The Transporter: Cloud Banks Carry Mercury

When the fog rolls in off the ocean, so does mercury pollution.

A cloud bank rolls into Ben Sur, California.

A cloud bank rolls in from the ocean near Big Sur, California.

Photograph by Jane J. Lee, National Geographic

Jane J. Lee

Published December 4, 2012

The towering redwood forests along California's coast are known for the clammy fog that rolls in from the ocean almost every night. Now scientists have discovered an unwelcome stowaway in these cloud banks: mercury.

Previous research had shown that fog sampled in and around Santa Cruz, California, contained the heavy metal mercury—the silvery liquid found in old thermometers—but no one knew the source. New results presented at the American Geophysical Union conference in San Francisco today show that the mercury is likely coming from the ocean.

The minute amounts of mercury aren't enough to harm someone walking around in the fog—you could find more mercury in a can of tuna, said atmospheric chemist Peter Weiss-Penzias at the University of California, Santa Cruz. But fog is a big player in the central California coast's water cycle. And since mercury accumulates in plants and animals, getting more concentrated as it moves further up the food chain, the pollutant's presence poses a threat to the ecosystem. (Learn about Atlantic Bluefin Tuna.)

Weiss-Penzias and his colleagues measured seawater concentrations of dimethyl mercury—an unstable, gaseous form of mercury—at depths ranging from 3,300 feet (1,000 meters) to the surface of Monterey Bay this spring. They found the highest concentrations of the unstable form around 660 feet (200 meters). "Above 200 meters, you enter the zone where dimethyl mercury is less stable, so it decomposes and some of it escapes into the atmosphere," Weiss-Penzias said. (Learn about the Monterey Bay National Marine Sanctuary.)

Combining this research with measurements indicating that fog is the vehicle for 60 to 99 percent of all atmosphere-to-land mercury transmission, Weiss-Penzias said he thinks it's a good bet that the ocean is the source in California's case. He plans to collect measurements of dimethyl mercury in the atmosphere directly over the ocean to confirm his hypothesis and would also like to study other contaminants, including pesticides that could be carried around in the fog's cold embrace.

"It is a bit of a mystery where [the mercury's] coming from, but I think what we're seeing is a large-scale phenomenon that has to do with upwelling of deep ocean water along the coast," Weiss-Penzias said in a statement.

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