BEIJING, Oct. 21 (Xinhua) -- The air quality in 338 Chinese cities improved year on year in September, according to the Ministry of Ecology and Environment.

The average density of PM 2.5 stood at 25 micrograms per cubic meter in September, down 16.7 percent from the same period last year.

A total of 338 cities enjoyed good air quality on 90.8 percent of days last month, up 2.6 percentage points year on year, according to the ministry.

In the Beijing-Tianjin-Hebei region, the share of days with good air quality in September stood at 79.2 percent, a year-on-year increase of 27.8 percentage points, while the PM 2.5 density of the region dropped by 33.3 percent year on year to 36 micrograms per cubic meter.

In the first three quarters of this year, 7 out of 20 cities with the worst air quality were in northern China's Shanxi province.

In July, China released a three-year action plan on air pollution control.

According to the action plan, by 2020, emissions of sulfur dioxide and nitrogen oxide should have dropped by more than 15 percent compared with 2015 levels, while cities that fail to meet the requirement of PM 2.5 density should see a decline of more than 18 percent from 2015 levels.

Meanwhile, cities at prefecture level and above should see their numbers of good air days reach 80 percent annually and the percentage of heavily polluted days decrease by more than 25 percent from 2015 levels.