Scientists for years have been at a loss to explain unexpectedly high levels of mercury in fish swimming the rivers and streams of areas like eastern Oregon, far away from industrial sources of mercury pollution such as coal-fired power plants.
New University of Washington research suggests mercury can be carried long distances in the atmosphere, combining with other airborne chemicals as it travels. These compounds are much more water-soluble and therefore are more easily removed from the air in rainfall.
Mercury generally is present in the atmosphere in only very small amounts compared with other pollutants, said Philip Swartzendruber, a University of Washington doctoral student in atmospheric sciences. But mercury does not break down and after it washes out of the atmosphere it can be converted to a more toxic form, methyl mercury. Even in places thought to be pristine, the more toxic form can become very concentrated as it is passed up the food chain.
"By the time mercury gets to the top of the food chain, it can increase by a factor of a million," he said. "It can go from being nearly undetectable in the air to being toxic to larger organisms."
Swartzendruber presents his team's findings Thursday during the American Geophysical Union's fall meeting in San Francisco. He is part of a team that began taking measurements of atmospheric mercury levels early this year atop Mount Bachelor, near Bend, Ore. At about 9,000 feet, the station is high enough to take readings from the bottom of the free troposphere, where substances such as mercury, carbon dioxide and ozone can travel great distances and remain for a long time. The free troposphere extends from about 5,000 feet in altitude to about 40,000 feet.
The researchers recorded mercury levels that included significant concentrations of a type called reactive gaseous mercury. "After nearly half a year of results, it is pretty clear things are going on up there," Swartzendruber said.
The researchers are only able to pinpoint a source of mercury if the air mass arrives at the observation station within a few days after the mercury is emitted. Measurements of other chemicals in the air help provide evidence of the origin, but after a few days in the free troposphere air masses dilute and can change chemically, masking their origin.
The research indicates there is a lot more mercury than previously believed that is available to be washed out of the atmosphere, even far away from industrial sources, Swartzendruber said. He noted that a large portion of atmospheric mercury comes from eastern Asia, and it can survive in the free troposphere for a year, perhaps more. But even a relatively small amount of mercury emitted in North America can settle to the Earth in remote, unspoiled regions.
The type called reactive gaseous mercury -- basic, or elemental, mercury that has combined with another substance -- is important because rain washes it out of the air very easily. It is likely that ozone, a common pollutant usually associated with smog, or other oxidants most often combine with elemental mercury to form reactive gaseous mercury, Swartzendruber said.
Contrary to what researchers previously thought, he added, high concentrations of reactive mercury can be present in the atmosphere far from industrial sources. That means either that elemental mercury can transform to the reactive variety in just a week or two, a much shorter time than scientists have believed, or that there is a large pool of reactive mercury at the top of the troposphere.
"We need to be looking for reactive gaseous mercury when we do air studies," Swartzendruber said. "It is possible it is being produced more quickly than our estimates, based on laboratory experiments."
The work continues the effort to understand how a chemical once thought to be inactive has become a global toxin. It also sheds light on why fish in streams far from people and industry are tainted with mercury.
"It's definitely a step in understanding the global cycling of mercury and how this toxin is carried by the atmosphere and removed a great distance from its source," Swartzendruber said.
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