The 13 cities monitored in the area reported "good" air quality on 52.6 percent of days during the January-September period, down 8.7 percentage points from the ratio in the same period last year, according to Liu Zhiquan, head of environmental monitoring at the Ministry of Environmental Protection

The density of PM2.5 in those cities went up 10.3 percent percent from a year ago, and the density of PM10 climbed 10.7 percent.

In September, Beijing had "good" air quality on 53.3 percent of the days, down 13.4 percentage points from the share in the same period last year.

Density of PM10 surged by a staggering 53.8 percent in the capital.

Nationwide, the 338 cities monitored by the MEP reported good or excellent air quality on 79.4 percent of the days in the first three quarters, slight down from the same period last year.

Air quality usually worsens in winter in northern China where cold weather conditions and the burning of coal for heating combine to exacerbate the situation.

To improve air quality in the season, the MEP has started rounds of on-site environmental inspections in cities around the Beijing-Tianjin-Hebei area.