BEIJING, Dec. 12 (Xinhua) -- Beijing maintained an alert for heavy pollution on Monday.
Smog hit many parts of China on Sunday, with Beijing and neighboring Tianjin issuing blue and orange alerts.
China has a four-tier warning system for severe weather, with red being the most serious, followed by orange, yellow and blue.
A cold front was expected to dissipate Beijing's smog Monday afternoon, but the alert will be maintained for 24 hours.
Smog has also affected Henan, Hebei, Shanxi and Sichuan provinces.
Data on Monday from Beijing Municipal Environmental Protection Bureau showed the city's average PM2.5 density decreased in the first 11 months of this year.
The average density of PM2.5 from January to November was 67 micrograms per cubic meter, a year-on-year decrease of 9.5 percent.
The capital saw 185 days with good air quality during the 11 months, 10 days more than the same period last year.
Average densities of sulfur dioxide, nitrogen dioxide and PM10 from Jan. to Nov. dropped by 23.1 percent, 6.3 percent and 10.4 percent year on year.