The study, published in the journal Nature, probes a phenomenon that has puzzled scientists in the last decade: Ground-level ozone has dropped in cities thanks to tighter pollution controls, but it has risen in rural areas in the Western U.S., where there is little industry or automobile traffic.

"Ozone is a difficult gas to pin down," said Cooper, who works at the Earth System Research Laboratory of the National Oceanic and Atmospheric Administration in Boulder, Colo. "The study of intercontinental air pollution has been going on for a decade, but whether it was increasing overall was uncertain. And in places where it had spiked, along coasts and in national parks, we didn't know how much was from local sources and how much was from Asia.

Cooper said they have not yet determined exactly how much of the ozone increase comes from Asia, but they found that the increase was about twice as much when prevailing winds came from South and East Asia.

In Southern California, which has some of the highest pollution levels in the U.S, smog levels exceed health standards more than 80 days a year. Based on new studies, the EPA announced this month that it might tighten federal ozone rules.

The Environmental Protection Agency proposed the nation's strictest-ever smog limits Thursday, a move that could put large parts of California and other states in violation of federal air quality regulations.

The EPA proposed allowing a ground-level ozone concentration of between 60 and 70 parts per billion, down from the 75-ppb standard adopted under President George W. Bush in 2008.

That means cracking down further on the emissions from cars, trucks, power plants, factories and landfills. The [ozone] emissions bake in sunlight and form smog.

Though complying with the standards could cost up to $90 billion nationwide, according to the EPA, it could also save $100 billion in health costs over time.

A 65-ppb standard -- the middle of the proposed acceptable range -- would avert 1,700 to 5,100 premature deaths nationwide in 2020 compared with the 75-ppb standard, the EPA estimates.

Besides ratcheting up pressure on highly polluted parts of Southern California and the Central Valley, the revised standard would require several new areas to take measures to slash air pollution, including parts of the northern Sacramento Valley and the Central Coast that have been in compliance under the previous standards.

Unlike Eastern and Midwestern states, where much of the pollution comes from coal-fired power plants, three-quarters of California's ozone-forming emissions are from mobile sources such as cars, trucks, trains, ships, planes and construction equipment.

In the last three years, the state [California] has adopted the nation's strictest rules to control pollution from diesel engines in trucks and construction equipment, which emit nitrogen oxides, a precursor to smog.

The EPA's new standard could force the state to crack down further on vehicle pollution, on refineries and power plants, and even on volatile organic compounds coming from consumer products such as hair spray. Air districts also would be likely to increase efforts to control sprawl and force more concentrated land development.

"This is going to require us to look for new solutions," said Leo Kay, a spokesman for the California Air Resources Board. "On the ground we will be looking for where we can tighten the screws."

As for how exactly that will be done, he acknowledged, "We don't have all the answers yet."

The EPA also proposed setting a "secondary standard" to protect plants and trees from repeated smog exposure during growing season, a move environmentalists said would help national parks, forests and sensitive ecosystems.

We suggest that the observed increase in springtime background ozone mixing ratio may hinder the USA's compliance with its ozone air quality standard.