Straw burning is the main culprit in the recent severe pollution in Northeast China, according to experts from the National Joint Research Center for Tackling Key Problems in Air Pollution Control, China Youth Daily reported on Thursday.

Northeastern China has been experiencing heavy PM2.5 pollution — health-damaging particulate matter with a diameter of 2.5 mm or less — since Saturday.

According to the National Environmental Air Quality Monitoring Network, the cities of Daqing, Heilongjiang province, and Chaoyang, Liaoning province, have experienced severe PM2.5 pollution since 4 pm Tuesday, while Harbin and Suihua in Heilongjiang have experienced moderate pollution.

Eleven cities in Northeast China recorded PM2.5 concentrations of 150 micrograms per cubic meter per hour, with Harbin and Suihua recording more than 500 micrograms per cubic meter.

Peak concentration in Suihua reached 735 micrograms per cubic meter at 8 am on Tuesday.

From Saturday to Monday, satellites detected a large number of open straw-burning points in northeastern China, mainly concentrated in central Jilin province, central Liaoning and southern Heilongjiang.

Cities such as Harbin and Suihua; Changchun and Jilin in Jilin province; and Liaoyuan and Fushun in Liaoning had many straw burning points. Experts from the center said the intensity of open straw-burning resulted in the accumulation of pollutants at night, which exacerbated the PM2.5 pollution.

They said the straw burning took place most heavily from noon to 6 pm daily, with the greatest number of burning points reached at 3 pm on Monday.

Around three to four hours after the concentrated burning of straw, pollutants were rapidly accumulated and transported by air currents in the region, leading to varying degrees of air quality deterioration in cities.

Unfavorable topography and meteorological conditions facilitate the accumulation of pollutants, experts said. Moreover, the strong temperature inversion in the early morning within a range of 0 to 500 meters from the ground can also cause the accumulation of pollutants.

From the Friday to Tuesday, influenced by cold air, most parts of the region were expected to have good air quality because of better diffusion conditions.