Is Asia polluting the US?

Published: Saturday 15 February 2003

industrial emissions of Asia, particularly China, are a major source of mercury in rainwater that falls along the California coast of usa. So says a new study by researchers from the Santa Cruz-based University of California (ucsc). However, the researchers acknowledge that mercury in rainwater is not a health threat.

Interestingly, it is not just the mercury itself, but a whole cocktail of atmospheric pollutants that contribute to the deposition of mercury in rainfall, admits Russell Flegal, co-author of the paper. Elemental mercury behaves as a gas in the atmosphere and is not washed out in rain until it has been oxidised 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. Air loaded with mercury and ozone moves off China towards the western Pacific Ocean, where it is incorporated into developing storms.

The researchers collected rainwater samples from two places in central California: ucsc's Long Marine Laboratory (a coastal site) and Moffett Field (an inland site). For each rainfall event, the researchers studied data pertaining to the movement of the storms. They found that rainwater collected from the coastal site had mercury concentration about three times higher than that of pre-industrial times. But rainwater from the inland site had mercury concentrations 44 per cent higher than that of the coastal site. Douglas Steding, the lead researcher, tentatively attributed these to ozone in the area's smog.

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