Coal burning is no longer a major source of PM2.5 in Beijing, with car emissions having become the top cause, according to a study released by Beijing's environment watchdog on Monday.

The new findings come as Beijing encourages the use of natural gas instead of coal for winter heating and shuts down polluting smokestack factories.

Emissions from vehicles and construction machinery are the top sources of PM2.5 (particulate matter of diameter less than 2.5 micrometers), contributing up to 45 percent of the total pollutants in Beijing, according to the study by the Beijing Environmental Protection Bureau.

Beijing has the highest level of car ownership in the country, with 5.64 million privately owned cars at the end of 2017.

The study also shows that Beijing's surrounding regions contributed to more than 50 percent of the pollutants on heavily polluted days when the air quality index exceeded 200 in the city.

Air samples were gathered daily in 11 different locations around Beijing and more than 300,000 sets of data were analyzed, said Liu Baoxian, deputy director of Beijing Municipal Environmental Protection Monitoring Center.

The level of PM2.5 in Beijing has dropped to its lowest level since 2013 when the national air pollution control campaign began. The average PM2.5 density in 338 cities fell by 6.5 percent in 2017.