BEIJING - Beijing's air quality has improved for 14 straight years, with major pollutants down in the city, a municipal government spokesman said Monday.

The annual average concentration of major pollutants PM10 (particulate matter under 10 micrometers in diameter), sulphur dioxide and nitrogen dioxide dropped by 4 percent year on year in 2012, said Fang Li, spokesman for the municipal environmental protection bureau.

"The biggest challenge for air quality control in Beijing is the increase of pollutants," said Fang. "We can only improve air quality by making sure that pollution reduction outpaces increases in pollutants."

Fang cited the city's growing population, as well as an increasing number of cars and construction projects, as problems for improving air quality.

Replacing coal with natural gas as a fuel source reduced the consumption of coal by 700,000 tonnes last year, Fang said, adding that 122 heavily polluting companies were closed as well.