BEIJING - Out of concern for its air quality, Beijing has stepped up efforts to phase out old, heavy-polluting motor vehicles, the local environmental watchdog said Tuesday.

A total of 515,000 obsolete motor vehicles have been taken off the city's roads over the past two years, exceeding the goal of removing 400,000 such vehicles that was set in a municipal five-year development plan (2011-2015), according to the Beijing Municipal Environmental Protection Bureau.

Motor vehicles contribute to about 22.2 percent of a city's PM2.5 data, a gauge monitoring airborne "fine" particles 2.5 microns or less in diameter, and old vehicles release more pollutants, said Fang Li, a spokesman with the bureau, citing an analysis of the city's pollution sources.

Beijing is suffering from poor air quality, with this year's average PM2.5 data reaching 70-80 micrograms per cubic meter, twice the regulated standard of 35 micrograms per cubic meter.

Fang said motor vehicles that have been in use for more than eight years account for about 20 percent of the city's car population, but they create more than 60 percent of the air pollution.

"Phasing out obsolete motor vehicles is of great significance in lowering Beijing's PM2.5 data," said Fang.

In the meantime, Beijing has been working to improve the quality of its fuel supply and rolling out stricter emissions standards.

According to a municipal plan on air pollution control, Beijing hopes to implement the stricter State VI emissions standard, which is equivalent to the Euro VI standard, by 2016.

The Beijing municipal government will also push for the use of green vehicles in the future and expand public transport to make it account for half of all traffic.

In a bid to ease the city's chronic traffic jams, Beijing started a car plate lottery scheme in 2011, allowing a maximum of 240,000 new cars to be put on the road each year.