Major cities in the Pearl River Delta have seen a year-on-year increase of fine particulates in air pollution during the third quarter, the national environmental watchdog said on Wednesday.

The Ministry of Environmental Protection released its quarterly report on air quality in 74 major cities, saying the concentration of PM2.5 - particulates with a diameter of less than 2.5 microns –increased 16 percent year-on-year in the third quarter in the Pearl River Delta.

The concentration of PM10 also increased by 7.1 percent than the same period last year, Luo Yi, head of the environment monitoring department under the ministry said.

The increase of PM2.5 and PM10 in the southern region may be related to strong sunshine during the summer which accelerated chemical reactions to generate more fine particulates into the air, said Li Hong, a researcher of the China Research Academy of Environmental Sciences.

In the third quarter, seven cities from Hebei province, a major iron and steel producer in China, were listed in the top ten with severe air pollution, including Xingtai, Tangshan and Shijiazhuang, the report said, adding that Xingtai city had the worst air quality.

Less than half the days (45.7%) between July and September failed to reach the national standards on air quality in Beijing but there were three days with heavy smog.

On average, the Beijing-Tianjin-Hebei region, though still having severe air pollution, has reduced major air pollutants steadily, the ministry’s Luo said.

The concentration of PM2.5 in the trilateral region has been reduced by 18.5 percent year-on-year, the report said.