Doing a ＂physical examination＂ for the atmospheric environment makes ＂secondary pollution＂ nowhere to follow.
Late autumn and early winter are high season for air pollution, but the winter in Beijing, Tianjin and Hebei in 2019 is far more "blue" than in previous years. Under this blue sky and white clouds, there is an important technology that is "escorting" to win the blue sky defense war, which is — — Atmospheric monitoring and early warning technology.
"Atmosphere ‘ Monitoring and early warning ’ Just like the laboratory in the hospital, it tells you carefully, accurately and clearly whether there is pollution in the atmosphere and how the whole pollution level is. " At the recent "air pollution monitoring, forecasting and early warning technology exchange meeting" held by China 21st Century Agenda Management Center, Liu Wenqing, an academician of China Academy of Engineering, said that although the current average concentration of PM2.5 has dropped significantly, the "secondary pollution" represented by ozone and volatile organic compounds is constantly emerging, which challenges air monitoring and is also a difficult problem that must be overcome to win the blue sky defense war.
At the exchange meeting, experts including Zhang Yuanhang and Academician Liu Wenqing conducted in-depth discussions on the innovation and breakthrough of fine monitoring and early warning technology of air pollution in the "Special Project of Causes and Control Technology of Air Pollution" (hereinafter referred to as "Special Project of Air Pollution"). They believed that the internal cause of regional air quality regulation is emission, and the external cause is meteorology, and the control should focus on monitoring and emission reduction at the source.
Capturing the invisible "killer" of air pollution
Free radical, an active name, plays the role of "yin and yang teacher" in the prevention and control of air pollution.
On the one hand, it is the "scavenger" of atmospheric toxic substances, and the existence of free radicals constantly regulates the toxic components in the atmospheric environment, so that they cannot be aggregated. On the other hand, free radicals will oxidize with volatile organic compounds emitted by humans, and then react with nitrogen oxides to generate ozone and secondary organic aerosol, which play a "core" role in ozone pollution and particulate pollution.
A double-edged sword stands upright in the atmosphere. How should we examine it? The attitude given by the atmospheric expert group is objective and accurate, and the finer the better.
Accurate monitoring of free radicals is based on the changing trend of the current air pollution pattern. Li Xin, a researcher at Peking University Institute of Environment and the project leader of "On-line Measurement Technology of Atmospheric Free Radicals and Active Precursors", said in an interview with the Science and Technology Daily reporter that since the State Council promulgated the Air Pollution Prevention and Control Action Plan in 2013 (referred to as "National Ten Articles"), various measures have significantly alleviated primary pollution such as PM2.5. However, from the chemical composition of particulate matter, secondary components (that is, the first ten articles), Moreover, the concentration level and over-standard rate of ozone generated by the chain reaction of volatile organic compounds and nitrogen oxides under light conditions are also on the rise in major urban areas in China.
Free radicals are the indispensable "catalyst" in the above-mentioned "trade-off" pollutant transformation. Therefore, free radicals have become an important indicator to monitor the change of air pollution, and its concentration and activity have become a marker to measure the level of atmospheric oxidation in the eyes of Li Xin’s team.
Can free radicals be easily detected? Compared with conventional gaseous pollutants, the concentration of free radicals is very low. Li Xin introduced that the concentration standard of ozone in the atmosphere is 160μ G/m3 (microgram per cubic meter), and the converted concentration is 2×1012 cm-3 (molecule per cubic centimeter). However, the peak concentration level of hydroxyl radical (OH), which contributes the most to atmospheric oxidation, is only about 107cm-3 (molecule per cubic centimeter), which is equivalent to one hundred thousandth of the concentration of traditional gaseous pollutants. Moreover, the activity of free radicals is very strong, and the surface of OH radicals will be annihilated when it is touched by any substance, which means that it is in a state of change all the time, which makes it more difficult to detect free radicals.
In fact, the measurement of OH radicals has been studied since 1970s, and there are still some technical problems that have not been overcome until now. Li Xin’s team also adopts the internationally mature technical route — — The method of laser-induced fluorescence is that a laser beam hits the OH radical to stimulate the OH radical to release fluorescence, and the concentration of the OH radical can be judged by monitoring the intensity of the fluorescence signal.
The innovation of their method lies in making the laser measurement stable, accurate and accurate. Behind it is a set of self-designed measurement and feedback system, a series of conditional experiments and software simulations, and a self-built free radical concentration emitter & HELIP; … It is also the complete application of this technology and equipment that makes China the sixth country in the world to independently master this technology to capture the invisible "killer" — — Free radical country.
Monitoring ultra-fine particles from stationary pollution sources
With the introduction of the "National Ten Articles" and other policies, the emission standards of pollutants have become more and more strict. Nowadays, the atmospheric environment monitoring technology for fixed pollution sources has already entered the era of ultra-fine particle monitoring.
In the project report of Ding Yanjun, a professor in the Department of Energy and Power Engineering in Tsinghua University, the reporter saw that although the particulate emission detection equipment of stationary pollution sources such as thermal power plants in China and the world can monitor the ultra-low emission below 5mg/m3 (mg/m3) on line, it can only measure the total mass concentration of discharged smoke and dust, and cannot realize the high-precision on-line monitoring of the particle spectrum distribution of fine and ultrafine particles.
"The traditional one-way light particle optical scattering technology can only obtain one-dimensional optical information corresponding to the total mass concentration of smoke and dust, and when the environmental conditions change, the particle spectrum and refractive index of smoke and dust will change, which will affect the accuracy of the measurement results because it cannot perceive the change of smoke and dust characteristics." Ding Yanjun told the reporter that according to the scattering relationship between light waves and particulate matter, the project team used signal measurement and inversion signal processing algorithms to obtain PM10, PM2.5 and PM1 values in the smoke concentration, and finally realized the joint measurement of particle spectrum and mass concentration of fixed pollution sources.
It is noteworthy that the theoretical methods and key components obtained in this project have all been patented, among which the method of obtaining aerosol characteristics of scattered light signal and its application have been patented in PCT international invention, and both Japanese and American patents have been authorized.
For the monitoring of stationary pollution sources, in addition to sophisticated technical standards, there must also be pollutant monitoring technology that conforms to China’s actual working conditions.
Ding Yanjun told the Science and Technology Daily reporter that the imported instruments, whether from the United States, Europe or Japan, can not meet the monitoring of ammonia escape in the flue gas of a large number of high-ash coal-fired units in China, because a large number of fly ash in the flue gas makes the measurement laser unable to penetrate and fail.
In view of this situation, the project team developed a high-precision, high-sensitivity, online-calibration in-situ sampling ammonia escape monitoring technology, which realized ammonia escape monitoring of flue gas denitrification in a large number of high-ash coal-fired units in China. In addition, in view of the latest research results of fly ash adsorbing a large amount of escaped ammonia, the online monitoring technology of ammonia adsorbed by fly ash in flue gas was first developed in the world, and the total amount of ammonia escaping from flue gas denitrification was successfully monitored.
"This will provide key technical support and guarantee for the comprehensive monitoring and strict control of ammonia escape from flue gas denitrification widely existing in China’s industrial process." Ding Yanjun said.
Exploring three-dimensional information of atmospheric boundary layer pollution
In the winter of 2018, a 15-day large-scale meteorological observation experiment spanning 2,000 kilometers was held as scheduled.
"This is a large-scale, multi-platform and multi-factor comprehensive observation experiment of air pollution." Hu Fei, a researcher in lasg, who is in charge of this experimental project, said that the purpose of this experiment is to obtain the three-dimensional information of air pollution distribution in Beijing, Tianjin and Hebei, especially the high-resolution vertical distribution and time evolution characteristics of air pollutants and meteorological elements in the atmospheric boundary layer.
The difficulty of the experiment lies in high-altitude detection and keeping watch over the Beijing-Tianjin-Hebei region, so it was named "Wangdu Experiment". Different from the monitoring technology of fixed pollution sources on the ground, "Wangdu Experiment" mainly aims at the deficiency of simultaneous joint detection of pollutants in the atmospheric boundary layer and the vertical structure of meteorological elements, and carries out comprehensive detection and technological breakthrough.
The reason is that the atmospheric boundary layer, which is 1-2 kilometers away from the ground, dominates the material and energy exchange between the earth and the atmosphere, and is the main place for human life and engineering activities, as well as the main place for air pollution and meteorological disasters. Hu Fei told reporters: "In such a high range, it is a concrete difficulty to achieve full coverage in space, complete elements in composition and continuity in time."
An air pollution monitoring experiment integrating ground and air was staged in Beijing-Tianjin-Hebei region — — A 32-meter-long, 1,800-cubic-meter-long large air boat floats at an altitude of 1,000 meters like a kite. The "King of the Air" plane travels through the air with equipment, and the lidar conducts navigation observation with Beijing as the center, and a number of ground observation stations in Beijing, Tianjin and Hebei also implement synchronous collaborative observation & HELIP; … In order to support the comparison of air pollution characteristics between the north and the south of China, the monitoring equipment of the Pearl River Delta, the 356-meter-high Shenzhen Meteorological Tower and the 600-meter-high Guangzhou TV Tower, which are 2000 kilometers away, cooperated with the "Wangdu Experiment" to carry out observation at the same time.
In this observation experiment, the new ozone lidar, nitrogen dioxide lidar, high-altitude turbulence ultrasonic anemometer detection system and vorticity-related PM2.5 turbulence flux observation system independently developed by the project also made their debut.
Hu Fei said that this experiment has obtained new information and new phenomena about the fine structure of physical and chemical elements of pollutants that could not be obtained by ground observation or portable instrument sounding observation, which can provide scientific reference for revealing the formation mechanism of heavy pollution, pollutant source analysis and optimal control of air pollution in Beijing, Tianjin and Hebei. (Reporter Liang He)