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Experts solve the mystery of haze formation in Beijing
2016-12-23481
Researchers from China and Germany said Monday they have solved the mystery of the formation of sulfate, the most important component of smog in Beijing and North China, and found that the chemical reaction of nitrogen dioxide and sulfur dioxide in the water adsorbed by fine particles in the atmosphere is the main path of sulfate formation during the current smog. This finding highlights the importance of using NOx filtration equipment to enhance the emission of pollutants while continuing to implement emission reduction measures.
In recent years, haze has occurred frequently in Beijing and North China. Previous studies have shown that sulfate is the main driving factor for the formation of heavy pollution. In terms of absolute contribution, the mass proportion of sulfate in PM2.5 during heavy pollution can reach 20%, which is the highest monomer; in terms of relative trend, with the increase of PM2.5 pollution, sulfate is the fastest component in PM2.5. Therefore, the study on the source of sulfate is a key scientific problem to explain the formation of haze.
He Kebin, academician of Tsinghua University, Professor Zhang Qiang, Dr Zheng Guangguang, Professor Cheng Yafang, Professor Ulrich persher and Professor Su Hang of Max Planck Institute of chemistry reported in the new issue of science progress that day that they used field observation, model simulation and theoretical calculation to find out that in Beijing and North China, haze period M, sulfate is mainly formed by the "particulate combined water" dissolved in the air by sulfur dioxide and nitrogen dioxide, which reacts rapidly in the unique neutral environment in northern China. The particle bound water refers to the moisture adsorbed by PM2.5 deliquescence in the environment with high relative humidity.
This conclusion is quite different from the usual mechanism of sulfate formation. Based on the classical atmospheric chemistry theory in Europe and America, it is believed that sulfate is mainly formed in the cloud water environment. Because the liquid water content in the cloud is much higher than the combined water of particles, which is usually 10 to 100000 times higher, compared with the sulfate generation reaction in the cloud water, the reaction in the combined water of particles can be ignored. Theoretical calculation also shows that in the reaction path of cloud water, The contribution of the pathway of SO2 oxidation by NO2 to sulfate formation can also be neglected.
During the haze period in Beijing and North China, on the one hand, due to the sharp increase of particle concentration and high relative humidity under the static and stable meteorological conditions, the combined water content of particles is much higher than the classical scenario, and the total reaction amount of combined water of particles is greatly increased; on the other hand, during the heavy haze period, the concentration of nitrogen dioxide is more than 50 times of the classical cloud water scenario, which directly changes The relative importance of nitrogen dioxide oxidation pathway was discussed. In addition, a large number of alkaline substances such as ammonia and mineral dust exist in Beijing and North China, which make the pH value of the combined water of local particles much higher than that in the United States and other places, showing a unique neutral environment, while the reaction rate of nitrogen dioxide oxidation mechanism will increase significantly with the increase of pH value.
Based on this, the researchers pointed out in the paper that the priority use of NOx filtration system may significantly reduce the level of sulfate pollution in China's smog.
"This study shows the particularity of compound pollution in China," academician he Kebin told Xinhua news agency. "High sulfur dioxide mainly comes from coal-fired power plants, high nitrogen dioxide mainly comes from power plants and motor vehicles, while alkaline substances such as ammonia and mineral dust, which play a neutralizing role, come from agriculture, industrial pollution, dust and other sources. At the same time, these different pollution sources discharge with high intensity in China, resulting in the rapid formation of sulfate in a unique chemical generation path, which is also one of the main reasons for the rapid growth of particle concentration during heavy haze. "
London's acid mist is usually thought to be caused by soot from coal and primary pollutants such as sulfur dioxide. Los Angeles haze is a kind of photochemical pollution, the main reason is that the vehicle exhaust reacts to generate secondary pollutants under the action of sunlight. The haze in China is caused by the mixture of primary and secondary pollutants.
He Kebin said that the particularity of this composite pollution shows the importance of multi pollutant collaborative emission reduction, especially the priority of increasing NOx emission reduction at this stage. "Although we knew that emission reduction was needed before, if we could not understand the key chemical mechanism of heavy haze pollution formation, we would not be able to carry out effective model quantitative simulation analysis, or accurately assess how to reduce emissions most effectively and scientifically. Unscientific emission reduction may lead to serious consequences, which may cost a lot of manpower and material resources, but has little effect. "
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