Over 300 contrast photos taken each day in 2015 in Beijing by the China Meteorological Bureau are put together on December 31st, 2015 to review the environment in the capital city for the past year. [Photo/weather.com.cn]

Concentrations of four major air pollutants continued to decline over the last year, while further efforts are needed to reduce the level of unhealthy particulates known as PM 2.5, an annual report released by the Beijing municipal protection bureau said.

Beijing's efforts to control air pollution have resulted in some improvements. About half of the days in 2015 met national air standards, 14 days more than in 2014, the report said.

Among the four major pollutants, sulfur dioxide registered the largest decline, decreasing by 38 percent from the previous year.

PM2.5, particulate matter with diameter of less than 2.5 microns that can penetrate lungs and harm human health, needs to be cut further. Concentrations of PM 2.5 decreased by 6.2 percent and remained 1.3 times higher than the national standard.

PM10, a larger particulate matter than can be seen as soot or smoke, decreased by 12.3 percent, while nitrogen dioxide decreased by 11.8 percent last year, the report said.