Beijing will continue to lower emissions of major pollutants including sulfur dioxide, nitrogen oxides, chemical oxygen demand and ammonia nitrogen this year, local authorities said Tuesday.

The four pollutants are each expected to be cut by 2 percent from levels recorded last year, according to Qiao Shufang, an official with the Beijing Municipal Environmental Protection Bureau.

Last year, the city planned to cut emissions of sulfur dioxide, chemical oxygen demand and ammonia nitrogen by 2 percent and that of nitrogen oxides by 3 percent from the previous year.

Qiao said it is believed that last year's targets have already been met, but the Ministry of Environmental Protection is still examining Beijing's performance in the emission cut.

In recent years, the permanent population in Beijing has been growing by over 400,000 annually, which is nearly the size of a medium-sized city, Qiao said, adding that the growth has put great pressures on the city's pollution control efforts.

The growing population can raise chemical oxygen demand levels by 15,000 tonnes and ammonia nitrogen levels by 1,500 tonnes each year, and the city has to build a new sewage treatment factory every year to treat newly added pollutants, Qiao said.

There are also 400,000 new vehicles on the city's roads each year, increasing the difficulty of treating air pollution, Qiao added.