Air quality, which will be the key factor in evaluating the capital's overall environmental quality during the next five years, is mainly affected by motor vehicle pollution, officials said on Monday.

"Reducing pollutants released by coal burning, construction dust emission, heavy-polluting enterprises and motor vehicles are the four main ways to control air pollution," said Du Shaozhong, deputy head of the Beijing Municipal Environmental Protection Bureau.

He said pollution created by motor vehicles has now become the biggest threat to the city's air quality because impressive achievements have been made during the past five years in reducing pollutants from coal burning, construction dust emissions and enterprises.

More than 160,000 families in the city center began to use electric heaters instead of burning coal; some 300 heavy-polluting enterprises were shut down or moved out of Beijing; and a green construction regulation reduced dust pollution. From 2005 to 2009, the emission of sulfur dioxide was reduced by about 38 percent.

The number of "blue sky days" increased to 285 in 2009, compared with 234 in 2005. The city is aiming for 274 "blue sky days" in 2011, eight more than this year's target.

"However, the capital's air quality is still under huge pressure due to the large number of cars and the high growth rate of new cars on the road, more than 15 percent every year," Du said.

Nitrogen oxide has been added to the list of pollutants to be specially controlled in the city's 12th Five-Year Plan (2011-2015).

"Motor vehicles contribute more than 50 percent of nitrogen oxide emissions," Du said, adding that the bureau is taking action to reduce the pollution generated by motor vehicles, including eliminating heavy-polluting cars and raising the emissions standard for new cars.

Between 1999 and 2008, Beijing upgraded its emission regulations from the European No 1 standard to the European No 4 standard.

"Each time the regulation was upgraded to a stricter standard, we experienced a decrease in emissions by 30 to 50 percent," said Du.

He said the bureau is working on upgrading the standard to European No 5 by the end of 2012.

The number of cars in Beijing exceeded 4.76 million before Christmas.