The quality of air quality is mainly affected by two factors: pollutant emissions and atmospheric environmental capacity.
The "2+26" cities in Beijing-Tianjin-Hebei and its surrounding areas are among the regions with the worst air quality in mainland China, with a heavy energy structure, industrial structure and transportation structure: about 20% of the country's coal is consumed, 30% of the country's steel is produced, and 20% of the country's cargo transportation is accounted for. The emission of major air pollutants per unit area is as high as 2-5 times of the national average emission intensity, which is the root cause of the relatively poor air quality in "2+26" cities.
Atmospheric environmental capacity refers to the maximum emission of pollutants that can be accommodated by the regional atmospheric environment in a certain period of time in order to achieve ambient air quality standards or specific control objectives under given natural conditions and pollution source characteristics. Under the current level of pollution emissions, the fluctuation of atmospheric environmental capacity has become the main factor affecting air quality.
The capacity of the atmospheric environment is mainly affected by natural factors and the characteristics of pollution sources. Natural factors mainly include: meteorological conditions, topography and landforms. The characteristics of pollution sources mainly include: the layout of pollution sources, the composition of pollutants, etc. Under the combined action of these factors, pollutants are transported, transformed, and accumulated in the region, forming a pollution process.
For a specific area, the topography is basically stable, and the characteristics of pollution sources change little over a period of time. Therefore, the atmospheric environmental capacity mainly changes with the changes of meteorological conditions such as vertical self-cleaning capacity, horizontal self-cleaning capacity, sedimentation, and humidity in the short term.
The vertical self-cleaning capacity is generally characterized by the boundary layer height, which is mainly affected by temperature stratification and ground roughness, and is positively correlated with environmental capacity. During heavy pollution periods, the height of the boundary layer decreases significantly, from more than 1 km to 300 to 400 m, about one-third of the usual level. The environmental capacity has been significantly reduced, resulting in a significant increase in the degree of pollution.
The horizontal self-cleaning capacity is mainly characterized by the magnitude of wind speed, which is also positively correlated with environmental capacity. During the period of heavy pollution, the wind speed decreases significantly, often less than 2 m/s, the dilution rate of pollutants slows down, and the environmental capacity is significantly reduced. When pollutants accumulate in the air much faster than they dilute, air pollution tends to worsen rapidly.
Sedimentation includes: dry sedimentation and wet sedimentation. The process of wet sedimentation, represented by precipitation, can accelerate the removal of pollutants, which is the main reason for the feeling of fresh air after rain.
Humidity is also one of the main factors affecting the capacity of the atmospheric environment. The high humidity environment is conducive to the conversion of gaseous pollutants such as sulfur dioxide, nitrogen oxides, and ammonia into particulate matter, which aggravates PM2.5 pollution.
Due to the high pollutant emission intensity in the Beijing-Tianjin-Hebei region and the surrounding areas, coupled with the special topographic characteristics of "Taihang Mountain in the west and Yanshan Mountain in the north", the atmospheric self-purification capacity is weakened. Driven by southerly winds, air pollutants are transported over long distances and are easy to accumulate in the piedmont area, forming a heavy pollution process.
In 2020, during a pollution process around the Lantern Festival, in just two days, the atmospheric environmental capacity changed dozens of times.
Meteorological conditions and other natural factors are difficult to change, and we must improve air quality and reduce heavy pollution, the core of which is to drastically reduce the emission of air pollutants to the allowable range of atmospheric environmental capacity.
After continuous efforts in recent years, the frequency, scope and intensity of heavy pollution in "2+26" cities have been significantly reduced. In particular, in Beijing, the number of heavily polluted days has decreased significantly. However, when encountering extremely unfavorable meteorological conditions, it is still difficult to avoid large-scale and continuous heavy pollution.
Quantitative analysis shows that the emission of major air pollutants in "2+26" cities should be reduced by about 50% compared with usual in order to basically avoid the occurrence of regional heavy pollution.
Therefore, we must fully realize that the work of air pollution control is long-term and arduous, especially the "2+26" urban air pollution control, there is still a long way to go.
Source: Chinese Academy for Environmental Planning, Ministry of Ecology and Environment