China’s war on PM 2.5 has had a side-effect: ozone pollution

Air pollution levels in many Chinese cities have long been exceedingly high, leading to a national health crisis and dampened moods. Yet officials have not been idle. In 2013 they set about enacting far-reaching policies to reduce emissions of fine particulate matter, or PM 2.5.

Concentrations of PM 2.5 in eastern China, the country’s industrial heartland, have since fallen by nearly 40%. Time to celebrate? Yes and no.

Even as levels of fine particulate matter have declined in China, ozone levels have risen in their place. “There was so much particulate matter in Chinese cities that it stunted the ozone production,” explains Daniel Jacob, a professor of atmospheric chemistry and environmental engineering who is an author of a study on ozone pollution in Chinese cities.

Because PM 2.5 concentrations have been reduced so fast the chemistry of the atmosphere has been drastically altered, which has left more radicals available in the air for 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,” Jacob notes. “It took China four years to do what took 30 years in the U.S.”

A primary ingredient in smog, ozone is a gas composed of three atoms of oxygen (O3). Ozone at ground level is generated through a series of chemical reactions starting with the oxidation of volatile organic compounds, or VOCs. This results in chemical radicals that drive reactions among oxides of nitrogen (NOx) and VOCs to produce ozone when sunlight in present, such as during summer months.

The burning of fossil fuels leads to emissions of both NOx and VOCs. Ironically, high levels of fine particulate matter acts like a sponge for radicals that help generate ozone pollution by sucking them up, the researchers explain. Yet once PM 2.5 levels drop significantly, more ozone can be created.

Ground-level ozone pollution is especially likely to reach unhealthy levels on hot sunny days in towns and cities. Worse: wind can blow ozone away from cities, contaminating rural areas as well.

“Breathing ozone can trigger a variety of health problems including chest pain, coughing, throat irritation, and airway inflammation,” the Environmental Protection Agency in United States explains. “It also can reduce lung function and harm lung tissue. Ozone can worsen bronchitis, emphysema, and asthma, leading to increased medical care.”

And the situation is bound to get worse in China. “As PM 2.5 levels continue to fall, ozone is going to keep getting worse,” said Ke Li, a postdoctoral fellow who was first author of the study. The next step in China, the researchers say, must now involve dedicating extra efforts are needed to reduce NOx and VOC emissions so as to rein in ozone pollution.