[Repost] German and Chinese researchers announce solving the mystery of Beijing's smog formation
Researchers from China and Germany said on the 21st that they have solved the mystery of the formation of the main component of smog in Beijing and the surrounding areas, namely sulfate. They found that the chemical reaction between nitrogen dioxide and sulfur dioxide in the water adsorbed by fine particulate matter in the atmosphere is the main generation path of sulfate during the current smog period. This discovery highlights the importance of prioritizing the reduction of nitrogen oxide emissions while continuing to implement emission reduction measures to alleviate air pollution problems.
In recent years, smog has occurred frequently in Beijing and the surrounding areas. Studies have shown that sulfate is the main driving factor for heavy pollution. In terms of absolute contribution, sulfate accounts for 20% of the mass of PM2.5 in heavy pollution periods, making it the single component with the highest proportion. In terms of relative trend, as the PM2.5 pollution level increases, sulfate is the component with the fastest relative increase in PM2.5. Therefore, the study of sulfate sources is a key scientific issue to explain the formation of smog.
On the 21st, academicians He Kebin from Tsinghua University, Professor Zhang Qiang, Dr. Zheng Guangjie, and professors Cheng Yaofang, Ulrich Perschel, and Su Hang from the Max Planck Institute of Chemistry in Germany, and others reported in the latest issue of the American journal Science Advances that they used field observations, model simulations, and theoretical calculations to discover that during smog periods in Beijing and the surrounding areas, sulfate is mainly formed by the combination of sulfur dioxide and nitrogen dioxide with the "particle-bound water" in the air. In the unique neutral environment of northern China, the reaction occurs rapidly.
This conclusion is quite different from the commonly held mechanism of sulfate formation. The existing classic atmospheric chemical theories based on regions such as Europe and the United States believe that sulfate is mainly formed in cloud water environments. Since the liquid water content in clouds is much higher than the "particle-bound water" in particles, it is usually 1000 to 100,000 times higher, so the reaction in "particle-bound water" can be ignored compared to the formation reaction of sulfate in cloud water. Theoretical calculations also show that in the cloud water reaction path, the contribution of the reaction of nitrogen dioxide oxidizing sulfur dioxide to form sulfate can also be ignored.
During smog periods in Beijing and the surrounding areas, on the one hand, due to the significant increase in particle concentration and the relatively high relative humidity under stable meteorological conditions, the content of particle-bound water is much higher than the classic scenario. The total amount of reactions in particle-bound water has greatly increased. On the other hand, during severe smog periods, the concentration of nitrogen dioxide is more than 50 times higher than that in the classic cloud water scenario. This directly changes the relative importance of the nitrogen dioxide oxidation path. In addition, the large amount of alkaline substances such as ammonia and mineral dust present in Beijing and the surrounding areas make the pH value of particle-bound water much higher than that in the United States and other places, presenting a unique neutral environment. The reaction rate of the nitrogen dioxide oxidation mechanism will increase significantly with the increase in pH value.
The researchers pointed out in the paper that prioritizing the reduction of nitrogen oxide emissions may help significantly reduce the sulfate pollution level in smog in China.
"This study shows the particularity of the composite pollution in our country," said academicians He Kebin to Xinhua News Agency reporters. "High sulfur dioxide mainly comes from coal-fired power plants, high nitrogen dioxide mainly comes from power plants and vehicles, etc., while the neutralizing substances such as ammonia and mineral dust come from agricultural, industrial pollution, and dust from other sources. These different pollution sources are simultaneously emitted at high intensity in our country, leading to the rapid formation of sulfate through a unique chemical generation path, which is also one of the main reasons for the rapid increase in particle concentration during severe smog periods."
London smog is usually considered to be caused by smoke dust emitted from coal combustion and primary pollutants such as sulfur dioxide. The smog in Los Angeles is a type of photochemical pollution. The main reason is that the exhaust from vehicles reacts with sunlight to form secondary pollutants. In China, the smog is a mixture of primary and secondary pollutants.
He Kebin said that the uniqueness of this complex pollution further highlights the importance of coordinated reduction of multiple pollutants. Especially at this stage, we should prioritize increasing the reduction of nitrogen oxides. "Although we previously knew that emission reduction was necessary, if we couldn't figure out the key chemical mechanism of the heavy smog pollution, we couldn't conduct effective quantitative simulation analysis of the model, and thus couldn't accurately assess how to reduce emissions most effectively and scientifically. Unscientific emission reduction may lead to serious consequences. It may cost a lot of manpower and resources, but the results will be insignificant."
(The source: Xinhuanet)
