Beijing is seen with blue skies. [File photo: baidu.com]

Air quality in Beijing has improved in the first four months of this year, with the density of a major pollutant particles down more than 20 percent year on year, local environment watchdog said on Tuesday.

The density of PM2.5, particulate matter measuring less than 2.5 microns in diameter and the main cause of smog in the city, dropped 21.8 percent between January and April, compared with the same period a year ago.

The density of sulfur dioxide, nitrogen dioxide and PM10 went down 31.2 percent, 14.3 percent and 22.5 percent, respectively.

The Beijing Municipal Environmental Protection Bureau said that it will continue to reduce the use of coal and take high-emission vehicles off the roads, among other measures, to improve air quality in the city.