The smog-filled Beijing-Tianjin-Hebei region has seen improvement in combating particulate pollution, such as PM2.5, since 2013, but faces worsening ground-level ozone pollution during the summers, a challenge many developed countries share, a new report from Peking University says.

The 13 cities in the Beijing-Tianjin-Hebei region saw the annual average concentration of ground-level ozone increase last year by 13.1 percent, compared with the level in 2013, the report says.

The report was compiled by the Center for Statistical Science and the Guanghua School of Management, both based at Peking University, using data released from the 73 State-controlled air quality monitoring stations and 25 meteorological stations in the region between March 2013 and May 2015.

"The analysis has clearly shown the sharp decline of the major airborne pollutants since 2013, especially of particulate matter, because of a package of strong control measures initiated in past years works," Chen Songxi, director of the statistics center and survey leader, said on Aug 8.

The annual average concentration of PM2.5 decreased last year by 27 percent since 2013, and PM10 fell by 31 percent last year compared with the 2013 level, according the report, released on Aug 5.

PM2.5 and PM10 refer to fine particulate matter, with diameters measured in microns, and are regularly monitored.

The annual levels of sulfur dioxide and carbon monoxide dropped dramatically in the past four years, and the nitrogen dioxide concentration dipped.

But among other major pollutants, ground-level ozone concentration has increased in the 13 cities between 2013 and 2016, the report says.

"In summers, this invisible pollution worsened, and governments should set reduction targets to control it, like they did in combating PM2.5," said Chen.

High ozone concentration at ground level could lead to breathing problems, lung diseases and asthma, the World Health Organization has said.

Since May 1, Beijing has had 53 days with excessive ground-level ozone concentration, 7 percent more than the annual average in the same period of the past five years, says Dong Xin, an engineer at the Beijing Environmental Monitoring Center.

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(China Daily European Weekly 08/11/2017 page14)