WASHINGTON — A new study further bolsters concerns that pollution blowing across the Pacific Ocean from China and other rapidly developing Asian nations may swamp efforts to clean up the air in the Western United States and make it difficult for states and cities to meet federal standards.
The study, based on 100,000 measurements over 25 years and a computer model tracking air-flow patterns, found that during the spring, ozone from Asia reaches Washington, Oregon, California and other states west of the Rocky Mountains.
For the first time, the study links ozone in the air above the United States with Asian pollution, said Dan Jaffe, a professor of atmospheric and environmental chemistry at the University of Washington-Bothell and one of the study's authors.
"It is possible that emissions from emerging economies like China, with relatively limited emissions controls, are outpacing reductions in the developing countries," the report concludes. It says that the Asian emissions may "hinder the USA's compliance with its own ozone air-quality standard."
Previous studies have detected such pollutants from Asia as mercury, soot and PCBs reaching the United States. A National Academy of Sciences study last year pointed to increasing unease among regulators about a growing problem.
"Any air pollutant with an atmospheric lifetime of at least three to four days may be transported across most of a continent, a week or two may get it across an ocean, a month or two can send it around the hemisphere and a year or two may deliver it anywhere on Earth," the National Academy of Sciences said last year.
The academy's new report, prepared by the National Research Council, says the problem involves not only trans-Pacific pollution but also trans-Atlantic pollution, with emissions from the United States reaching Europe. The study zeroed in on ozone, particulate matter, mercury and persistent organic pollutants, which have been tracked by ground-based monitors, airborne monitors and satellite-borne sensors.
Among the federal agencies that are interested in the issue are the Environmental Protection Agency, the National Oceanic and Atmospheric Administration, NASA and the National Science Foundation.
The academy called for increased "fingerprinting" of pollutants, so that it's easier to locate their sources, and more detailed studies of emission totals and the atmospheric conditions that spread the pollution.
"The relative importance of long-range pollutant contributions from foreign sources is likely to increase as nations institute stricter air-quality standards that result in tougher emission controls on domestic sources," the academy's report says.
The study on ozone from Asia, authored by an international group of scientists, appears in the Jan. 21 issue of the journal Nature. It comes as the EPA is considering tightening ozone standards.
Ozone is the main ingredient in smog, which can cause health problems that range from burning in the eyes and throat to pulmonary inflammation and increased risk of heart attack. It's created when sunlight mixes with oxygen and nitrogen from vehicle tailpipes and other sources of combustion.
The study focused on an area two to six miles above the Earth known as the mid-troposphere. Pollution in the troposphere could affect the ground level.
While emissions of nitrous oxide, a precursor compound in ozone, have declined in the United States by about one-third since 1985, the study found that ozone levels had increased by 29 percent over the same period.
The study notes that from 2001 to 2006, ozone precursor emissions in east Asia were up 44 percent, and 55 percent in China.
"The changes we have seen over the past 25 years coincide with when China was transforming itself into an economic powerhouse," Jaffe said.
The study didn't pinpoint which Asian countries the ozone might be coming from, Jaffe said. Among the possibilities are China, India and Vietnam.
"What we can say is there has been a strong and significant increase in ozone in the mid-troposphere in the West and it doesn't seem the U.S. is contributing to the increase," said Owen Cooper, a research scientist at the University of Colorado attached to NOAA's Earth System Research Laboratory in Boulder, Colo.
Cooper said the study found that Asian emissions were adding to the ozone pollution in the Western United States, but scientists couldn't say by how much and which countries were involved.
"This is only the first step," Cooper said of the study.
The EPA, which regulates emissions through the Clean Air Act, is looking at international ozone transport, the agency said in an e-mailed statement.
"However, as the National Academy of Sciences concluded in its recent report, our current ability to fully characterize the impact of foreign sources on air quality in the United States is somewhat limited," the statement said.
The EPA said it was too soon to tell how the latest study might affect ozone standards.