More than 20 percent of cities with regular air quality monitoring exceeded national safety standards last year, mainly in the southern regions, while severe air pollution continued to bedevil the northern region, the national environmental authority reported on Thursday.

Excessive concentrations of unhealthy particulate matter was the main culprit for air pollution in the 338 monitored cities, said Luo Yi, head of environmental monitoring bureau at the Ministry of Environmental Protection.

The average concentration of PM2.5 — particulate matter with a diameter less than 2.5 microns that is hazardous to human health — was nearly 43 percent higher than the national standards, while the concentration of PM10 was more than 24 percent higher, the ministry's annual air quality report noted.

Among the 338 cities included in the report, only 73 had air quality that exceeded the national safety standards, and they were concentrated in Fujian, Guangdong and Yunnan provinces and the Tibet autonomous region.

The numbers of cities with air quality that reached national standards grew by three from 2014, Luo said.

Average concentrations of PM2.5 and PM10 in the 338 cities decreased by more than 11 percent year-on-year, the report said.

But China's vast northern and northeastern regions continued to suffer from severe pollution, the ministry said.

Thirteen cities in the Beijing, Tianjin and Hebei province corridor reached the national standards for air quality on only half of the days last year, meaning residents spent many weeks in a haze.

Seven of the 10 cities with the worst air pollution last year were in Hebei province: Baoding, Xingtai, Hengshui, Tangshan, Handan, Shijiazhuang and Langfang. Baoding, a southern neighbor of the capital, ranked at the bottom.

The other three cities on the list were Zhengzhou in central China's Henan province, Jinan in East China's Shandong province and Shenyang in Northeast China's Liaoning province.