Exhaust emissions are still the main cause of air pollution in the Beijing-Tianjin-Hebei region and its surrounding areas in autumn and winter seasons, according to a report released by a national joint research center on air pollution causes and control on May 17.

The explosive growth of PM2.5 in Beijing is resulted from the combined influence of regional transmission, local accumulation and secondary transformation, according to the report.

The center is tasked with investigating the causes and sources of air pollution in the Beijing-Tianjin-Hebei region and surrounding areas, including the characteristics of regional PM2.5 pollution, physical and chemical causes of the explosive growth of PM2.5, the impact of atmospheric conditions on air pollution, and specific sources of PM2.5.

It has utilized resources from the Ministry of Ecology and Environment, the China Meteorological Administration, the Chinese Academy of Sciences, and various universities to complete the preliminary construction of a stereoscopic monitoring network for the atmospheric environment in China.

A data management and application platform has been established to provide data sets for the study of air pollution causes, aiming at improving the air quality of the Beijing-Tianjin-Hebei region and surrounding areas.

The report has shown that massive exhaust emissions from coal burning, manufacturing and automobiles are the main sources of PM2.5. Among them, coal burning is the primary source. Meanwhile, adverse atmospheric conditions are an important incentive for air pollution as well.

Analysis of pollution processes in the past three years reveals that the explosive growth of PM2.5 can be attributed to three causes: local accumulation, regional transmission, and secondary transformation. For example, air pollutants in cities with large emissions, such as Shijiazhuang, Tangshan, and Handan, first begin to accumulate when atmospheric conditions worsen. Then air masses flow downwind, transporting high-concentration PM2.5 to cities located in the downwind areas and causing regional transmission pollution. Gaseous pollutants, such as sulfur dioxide and nitrogen oxides, further react under suitable conditions and form secondary components ― sulfates and nitrates. These secondary components grow as they absorb water and aggravate PM2.5 pollution, which then begins the secondary transformation.

The report indicates that accumulation of particulate matter will result in the deterioration of near-surface atmospheric conditions as well as an increase in temperature inversion and near-surface humidity. The deteriorated atmospheric conditions, in turn, foster the explosive growth of PM2.5.