Ozone from Asia is wafting across the Pacific on springtime winds and boosting the amount of the smog-producing chemical found in the skies above the Western United States, researchers said in a study released Wednesday.

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.

The study, led by Owen R. Cooper, an atmospheric scientist at the University of Colorado, examined nearly 100,000 observations in the free troposphere — the region two to five miles above ground — gathered from aircraft, balloons and ground-based lasers.

It found that baseline ozone — the amount of gas not produced by local vehicles and industries — has increased in springtime months by 29% since 1984. The study has important implications both for the curbing of conventional pollution that damages human health and for controls on greenhouse gases that are changing the planet’s climate, experts said.

It shows the need, said Mary Nichols, chairwoman of the California Air Resources Board, for a transformation of global energy and transportation systems. “Atmospheric scientists keep finding more evidence that pollutants travel around the globe and move up and down as they travel,” she said. “There is not a bright line separating greenhouse gases from regular air pollution.”

The study, co-authored by researchers from nine institutes in the U.S. and abroad, is only a first step in understanding the complexities of cross-border pollution, Cooper said. More research will be needed to probe the sources of ozone at ground level and at other times of the year. The researchers began with the free troposphere because it is easier to eliminate local sources from baseline ozone calculations. They chose the months of April and May because that is when winds from Asia are strongest.