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 f
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Contact: Vince Stricherz
vinces@u.washington.edu
206-543-2580
University of Washington
7-Dec-2005