Mercury From China Rains Down on California
SANTA CRUZ, California, December 20, 2002 (ENS) – Industrial emissions in Asia are a major source of mercury in rainwater that falls along the California coast, a new study suggests.
The mercury in rainwater is not in itself a health threat, but mercury pollution is a problem in San Francisco Bay and other California waters because the toxic element builds up in the food chain. State regulatory agencies are looking for ways to reduce the amount of mercury entering the state’s waters from various sources.
It is not just the mercury itself but a whole cocktail of atmospheric pollutants that contribute to the deposition of mercury in rainfall. Elemental mercury behaves as a gas in the atmosphere and is not washed out in rain until it has been oxidized into a charged ionic form that can be captured by water droplets.
Ozone, a major component of urban and industrial smog, plays a key role in this oxidation process, said Douglas Steding, lead author of a paper published Thursday in the online edition of the “Journal of Geophysical Research – Atmospheres.” The report by Steding and other researchers from the University of California, Santa Cruz (UCSC) will appear in a later print edition of the journal.
“There is a relatively large reservoir of mercury in the atmosphere, and it’s the rate of oxidation that determines how much of it gets deposited in rainfall,” Steding said.
Mercury is a trace contaminant of most coal, and emissions from coal burning power plants are a major source of mercury pollution in many parts of the world. In the Pacific Basin, the main source of atmospheric mercury is coal combustion in China.
China relies on coal as a fuel and accounts for about 10 percent of the total global industrial emissions of mercury.
Air pollution in China also generates ozone, which peaks during the winter due to increased fuel consumption for heating. Air loaded with mercury and ozone moves off the continent into the Western Pacific, where it is incorporated into developing storms.
“The mercury we measured in rainwater results from a combination of mercury emissions and ozone production, as well as meteorological factors – the storm tracks that transport the pollutants across the Pacific,” Steding said.
Steding collected rainwater samples at two sites in central California: on the coast at UCSC’s Long Marine Laboratory and at Moffett Field near San Jose, on the inland side of the Santa Cruz Mountains. For each rainfall event, the researchers used air mass trajectories calculated by a national climate lab to trace the movement of the storms across the Pacific from Asia.
Rainwater collected at the coastal site showed the background concentrations of mercury in storms as they arrived off the Pacific Ocean. Those measurements were about three times higher than estimates of the natural, preindustrial level, Steding said.
Rainwater from the inland site showed mercury concentrations 44 percent higher than at the coastal site. Steding attributed the difference between the two sites to ozone in Bay Area smog, rather than local emissions of mercury.
“There is a local influence of urban smog on the mercury oxidation rate. We see a background signal of mercury blowing off the Pacific, then a local enrichment that’s probably due to urban smog,” Steding said. “If we want to reduce mercury deposition, it’s not enough to shut down local emissions of mercury, because other pollutants influence how much of the mercury in the atmosphere ends up in rainwater.”
Steding said people should not worry about health effects from the mercury in rainwater, because the concentrations are very low. But the deposition in rain does add mercury to surface waters, where the toxin enters the food chain and builds up to high levels in certain kinds of fish.
State health officials have issued advisories warning people not to eat fish from more than a dozen bodies of water in California, including San Francisco Bay.
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Posted by: Paul on December 21, 2002 at 14:21:11