BEIJING, July 3 -- In the first six months of 2015, Beijing's air quality improved significantly, registering fewer air pollutants and more clear days, according to the municipal environment protection bureau on Thursday.

Concentration of PM2.5, airborne particles measuring less than 2.5 microns, which are small enough to enter the bloodstream, dropped to 77.7 micrograms per cubic meter, down 15.2 percent year on year.

The density of sulfur dioxide, nitrogen dioxide and PM10 dropped 41.3, 14.5, and 12.7 percent, respectively.

The city has seen 88 days of sound air quality, nine days more than the same period last year. Air pollution was registered on 16 days, a decrease of nine days compared with the first half of last year.

"Conditions were favorable in the first half of the year," said Sun Feng with the bureau's monitoring center. "But not enough that there was a sharp cut to air pollution."

Sun said limits to emissions and clean production in Beijing and its surrounding areas had helped increase air quality.

Last year, Beijing reduced coal consumption by 2.8 million tonnes. In March, two coal power plants were shut down, 200,000 vehicles and more than 300 polluting factories will be phased out this year.

As the city is bidding for the 2022 Winter Olympic Games, Beijing issued a five-year plan on the environment, vowing to cut PM2.5 density by 20 percent by 2017, backed by investment of 130 billion U.S. dollars.