BEIJING — Chinese cities reported fewer days with “good air” in July compared the same period last year despite a decline in levels of a key pollutant PM2.5, official data showed on Aug 16.

The 338 cities monitored by the Ministry of Environmental Protection (MEP) enjoyed good or excellent air quality on 82.5 percent of the days in July, down 6.3 percentage points year on year, according to Liu Zhiquan, head of the environmental monitoring department at the MEP.

During the same period, the density of hazardous fine particle matter PM2.5 nationwide dropped 6.9 percent from a year ago. Notably, the density of PM2.5 slumped 24.6 percent in Beijing, helping the city enjoy more “good air” days in July.

In the first seven months of this year, Beijing’s PM2.5 density dropped 1.5 percent year on year to 64 micrograms per cubic meter, still above its target of around 60 micrograms per cubic meter in 2017.

Air quality in 13 cities in the Beijing-Tianjin-Hebei area was rated good or excellent on 50.4 percent of the days in July, down 4.9 percentage points year on year.

The density of PM2.5 and PM10 in these regions went down by 13.8 percent and 4.7 percent, respectively, indicating improved air quality in these areas.

Authorities have been undertaking a year-long on-site investigation to discover the sources of air pollution in Beijing-Tianjin-Hebei area, where winter smog frequently occurs.

Recent findings showed that more than half of the nearly 40,000 enterprises checked have violated environmental rules.

MEP data also showed that of the country’s 74 major cities, Haikou in southern China’s Hainan province had the best air quality in July while Tangshan in northern China’s Hebei province was the most polluted.