Atmospheric conditions caused by lower-than-normal sea temperatures have kept the worst of Asia’s air pollution from drifting over Hawaii during the past dozen years or so.

That’s one of the findings in a new study, published in the journal Nature Geoscience, that solves the mystery of why Hawaii has largely escaped the growing amounts of Asia-produced ozone pollution that drift eastward with springtime winds.

A team of researchers, led by Princeton University’s Meiyun Lin, found that since the mid-1990s these shifts in atmospheric circulation have caused Asian ozone pollution reaching Hawaii to be relatively low in the spring but rise significantly in autumn.

It turns out that La Niña-like conditions, featuring lower sea temperatures in the equatorial Pacific Ocean, have weakened the springtime flow of ozone-rich air from Eurasia toward Hawaii since the 2000s, according to the study, while atmospheric circulation variability known as the Pacific-North American positive pattern works to create stronger flow of Asian pollution to Hawaii during the autumn going back to the mid-’90s.

Lin, an atmospheric chemist with the National Oceanic and Atmospheric Administration’s Geophysical Fluid Dynamics Laboratory, was the lead researcher in a study last year that showed for the first time that Asian emissions directly contribute to smog in the United States, increasing ground-level ozone pollution by as much as 20 percent.

That study also found that on days when ozone levels in American cities were higher than the maximum levels for health concern, Asian emissions pushed them over the top more than half the time.

Ozone is not directly emitted but produced from chemical reactions of air pollutants from power plants, factories, vehicles and other sources. Ozone pollution occurs in the lower atmosphere and is the main component of smog.

Exposure to ozone — a greenhouse gas that traps heat at the Earth’s surface — can worsen asthma and lung function as well as harm plant tissue and damage crops and forests.

It takes about one week for pollution from the booming industrial towns of Asia to reach Hawaii, Lin said. The smog travels across the Pacific in westerly winds that are stronger in spring than autumn. Pollutants are transported at greater speeds and over greater distances during the spring months, she said.

In the latest study, the research team used chemistry-climate models and observations, including 40 years of continuous ozone measurements from atop Hawaii island’s Mauna Loa.

As the longest, high-quality ozone record in the U.S. — and maybe the world — the Mauna Loa data provided an unprecedented opportunity to deconstruct the observed ozone changes into components driven by trends in Asian emissions and shifts in atmospheric circulation, she said.

Another study researcher, Samuel Oltmans of the University of Colorado and NOAA, said he started collecting the Mauna Loa ozone measurements at a time when it was believed that ozone was primarily created only in the upper atmosphere. But the Mauna Loa measurements, he said, helped prove that ozone could be formed in the lower atmosphere in a clean environment.

Oltmans said it was surprising that Hawaii had not seen a recent springtime boost in ozone levels even though Asian emissions were continuing to explode. Ozone production has tripled in Asia in recent decades.

While Oltmans suspected global-scale wind conditions, he said, the new study pinpointed the cause: the La Niña conditions. But that could change if sea temperatures become higher than normal under El Niño-like conditions, he said.

"If there’s a period of more and stronger El Niño conditions, more pollution could be transported to Hawaii in the spring," he said.

How much more is unknown. While it’s unlikely to create an environment in Honolulu similar to, say, smog-choked Beijing, it may raise the chances for more smoggy days, especially when winds are light.

The American Lung Association gave Honolulu an A for ozone levels in its State of the Air 2013 survey. Hawaii’s abundant wind and rain, as well as Oahu’s relatively small population and lack of heavy industry, help to keep Honolulu’s air clean.

The U.S. national threshold of health concern for ground-level ozone is 75 parts per billion by volume for an eight-hour average. During the past 40 years, ozone measured at Mauna Loa exceeded the national threshold only 84 days in total, Lin said, and the impact is smaller at lower elevations.

Lin and Oltmans conducted the research with Larry Horowitz and Songmiao Fan of Geophysical Fluid Dynamics Laboratory and Arlene Fiore of the Lamont-Doherty Earth Observatory at Columbia University.