Guangzhou recorded a big decrease in harmful fine particulates known as PM2.5 last year, and boasted the best air quality among China's five national central cities, officials from the environmental protection bureau reported on Monday.

PM2.5 density, which measures airborne particles smaller than 2.5 microns in diameter that can penetrate the lungs, dropped 20.4 percent from 2014, according to a government work report released at the South China city's annual local legislative session, which opened on Monday.

Guangzhou, capital of Guangdong province, enjoys better air quality than the other four national central cities of Beijing, Shanghai, Tianjin and Chongqing, said Zhang Wei, deputy director of the city's environmental protection bureau.

PM2.5 density fell for the second consecutive year. Zhang attributed the improvement to the bureau's fight against pollution from factories and vehicles, which are responsible for at least 20 percent of the city's PM2.5 pollution.

Last year, 21 coal-burning plants with 4.63 million kilowatts of installed electrical capacity were transformed to clean discharge units and 101,000 vehicles that failed national emission standards were phased out, according to the 2015 Report on the State of Environment in Guangzhou.

Despite the big decrease, the PM2.5 density in Guangzhou's air was still too high at 39 micrograms per cubic meter on average last year. The national standard for urban districts is 35 micrograms per cubic meters.

Zhang told China Daily that the environmental protection bureau will focus on controlling volatile organic compounds in the air in 2016. According to the government work report, 209 major VOC-polluting enterprises will be remediated this year.