Mercury & Heavy Metal Rains

Why the Mercury Falls

Heavy-metal rains may trace to oxidants, including smog

by Janet Raloff

Science News  Feb 1, 2003  vol 163 # 5

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

The melting of Barrow's annual polar ice and snow

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.

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.

MERCURIAL RAINS. Downpours, like this one crossing 

the Everglades, effectively scrub water-soluble mercury from the air, thus contaminating the food chain.

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.