Causes of heavy air pollution in Beijing-Tianjin-Hebei released

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Researchers have identified the causes of heavy air pollution in the Beijing-Tianjin-Hebei region and the surrounding areas, and released quantified descriptions of the contributing processes, the National Air Pollution Prevention and Control Research Center announced on March 3.

According to the center’s study, the heavy atmospheric pollution in the Beijing-Tianjin-Hebei region is the combined result of local pollutant accumulation, regional transmission and secondary conversion.

Since April 2017, China has gathered more than 2,000 scientists and researchers in environmental science, atmospheric science, meteorological science and industrial governance to work on the study. They have built elaborate observation networks and conducted research through field observation, laboratory analysis and numerical simulations.

According to the center, the main cause of heavy air pollution in the Beijing-Tianjin-Hebei region and the surrounding areas is the accumulation of emissions, which far exceeds the environmental carrying capacity. The industrial infrastructure featuring heavy industries, the coal-based energy structure, and the highway-based transportation system together have resulted in high coal consumption per unit of land, at four times the national average. The production of steel, high-carbon coke, and glass accounts for more than 40 percent of the country's total output, and 80 percent of bulk materials are transported by diesel-fueled trucks.

According to the study, the geography of the region causes these pollutants to become trapped. The Beijing-Tianjin-Hebei region is a semi-enclosed terrain formed by the Taihang Mountains to the west and the Yanshan Mountains to the north, which lessens the impact of the northwest monsoon prevalent in autumn and winter. Meanwhile, this region is characterized by "weak winds" resulting in poor diffusion of pollutants.

Given the high emissions in the region and its geographical features, once certain meteorological conditions occur — such as on-the-ground wind speeds of less than 2 meters per second, relative humidity over 60 percent, or temperature inversions — the localized accumulation of pollution is likely to appear.

Additionally, the transmission of pollutants within the Beijing-Tianjin-Hebei region and surrounding cities accounts for 20 to 30 percent of the annual recorded pollution. During the heavy pollution period, the contribution spikes by about 15 to 20 percent. For Beijing, the regional transmission contribution can reach 60 to 70 percent. The transmission travels mainly along three passages: the southwest passage in front of the Taihang Mountains, the southeast passage from Jinan to Cangzhou and Tianjin and the eastern passage in front of the Yanshan Mountains. Quantitative analysis of the transmission via the southwest passage shows that at the beginning of a typical pollution cycle, the flow to Beijing can reach 500 to 800 micrograms per square meter per second, while the volume during the pollution formation phase preceding that is 100 to 200 micrograms per square meter per second.

The final factor leading to heavy atmospheric pollution in the tri-city region, as shown in the study, is secondary conversion. Nitrates have exceeded sulfates as the most significant secondary inorganic component of the PM2.5 readings. The shift is attributable to the coal-for-gas rectification program targeting households and coal-fired boilers in the region, which has dramatically reduced the concentration and proportion of sulfates in emissions. This, however, also highlights the importance and urgency of nitrogen oxide control.

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