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An article published on Monday in Nature Geoscience argues that the reason the American West is falling short on ozone pollution reduction may have less to do with their own efforts, and more to do with China.

NASA researchers teamed up with scientist from the Netherlands to conduct the study, using a combination of satellite data and computer modeling to reach their conclusion. In a statement, NASA explained why the scientists undertook the study at all:

Western U.S. emissions of ozone-forming pollutants decreased by 21 percent. The benefits of this large decrease will continue to accumulate for many years, like compound interest. By 2010, however, the decrease should have created a drop of more than two percent in mid-tropospheric ozone. Instead, there was no drop at all.

Researchers found that the concentration of ozone in the tropospheric ozone over China increased by seven percent from 2005 to 2010, both because of a rise in Chinese ozone emissions and because of how ozone travels in the atmosphere. That increase led to a 43 percent offset of U.S. efforts to cut down on ozone pollution.

The mid-tropospheric zone is roughly 10,000 to 30,000 feet above ground level, NASA explains. Ozone in that area causes a greenhouse effect on Earth, making it a major contributor to global warming. According to study co-author Jessica Neu, some of what their analysis shows is a result of ozone that had been in the stratosphere descending into the troposphere—a naturally occurring process. "The large contribution from the stratosphere is part of a natural up-and-down cycle of upper-atmosphere winds. We know pretty well what will happen to the stratospheric contribution in the next few decades; it will continue to go up and down every two years or so," she said.

But, she added, China's pollution played a significant role in our own ozone woes: "The contribution from China increased steadily throughout the study." Another co-author, the Royal Netherlands Meteorological Institute's Willem Verstraeten, put it more bluntly: "The dominant westerly winds blew this air pollution straight across to the United States…In a manner of speaking, China is exporting its air pollution to the West Coast of America."


This is not the first time that China has affected the United States' environment.

Back in March, NASA scientists suggested that pollution in Asia could have been one of the reasons for record-breaking cold winters. Researcher Yuan Wang said at the time, "We found that pollution from China affects cloud development in the North Pacific and strengthens extratropical cyclones." NASA added:

Wang thinks the cold winter that the U.S. East endured in 2013 probably had something to do with these stronger extratropical cyclones. The intense storms could have affected the upper-atmosphere wind pattern, called the polar jet stream.


And a study conducted by researchers at UC Irvine and others that was published last year also found that China's pollution travels to the American West. Rather than focus on satellite data, however, the authors of that paper looked at the effect of China's robust (and smoggy) factory production on Americans. UC Irvine explained at the time:

Los Angeles experiences at least one extra day a year of smog that exceeds federal ozone limits because of nitrogen oxides and carbon monoxide emitted by Chinese factories making goods for export, the analysis found.

Co-author Steve Davis, however, pointed out that the U.S. is far from blameless. He said in a release: "We’ve outsourced our manufacturing and much of our pollution, but some of it is blowing back across the Pacific to haunt us…Given the complaints about how Chinese pollution is corrupting other countries’ air, this paper shows that there may be plenty of blame to go around.”


Fair enough.

Danielle Wiener-Bronner is a news reporter.