BEIJING - The average PM 2.5 reading for Beijing in 2013 was more than double the national standard, the city's environmental authority reported Thursday, suggesting tough challenges ahead for the pollution-plagued Chinese capital in its anti-smog campaign.

Beijing's average PM 2.5 index, which measures airborne particles smaller than 2.5 micrometers, stood at 89.5 micrograms per cubic meter in 2013, the Beijing Municipal Environmental Protection Bureau said in a press release.

The reading was 2.56 times the national standard of 35, which was set by the State Council in 2012 in a set of revised air quality standards that included indices for ozone and PM 2.5.

Beijing began PM 2.5 monitoring in 2013 amid mounting public complaints about frequent bouts of smog.

The city reported 58 days of heavy pollution last year, compared with 176 days of excellent or fairly good air, said Zhang Dawei, director of the Beijing Municipal Environmental Monitoring Center.

"It suggests that, on average, heavy air pollution occurred every six or seven days," Zhang said.

Finer particles as measured by PM 2.5 had become the city's primary polluter, accounting for 77.8 percent of air pollution, followed by ozone, which made up 20.1 percent, according to Zhang.

The tiny PM 2.5 can go deep into the lungs, thus posing a greater health threat than PM 10.

The Beijing Municipal Environmental Protection Bureau said it aims to bring down the city's average PM 2.5 index to 60 in 2017 by reducing coal burning, limiting car ownership and other industrial polluters.

The city slashed its coal use by 1.3 million tonnes, phased out 35,000 motor vehicles, and overhauled or closed 288 factories in 2013 in its campaign to cut air pollution, according to city officials.