About one third of Beijing's PM 2.5 is from neighboring cities, Beijing Municipal Environmental Protection Bureau said on Wednesday.

According to latest research on the source of the capital's PM 2.5, 28 to 36 percent of pollutants were from nearby cities.

Motor vehicles, coal burning, industrial production and dust account for 31.1 percent, 22.4 percent, 18.1 percent and 14.3 percent of Beijing's PM 2.5 respectively. Cooking, live stock raising, vehicle repair and paint altogether takes up to 14.1 percent.

PM 2.5 particles are airborne particles measuring less than 2.5 microns in diameter that cause hazardous smog.

"Vehicle emissions are the major pollutant to be blamed," said Zhang Dawei, the head of Beijing Municipal Environmental Monitoring Center. "The gaseous pollutants discharged by cars will worsen air quality after mixing with dust."

The municipal government has pledged to reduce PM 2.5 density by 25 percent or more by 2017.