As particulate matter has declined in China, ozone pollution has increased, new research shows. According to chemists, the first trend explains the second.

China's government has made a concerted effort to reduce particle pollution in its biggest cities, spearheading the installation of more than 1,000 air quality stations.

When researchers from Harvard's John A. Paulson School of Engineering and Applied Sciences and the Nanjing University of Information Science and Technology analyzed the wealth of data collected by China's air monitoring stations, they found evidence of two types of pollution trending in opposite directions -- PM 2.5 pollution declining, and ozone levels increasing

Ozone is the main ingredient in smog, and it is formed through a series of atmospheric chemical reactions. First, volatile organic compounds, or VOCs, become oxidized, forming chemical radicals. These chemical radicals fuel reactions between oxides of nitrogen, or NOx, and volatile organic compounds.

Both nitrogen oxides and volatile organic compounds are emitted into the air via various industrial processes. Fossil fuel combustion also releases VOCs.

According to the new research, published in the journal Proceedings of the National Academy of Sciences, particulate matter in the air helps quell ozone production by acting as a sponge that collects chemical radicals.

"There was so much particulate matter in Chinese cities that it stunted the ozone production," Daniel Jacob, professor of atmospheric chemistry and environmental engineering at Harvard, said in a news release.

With the reduction in particle pollution in the skies above Chinese cities, chemical radicals have proliferated, accelerating the production of ozone.

"We haven't observed this happening anywhere else because no other country has moved this quickly to reduce particulate matter emissions," said Jacob. "It took China four years to do what took 30 years in the U.S."

Of course, it's good that China's air has fewer harmful pollution particles than it had just a few years ago, but now researchers argue China must do more to curb the emissions of pollutants response for the formation of ozone.

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