What is the main air pollution in winter in China?

On the morning of September 9th, 2020, the State Council Press Office held a routine briefing on the State Council policy, and invited Zhao Yingmin, Vice Minister of the Ministry of Ecology and Environment, Chai Fahe, Deputy Director of the National Joint Center for Air Pollution Prevention and Control, and chief scientist in the atmospheric field of china environmental science Research Institute, to introduce the situation of science and technology helping to win the blue sky defense war and answer reporters' questions.

Some media asked: Through the research of key projects, what are the main causes of air pollution in Beijing, Tianjin and Hebei and surrounding areas in autumn and winter? What is the main source?

Zhao Yingmin, Vice Minister of the Ministry of Ecology and Environment, said that this issue is also of great concern to the public, and frankly, it is also the focus of social attention. The core task of this key project is also to clarify this issue. In order to fully understand the causes of regional heavy pollution, a closed research technology system combining comprehensive observation, experimental analysis and digital simulation has been designed for key projects, and the largest three-dimensional observation network integrating space, sky and earth in China has been built to track the occurrence, development, transmission and dissipation of heavy pollution in real time and dynamically grasp the changes of components and pollution sources in the polluted atmosphere. The causes of regional heavy pollution in autumn and winter are comprehensively expounded from four aspects: pollutant discharge, chemical transformation, meteorological condition change and pollution transmission.

To sum up, there are four factors * * * working together, leading to heavy polluted weather in this area:

First, the discharge of pollutants exceeds 50% of the environmental capacity, which is the root cause of frequent heavy pollution weather. Beijing-Tianjin-Hebei and its surrounding areas, which are usually called "2+26" cities, have a high concentration of heavy chemical industry. The energy utilization mode mainly based on coal and the freight transportation mode mainly based on road transport in this area lead to high emissions of major air pollutants in this area. The discharge of major pollutants per unit land area is 2-5 times of the national average, and the discharge multiples of different pollutants are different. Except sulfur dioxide, the emissions of major pollutants in the region all exceed 50% of the environmental capacity, and some cities even exceed 80%- 150%. In addition, due to heating, the discharge of major pollutants in autumn and winter is about 30% more than usual. Generally speaking, regional pollutant discharge exceeds the environmental capacity, which should be the root cause of frequent heavy pollution.

The key project set up 109 sampling points in the "2+26" city, collected more than 58,000 samples, and determined 490,000 chemical composition data online. 20 18-20 19 autumn and winter PM2.5 source analysis shows that the contribution of industrial and civil loose coal and diesel vehicles to regional PM2.5 reached 36%, 17% and 16% respectively, that is, the three largest contributions and the main source of heavy pollution. Among the industrial sources, the steel coking industry contributes the most, followed by the cement industry.

Second, the high concentration of nitrogen oxides and volatile organic compounds in the atmosphere increases the oxidation of the atmosphere, which is the key factor for the rapid growth of secondary PM2.5 during heavy pollution. As we all know, some of PM2.5 is directly emitted, but some of it is also due to the emission of gaseous pollutants into the air, such as sulfur dioxide and nitrogen oxides, which form fine particles in the air through secondary transformation. We converted this part of PM2.5 into PM2.5 for the second time. With the deepening of air pollution control, the proportion of primary components decreased obviously, and the proportion of secondary components converted from pollutants in the air to particulate matter increased continuously. From 20 13, the proportion of secondary components gradually increased from 40% to about 50%. During the period of heavy pollution, secondary pollutants are the main components of particulate matter, and the proportion can reach 60% or even higher. High concentrations of nitrogen oxides and VOC in the region will lead to rapid photochemical reactions in the atmosphere, leading to a high degree of atmospheric oxidation, which is the decisive factor to promote the transformation of gaseous pollutants in the air into secondary PM2.5. The degree of atmospheric oxidation in Beijing is about 2-3 times higher than that in cities such as London and Tokyo. Such a high oxidation rate also led to a 3-5 times increase in the secondary conversion rate during heavy pollution in this area. On the one hand, atmospheric oxidation promotes the transformation of gaseous pollutants into PM2.5, and at the same time promotes the formation of ozone in summer. Therefore, controlling atmospheric oxidation is the intersection and focus of the next step of collaborative control of PM2.5 and ozone, which has the effect of "killing two birds with one stone".

Third, unfavorable meteorological conditions greatly reduce the regional environmental capacity, which is a necessary condition for the formation of heavily polluted weather. Beijing-Tianjin-Hebei and its surrounding areas are located in the semi-closed terrain on the east side of Taihang Mountain and the south side of Yanshan Mountain, which objectively has a "weak wind zone" and a "warm cover" structure in the troposphere over this area. The "congenital deficiency" of atmospheric diffusion conditions is mainly due to the fact that the diffusion conditions in this place are worse than those in other places, resulting in a small environmental capacity. Affected by climate change, the regional environmental capacity has generally declined since 2000. In addition, the environmental capacity also shows seasonal and monthly differences. It is well understood that an important reason for good air in summer is good air diffusion conditions. In addition, due to the change of meteorological factors, every month and every year is different. Generally speaking, the environmental capacity in autumn and winter is about 30% smaller than that in spring and summer, and the environmental capacity in June 5438+ 10 is about half that in July. Therefore, once the near-surface wind speed is less than 2m/s, the height of the boundary layer caused by inversion drops below 500m and the relative humidity is higher than 60%, the atmospheric environmental capacity will be further reduced by 50%-70%, which is easy to induce heavy polluted weather. The first factor is high emissions. At the same time, due to the mountainous areas in the west and north of the region, the environmental capacity is inherently insufficient, and the capacity in winter is reduced by half compared with that in summer. The superposition of two factors is easy to cause heavy pollution.

Fourth, regional transmission has a significant impact on PM2.5 The average contribution rate of each city is about 20%-30%, and it is further increased to 35%-50% when it is heavily polluted. In the initial stage of air pollution control, due to a large number of local emissions, local sources often have a greater impact on local environmental quality. With the deepening of pollution control, the proportion of local sources is gradually decreasing, and the external input is gradually increasing. The key projects analyzed nearly 100 heavy pollution weather processes since 20 13. It shows that during the period of heavy pollution, the average contribution rate of regional transmission to PM2.5 in Beijing is about 45%, and individual processes can reach 70%. There are mainly three transport channels for pollutants in the region, which is also the conclusion of this key project through research and observation. One passage is the southwest passage, that is, the northern part of the Henan-Handan,-Shijiazhuang-Baoding-Beijing line. This channel has the highest transmission frequency and maximum transmission intensity. The average contribution rate of heavy pollution process is about 20%, and the individual heavy pollution process can reach 40%. The second channel is the southeast channel, along the central Shandong-Cangzhou-Langfang-south-central Tianjin. The third passage is the eastward passage, which is the Tangshan-Tianjin North-Beijing line.

These are the four main causes of heavy pollution in autumn and winter in this study. These four reasons are superimposed on each other, resulting in heavy polluted weather in autumn and winter in this area. Of course, we should pay more attention to man-made emissions and greatly reduce pollutant emissions, so that even if we encounter extremely unfavorable meteorological conditions, heavily polluted weather will not happen again.