As PM2.5 Falls, Ozone Pollution Rises In China

Fine particulate matter in the air, while harmful for health, act as a sponge for free radicals that help produce ozone, according to research by scientists in China and the US.

AsianScientist (Jan. 10, 2019) – Scientists in China and the US report that decreased levels of fine particulate matter (PM2.5) in the air are associated with higher concentrations of harmful ozone. They published their findings in the Proceedings of the National Academy of Sciences.

In early 2013, the Chinese government declared a war on air pollution and began instituting stringent policies to regulate the emissions of PM2.5 pollutants. Cities restricted the number of cars on the road, coal-fired power plants reduced emissions or were shuttered and replaced with natural gas power plants. Over the course of five years, PM2.5 concentrations in eastern China have fallen by nearly 40 percent.

Meanwhile, the number of air quality monitoring stations across the country has grown to over 1,000, allowing researchers to collect unprecedented amounts of environmental data. Sifting through that data, researchers from the John A. Paulson School of Engineering and Applied Sciences (SEAS), US, and the Nanjing University of Information Science & Technology (NUIST), China, were surprised to discover that while PM2.5 pollution is falling, harmful ground-level ozone pollution is on the rise, especially in large cities.

Ozone is the main ingredient in smog and is formed through a series of chemical reactions, starting with the oxidation of volatile organic compounds (VOCs). This reaction forms chemical radicals, which drives reactions among oxides of nitrogen (NOx) and VOCs to produce ozone in the presence of sunlight. Both NOx and VOCs are emitted from fossil fuel combustion, and VOCs can also be emitted from industrial sources.

The researchers found that particulate matter acts like a sponge for the radicals needed to generate ozone pollution, sucking them up and preventing them from producing ozone.

“There was so much particulate matter in Chinese cities that it stunted the ozone production,” said Professor Daniel Jacob of SEAS, the co-corresponding author of the study.

But the rapid reduction of PM2.5 dramatically altered the chemistry of the atmosphere, leaving more radicals available to produce 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.”

As it turns out, when it comes to the war on air pollution, chemistry is a formidable foe, said the researchers.

The article can be found at: Li et al. (2018) Anthropogenic Drivers of 2013–2017 Trends in Summer Surface Ozone in China.


Source: Harvard School of Engineering and Applied Sciences; Photo: Pixabay.
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