Science News - October 20, 2004

Rethinking atmospheric mercury

Instead of spending up to two years in the atmosphere, elemental mercury is being oxidized in a matter of months, according to a spate of recently published and soon-to-be-published studies. The new research is having a major impact on our understanding of the global mercury cycle, say the scientists involved.

The scientists agree that the shorter residence time means that mercury is deposited faster than previously estimated, but they can’t yet fully assess the impact of this change on global modeling results. Even a few months “is still more than enough time for elemental mercury to travel around the globe, so it’s still a global issue,” says modeler Ashu Dastoor at Environment Canada, the Canadian EPA, in Dorval, Quebec. “Faster rates of deposition must also mean higher rates of emission to sustain the measured level of background elemental mercury," she says. "Better estimates of global emissions and re-emissions of mercury will be essential to assess the impact of these shorter residence times on the global environment," she adds.

Mercury speciation is a critical key to understanding the mercury cycle and atmospheric deposition, says Steve Lindberg at Oak Ridge National Laboratory in Tenn. Two types of gas-phase mercury species occur in the atmosphere: Most (>95%) is elemental mercury or Hg(0), and the other important species is reactive gaseous mercury (RGM) or Hg(II). Speciation is critical because it influences how far emitted mercury will travel. Hg(0) has generally been considered to be fairly stable in the atmosphere, with an estimated tropospheric residence time of one to two years. RGM rapidly deposits out of the lower atmosphere.

New field and laboratory studies point to the role of airborne halogens and oxidants that convert elemental mercury into RGM via photochemical reactions.

Tropospheric measurements show that the abundance of RGM increases with altitude. Laboratory and field studies also suggest that certain atmospheric reactions rapidly oxidize elemental mercury to its reactive, easy-to-deposit form. For example, the halide–mercury reactions that deplete elemental mercury from both poles (Environ. Sci. Technol. 2001, 35, 434A–435A) also appear to take place in the temperate marine boundary layer. Urban pollutants, most likely ozone, are also able to oxidize elemental mercury.

The evidence is so strong that a “short half-life for elemental mercury is our working hypothesis now,” says U.S. EPA researcher Matthew Landis in Research Triangle Park, N.C.

Landis notes that EPA scientists sampling atmospheric mercury by plane off the coast of Florida in 2000 found that the levels of RGM increase with altitude. Observations started in 2001 at Hawaii’s four-kilometer-high Mt. Mauna Loa confirm the aircraft observations.

Meanwhile, Robert Mason of the University of Maryland’s Chesapeake Biological Laboratory in Solomons, Md., and his colleagues have observed high concentrations of RGM at the open-ocean marine boundary layer. Mason says that the reactions in this layer are similar to the halogen-mediated chemistry that causes Arctic mercury-depletion events. These observations have also been confirmed at a number of coastal sites, he adds.

In data gathered at Cheeka Peak Observatory at the northwestern tip of Washington state, Peter Weiss-Penzias and colleagues from the University of Washington, Bothell, carefully tracked losses of elemental mercury, which they associate with increasing levels of air pollution from Seattle and Vancouver. They speculate that urban emissions might react with elemental mercury.

Laboratory experiments and modeling studies by physical chemist Parisa Ariya at McGill University in Montreal, Canada, have produced rate constants and identified the reaction products in gas phases, aerosols, and condensed phases for reactions involving elemental mercury oxidants, including ozone and the hydroxyl radical. These estimates support residence times of several months.

The U.S. EPA and the Canadian Meteorological Service are currently evaluating the impact of the shorter mercury residence time, according to Landis. For example, EPA is developing mercury oxidation rates with a variety of halogens. These new reaction rate constants will be incorporated into EPA’s mercury model to provide a more accurate picture of the importance of local, regional, and global sources to atmospheric mercury deposition, as well as the benefits of source-reduction scenarios, he says. —REBECCA RENNER