Stringent winter restrictions on industry, transportation and coal consumption enabled smog-prone northern Chinese regions to meet crucial air quality targets for the 2013-2017 period.
But while concentrations of hazardous airborne particles known as PM2.5 fell, average daytime ozone levels jumped sharply, according to a study of pollution data in 33 northern cities by Peking University's Guanghua School of Management and the Center for Statistical Science.
Also known as "sunburn for the lungs," ozone is caused by the interaction of sunlight with nitrogen oxides and vast amounts of uncontrolled volatile organic compounds (VOCs) produced by burning fossil fuels. Traffic congestion is a major cause.
Ozone is one of six components of China's official air quality index. The others are sulphur dioxide, nitrogen oxides, carbon monoxide, PM2.5 and larger airborne particles known as PM10.
Ozone concentrations were up 40 percent or more in 10 northern cities from 2014 to 2017. Declines in the amounts of PM2.5 and PM10 have reduced the volume of floating particles and increased the strength of sunlight required to produce ozone, the study said.