BEIJING, Jan. 4 -- Beijing's average PM2.5 density in 2015 was more than double the official level targeted nationwide, an official said on Monday.

Despite Beijing's attempts to limit air pollution, the average PM2.5 reading stood at 80.6 micrograms per cubic meter, 1.3 times more than the national standard, Zhang Dawei, director of the Beijing Municipal Environmental Monitoring Center (BMEMC), said at a news conference.

However, the reading has seen a year-on-year drop of 6.2 percent, Zhang said.

With a diameter of less than 2.5 micrometers, PM2.5 is particulate matter that causes hazardous smog.

Beijing only saw 186 days with air quality meeting the national standard, statistics from the BMEMC showed.

Air pollution was particularly severe in November and December, with the capital shrouded in foul air for 22 days, 15 days more than the same period last year. During the period, average PM 2.5 density hit 239 micrograms per cubic meter, according to the BMEMC.

"Heavy air pollution mainly happens in autumn and winter, particularly in winter, when coal burning significantly increases," Zhang said.

Average densities of sulfur dioxide, nitrogen dioxide and PM10 dropped 38.1 percent, 11.8 percent and 12.3 percent year on year, respectively.

Beijing frequently suffers from thick smog, cutting visibility and posing health hazards. Local government measures to try to limit the problem have included restricting traffic and halting industrial production.