When the sky turned white last April, Thomas Cahill at the University of California at Davis knew the great Chinese dust cloud he was expecting had arrived. He and a number of other researchers had been following it through satellite data since it rose out of the Gobi Desert a few days earlier.
Their feat typifies how, for the first time, scientists are proving that pollution from one continent can affect air quality on another.
Riding a channel of air that forms under specific atmospheric conditions, Asian pollution can cross the Pacific Ocean in four to 10 days, adding to local pollution, says Dan Jaffe of the University of Washington at Bothell. While he adds that the pollution is usually very diluted by the time it reaches North America - and there is much more research to be done - the findings may add to the call for global pollution controls, as nations try to improve their air-quality standards.
Dr. Jaffe says that the dust-cloud study and other research presented this week at a meeting in San Francisco of the American Geophysical Union represent the "first time anyone has [actually] documented" that pollution released on one continent can travel all the way to another continent.
Giving an overview of the research at a press conference, Jaffe noted that Asian pollution occasionally has significant impact in North America. Fine particles settling out of the Chinese dust cloud raised pollution levels to two-thirds of the Environmental Protection Agency air-quality standard in a number of locations over the western United States. Jaffe added that even low levels of intercontinental contamination can enhance pollution from local sources.
Asian pollution comes from many different sources. Some is desert dust. Some is industrial. Some is from pesticides. Jaffe and several colleagues found clear signs of industrial pollution in measurements taken March 29, 1997, at the Cheeka Peak Observatory in Washington. They found contamination from the burning of fuels, for example. Furthermore, extensive research into prevailing meteorological conditions show the pollution came from Asia, Jaffe says.
Some effects of long-range pollution are subtle. George Murray and colleagues at the University of Washington are monitoring conditions at the West Twin Creek watershed in Olympic National Park. They found substantial increases of nitrate, sulfate, and acidity in precipitation in four of the past six years. In this case, they can't prove the pollution came from Asia. But they suspect that it did. Whatever the source, they are finding that low levels of long-range pollution are making the local streams acidic.
Jaffe stresses that scientists still have a lot to learn about intercontinental pollution - especially Asian pollution. They need to find sources and identify the full range of contaminants. They need to learn when, where, and how often significant pollution reaches North America. "What we know for sure is that we see pollutants coming from Asia under certain specific conditions," he says.
Those conditions involve a low pressure system over the Aleutians and high pressure near Hawaii. These set up an air flow that channels Asian pollutants to North America. The winds blow at heights between 6,000 and 10,000 feet. They are not the jet stream, which is higher.
In tracking some of the pollutant flow, scientists are getting help from an unexpected source. An instrument called TOMS (Total Ozone Mapping Spectrometer) has been keeping track of the ozone layer for many years. A couple of years ago, scientists discovered it also could track dust and other aerosols. Jaffe says this "now has become a very powerful tool" for intercontinental pollution research.
Douglas Westphal from the Naval Research Laboratory in Monterey, Calif., is looking forward to using this for detailed monitoring of dust flows. He said spectacular events such as last April's Chinese dust cloud are not the whole story. There may be many more smaller episodes. "I think we're being bombarded by the dust more than we expected," he said.