm coastal areas,
such as Florida.
However, he points out that halogen oxidants also form during
the sunlight-triggered breakdown of industrial chemicals such as
chlorofluorocarbon refrigerants. What would be the source of
such industrial oxidants at Mauna Loa? Plumes of pollution from
Asia, Stevens suspects.
Ozone, too
California researchers have been examining the impact of
Asian air masses that travel to the U.S. mainland. In this case,
ozone and several other compounds carried in that air appear to
directly oxidize elemental mercury.
Steding and A. Russell Flegal of the University of
California, Santa Cruz, measured mercury in coastal rains and
compared events when the air had been relatively clean with
storms when plumes of ozone-rich Asian pollution were present.
In the Dec. 19, 2002 Journal of Geophysical Research,
they report that when pollution from China coincides with a
California rainstorm, up to nine times as much mercury rains out
of the atmosphere as it does during other storms.
Eight years ago, when Lindberg reported data suggesting that
ozone might be oxidizing elemental mercury in ambient air,
"nobody believed me," he recalls. Now, he observes, a growing
number of scientists are indicting ozone pollution as a
potentially important factor in creating reactive gaseous
mercury.
After years of study, the Everglades' high mercury fallout
appears to have resulted from many unusual factors, such as its
shallow waters and South Florida's especially large number of
municipal- and medical-waste incinerators, which emitted mercury
at a relatively low height. In recent years, new controls on
their emissions cut mercury releases to 1 percent of those in
the mid-1980s. "Largemouth bass are now carrying around
one-third of the mercury that they had in 1990," says Atkeson.
As it turns out, for the Everglades, "everything was in the
right juxtaposition to create an exaggerated response," he
observes. However, because of the wetlands' heavy mercury
contribution from local emitters. cleaning up their releases
"allowed these waters to clean up more rapidly than you would
expect with other water bodies. So, the Everglades is not a good
example of how much success you could have with lakes elsewhere
in North America," Atkeson says. "But it does show there is hope
for them." The newly recognized role of other pollutants in
mercury's fallout increases the challenge. Indeed, Steding says,
that task will likely require political and diplomatic solutions
that transcend national borders.
Mercury Retirement
The ultimate solution may be to store the metal, not sell it
To limit mercury's fallout, society must reduce the metal's
release. Environmentalists have proposed limits on mercury use,
but another idea gaining interest is the collection of excess or
recovered mercury for long-term—potentially permanent—storage.
Indeed, at a United Nations–sponsored meeting on mercury in
Geneva last September, the U.S. State Department supported a
proposal asking nations to formally consider "retiring excess
mercury through long-term waste management (terminal storage)."
Not so long ago, mercury was mined throughout the world to
meet a growing demand for the metal. What little was retired
from use often ended up in landfills, from which it can escape
into the atmosphere (SN: 7/7/01, p. 4:
http://www.sciencenews.org/20010707/fob1.asp).
But in the 1980s, biologists recognized the toxic impact of
chronic, low-level mercury exposure. Now, landfills frequently
prohibit products containing mercury. Moreover, use of the metal
is falling as recovery programs mushroom. For instance, U.S.
mercury demand has decreased to 20 percent of its 1980 level at
the same time that recycling of the metal has nearly tripled,
notes Michael T. Bender, director of the Mercury Policy Project
in Montpelier, Vt. Today, industrial countries—including the
United States—usually end up with more mercury than they need.
At issue is what to do with the excess.
A Department of Defense strategic stockpile of almost 5,000
metric tons of mercury—holdings no longer deemed
essential—constitutes the nation's largest store. Another 3,000
metric tons of mercury is employed in aging chlorine-production
facilities using a so-called chlor-alkali process. These plants
are expected to shut down in the coming decades, says Art Dungan
of the Chlorine Institute in Rosslyn, Va., an industry group.
Until recently, the Defense Department and the owners of
retiring chlor-alkali plants had expected to sell their mercury.
Buyers of such low-cost recycled mercury tend to be in the
developing world, where few regulations exist to encourage only
essential uses and careful management of the toxic material,
observes John Gilkeson of the Minnesota Office of Environmental
Assistance in St. Paul.
To keep mercury from re-entering the atmosphere, such sales
must be prohibited, he argues. Indeed, Bender advocates that the
United States halt mercury recycling and trade—"and provide
options for long-term, monitored storage."
The Chlorine Institute agrees, in part. "It may be prudent
for the United States to consider a national policy to identify
which worldwide outlets are acceptable," Dungan says, and halt
mercury trading with unacceptable ones. Last May, his institute
said that the chlor-alkali industry is willing to work with
federal officials on "how [it] can best ensure that any surplus
mercury from idled or converted sites is placed into . . .
permanent storage." However, Dungan says his industry wants
Uncle Sam to take possession—and responsibility for—its mercury.
The states want that, too. The Quicksilver Caucus—a
consortium of state officials—has begun lobbying for centralized
storage of excess mercury. For legal and financial reasons,
Gilkeson says, "the states believe this must be at the federal
level."
To date, Bender notes, the only step toward retiring mercury
is storage, last year, of 80 metric tons of the metal from a
shut-down Maine chlor-alkali plant. Environmental groups
convinced the plant's owner not to sell the mercury but to ship
it to a private company for safekeeping for at least 5 years.
****************
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References:
2002. Report of the Global Mercury
Assessment Working Group on the work of its first meeting.
Global Mercury Assessment Working Group. Sept. 23. Available
online at
http://www.chem.unep.ch/mercury/WGmeeting/K0262760-final-report.doc.
2002. Chlor-alkali industry principles
concerning the retirement of mercury. The Chlorine Institute.
May 9. Available at
http://www.cl2.com/whats_new/Hg%20Retirement%20Discussion%20-%20Industry%20Principles%20final.pdf.
2001. Mercury Update: Impact on Fish
Advisories. EPA-823-F-01-011. June. U.S. Environmental
Protection Agency Fact sheet. Available online at
http://www.epa.gov/ost/fishadvice/mercupd.pdf.
Bauer, D. . . . and A.J. Hynes. In press.
Gas phase elemental mercury: A comparison of LIF detection
techniques and study of the kinetics of reaction with the
hydroxyl radical. Journal of Photochemistry and Photobiology
A: Chemistry.
Ebinghaus, R., et al. 2002.
Antarctic springtime depletion of atmospheric mercury.
Environmental Science and Technology 36(March 15):1238-1244.
Hightower, J.M., and D. Moore. In press.
Mercury levels in high-end consumers of fish. Environmental
Health Perspectives. Abstract available at
http://dx.doi.org/10.1289/ehp.5837.
Lindberg, S.E. . . . M.S. Landis, R.K.
Stevens, et al. 2002. Dynamic oxidation of gaseous
mercury in the arctic troposphere at polar sunrise.
Environmental Science and Technology 36(March 15):1245-1256.
Steding, D.J., and A.R. Flegal. In press.
Mercury concentrations in coastal California precipitation:
Evidence of local and trans-Pacific fluxes of mercury to North
America. Journal of Geophysical Research 107(D24):4764.
Abstract available at
http://dx.doi.org/10.1029/2002JD002081.
Temme, C. . . . R. Ebinghaus, et al.
2003. Measurements of atmospheric mercury species at a coastal
site in the Antarctic and over the South Atlantic Ocean during
Polar Summer. Environmental Science and Technology
37(Jan. 1):22-31. Available at
http://pubs.acs.org/cgi-bin/sample.cgi/esthag/2003/37/i01/html/es025884w.html.
Further Readings:
1997. Mercury Study Report to Congress.
EPA-452/R-97-010. December. U.S. Environmental Protection
Agency. Available online at
http://www.epa.gov/oar/mercury.html.
Johnson, J. 2002. Too much of a bad thing:
As U.S. companies end mercury use, questions mount over need to
limit world access to surplus. Chemical & Engineering News
80(July 29):22-23.
Little, M. 2002. Reducing mercury pollution
from electric power plants. Issues in Science and Technology
Online. Summer. Available at
http://www.nap.edu/issues/18.4/p_little.htm.
Lutter, R., and E. Irwin. 2002. Mercury in
the environment: A volatile problem. Environment
44(November):24.
Natural Resources Council of Maine. 2002.
HoltraChem Mercury leaving Maine for good: Agreement reached to
place mercury in safe storage, prevent export. Environews.
Sept. 6. Available at
http://www.maineenvironment.org/
mercury/holtrachem_sept62002.htm.
Raloff, J. 2002. Mercurial effects of
fish-rich diets. Science News Online 162(Dec. 21).
Available at
http://www.sciencenews.org/20021221/food.asp.
________. 2001. Landfills make mercury more
toxic. Science News 160(July 7):4. Available at
http://www.sciencenews.org/20010707/fob1.asp.
Sources:
Thomas D. Atkeson
Mercury and Applied Science
Florida Department of Environmental Protection
2600 Blair Stone Road
Mailstop 6540
Tallahassee, FL 32399-2400
Michael Bender
Mercury Policy Project
1420 North Street
Montpelier, VT 05602
Web site:
http://www.mercurypolicy.org
Arthur E. Dungan
The Chlorine Institute
1300 Wilson Boulevard
Arlington, VA 22209
Web site:
http://www.CL2.com
Ralf Ebinghaus
GKSS Forschungszentrum Geesthacht GmbH
Institut für Küstenforschung
Physikalische und Chemische Analytik
Max-Planck-Str. 1
D-21502 Geesthacht
Germany
A. Russell Flegal
WIGS Laboratory Group
Department of Environmental Toxicology
University of California
Santa Cruz, CA 95064
John Gilkeson
Minnesota Office of Environmental Assistance
520 Lafayette Rd., N.
St. Paul, MN 55155-4100
Jane M. Hightower
2100 Webster Street
Suite 418
San Francisco, CA 94115
Anthony J. Hynes
Division of Marine and Atmospheric Chemistry
Rosenstiel School of Marine and Atmospheric Science
University of Miami
4600 Rickenbacker Causeway
Miami, FL 33149
Matthew Landis
E205-03
USEPA Mailroom
Research Triangle Park, NC 27711
Steve Lindberg
Environmental Sciences Division
Oak Ridge National Laboratory
P.O. Box 2008
Oak Ridge, TN 37831-6038
Chester W. Spicer
Atmospheric Science and Applied Technology Department
Battelle Memorial Institute
505 King Avenue
Columbus, OH 43201
Douglas Steding
c/o A. Russell Flegal
Environmental Toxicology
University of California, Santa Cruz
Santa Cruz, CA 95064
Robert K. Stevens
Florida Dept. of Environmental Protection
U.S. Environmental Protection Agency
NERL, HEASD
Mail Code: E205-01
109 T.W. Alexander Drive
Research Triangle Park, NC 27709
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