Reporter of China News Weekly/Peng Danni
Published in the 955th issue of China News Weekly on 2020.7.13
Duan Yusen, director of the atmospheric monitoring room of the Shanghai Environmental Monitoring Center, remembers that on May 23, 2019, the only day of the year was severe pollution in Shanghai. The pollutants were not smog or PM2.5, which people knew well, but Ozone (O3) is a kind of substance with good feelings. On that day, the ozone concentration in Shanghai reached 266 micrograms/cubic meter, while the national standard concentration was 160 micrograms/cubic meter.
In the stratosphere at a height of 20 to 50 kilometers from the surface of the earth, it is surrounded by a thin layer of ozone with an average thickness of about 3 centimeters. It can absorb most of the ultraviolet rays in sunlight and protect life on the earth from damage. However, near the ground, ozone is a pollutant. Although its damage is not comparable to PM2.5, it causes more than 1 million deaths and tens of billions of dollars in crop losses worldwide every year.
The clear blue sky of Beijing in the summer afternoon is often regarded as the best time of the year in this city suffering from smog. But this time is often the highest concentration of ozone in a day. Over the course of a year, ozone concentration generally starts to increase from May to the highest point in August, and gradually decreases after entering autumn. In Shanghai, Duan Yusen also noticed some signs of ozone aggravation: In March and November, when ozone exceeded the standard rarely before, ozone pollution began to appear in these two years.
This year is the year when the "Three-Year Action Plan for the Blue Sky Defence" ends. When China's PM2.5 governance begins to achieve significant improvements after years of hard work, ozone is becoming a new environmental challenge. On May 15, Liu Bingjiang, director of the Department of Atmospheric Environment of the Ministry of Ecology and Environment, said at a press conference that compared with last year, the biggest change in air pollution control in 2020 is to deal with the increase in ozone concentration.
A pollutant that does not distinguish between rich and poor
The emergence of ozone pollution in China is a slow and gradual accumulation process. On the one hand, the decline in PM2.5 has made the ozone problem stand out-the annual average concentration of PM2.5 in the city’s prefecture-level and above cities has continued to decline, and it has fallen by 21.7% from 2016 to 2020; Since the ozone concentration, China's ozone pollution has been rising steadily and continuously. In fact, of the six air pollutants being monitored, ozone is the only pollutant that does not fall but rises, and the number of cities that meet the standard is also declining slowly.
On June 2 this year, the Ministry of Ecology and Environment of the People's Republic of China released the "2019 China Eco-Environmental Status Bulletin". According to the Communiqué, in 2019, 30% of the 337 cities in the country exceeded the ozone standard, second only to PM2.5. Among them, ozone pollution in the Beijing-Tianjin-Hebei and Yangtze River Delta regions is particularly prominent.
A summary article published in the Engineering Journal of the Academy of Engineering by Hao Jiming, a professor at the School of Environment at Tsinghua University and an academician of the Chinese Academy of Engineering, in June 2020: From 2013 to 2017, the average concentration of ozone in 74 major cities across the country increased by 20% . According to a report by the South China Morning Post, although efforts to control other major pollutants have shown a good shift, ozone pollution in Hong Kong has increased by 20% from 2013 to 2018, setting a peak in two decades.
At the press conference on June 2, Bai Qiuyong, director of the Ecological Environment Monitoring Department of the Ministry of Ecology and Environment, pointed out that the global ozone background concentration is increasing due to factors such as global warming, anthropogenic pollution emissions, and large-scale regional transmission. The average annual increase is about 1 microgram, which is more consistent with the situation in China. Increased temperature is conducive to the formation of ozone. In actual monitoring, Duan Yusen saw that the peak of ozone in Shanghai in recent years was in 2017. The temperature was particularly high that year. In 2018 and 2019, due to more summer typhoons, the number of days with ozone exceeding the standard has decreased relatively.
Increasing ozone pollution caused by extreme high temperature weather and climate warming is a global and universal situation. The "2019 Global Air Status Report" shows that, unlike PM2.5, which has developed differently in different economic and social developed countries, whether in developed or developing countries, the ozone concentration in various countries continues to maintain a high level. Even in the United States and Japan, where ozone treatment started earlier and took great effort, there has been a slight increase in recent years.
Lei Yu, director of the Atmospheric Institute of the Environmental Planning Department of the Ministry of Ecology and Environment, told China News Weekly that because of the higher latitudes and relatively low temperatures and radiation in Europe, the overall ozone concentration is lower, and China is more comparable to the United States at the same latitude. Sex. Statistics show that from 1980 to the present, the ozone concentration in the United States has been declining slowly. The maximum average 8-hour ozone concentration in 2019 is about 130 micrograms/cubic meter, which is a 35% decrease from 1980.
Wan Wei, China project manager of the Asian Clean Air Center, an international non-profit organization, is responsible for the ozone project. She said that the average ozone concentration in China in 2019 is close to the average value of the United States more than a decade ago from 2005 to 2010.
According to an article published in late 2017 by Chen Renjie, a professor at the School of Public Health of Fudan University, the national average is that for every 10 micrograms/cubic meter increase in ozone concentration, although it is only short-term exposure, it is caused by cardiovascular disease, hypertension, coronary heart disease, stroke The average daily mortality rate will increase by 0.24%, 0.27%, 0.60%, 0.24% and 0.29% respectively.
The International Non-Profit Organization Health Impact Institute and other organizations released the 2019 Global Air Status Report in April 2019 to estimate the disease burden caused by three pollutants: indoor air pollution, PM2.5 and ozone: from the global Look, among the deaths caused by air pollutants, 8% is related to PM2.5 and 1% is related to ozone. 1% is still not a small absolute value. In 2017, ozone killed approximately 472,000 people, most of which occurred in China (38%) and India (31%).
Chen Renjie told China News Weekly that the current mainstream conclusion is that ozone is harmful to people's respiratory tract, such as causing respiratory tract inflammation, decreased lung function, and current views on cardiovascular damage are inconsistent. Unlike particulate matter, ozone is a very active gas that easily reacts and disappears with the surface of the object. It quickly reacts with objects such as walls, desktops, furniture, and floors in the room, so the indoor ozone concentration is relatively low. High ozone pollution weather, staying indoors is the most effective way to prevent and control ozone. There is no rigorous scientific evidence to show that wearing a mask and using an air purifier can effectively protect the health hazards caused by ozone.
Ozone will also hinder the ability of plants to photosynthesize, which in turn causes damage to ecosystems and crops. A document published by researchers at York University and other institutions in the UK in late 2016 found that in 2000, nearly half of the world’s representative ecosystems were exposed to ozone levels that constitute ecological risks, and predicted that this risk will be reached by 2050. increase.
Complex generation mechanism
Liu Qihan, a senior atmospheric scientist at the Hong Kong University of Science and Technology, said, "Ozone is a huge problem. But our understanding of the components that make up it and how to control them is so inadequate." Under the background of increased extreme weather and accelerated industrialization of cities, the complexity The atmospheric chemical reaction has exacerbated the challenge of ozone control.
Most of the ozone is formed by man-made emissions of "NOx (Nitrogen Oxide)" and "VOCs (Volatile Organic Compounds)" under high temperature and light conditions. NOx mainly comes from motor vehicles, power plants, coal-fired boilers and cement kilns. VOCs have the same source as NOx. In addition, they also exist in a wide range of organic solvents such as paints, printing inks, adhesives, and sealants. Plant emissions account for about 30% of VOCs in the atmosphere.
The precursors of PM2.5 also include NOx, VOCs, and SO2, which have a lot of overlap with ozone components. However, China's effective control of PM2.5 has not brought ozone a "two birds with one stone" effect. This is because the relationship between PM2.5 and its precursors is linear, and ozone does not follow this rule.
Scientists studying atmospheric chemistry use a curve called EKMA to specifically describe the nonlinear relationship between ozone concentration and its two precursor pollutants. In this regard, Lei Yu explained that photochemical pollution is a relatively complicated process. Within a certain range, if the reduction ratio of NOx and VOCs is not well controlled, the ozone concentration will rise.
Specifically, if VOCs did not decrease in the VOCs control area, but NOx decreased, the ozone concentration may increase. For example, during this new crown epidemic, due to the reduction of automobile exhaust and industrial emissions, the nationwide NOx NOx has dropped substantially year-on-year, but VOCs have not been reduced in a proportional manner. As a result, many cities have found an increase in ozone concentration-this year In the first quarter, domestic ozone concentration increased by 3.4% compared with the same period last year, and in April it increased by 8.1% year-on-year. In the first three months of this year, the national PM2.5 concentration dropped by about 15% year-on-year.
Taking Shanghai as an example, Duan Yusen explained to China News Weekly that because Shanghai has a large number of emission sources such as motor vehicles, ships and airplanes, the NOx in the air has always been at a relatively high level, and at the same time, there are many factories, so VOCs The concentration level is also relatively high, and the overall ozone pollution problem is becoming more and more prominent, but the rules behind it are complicated and changeable.
When the NOx concentration is very high, such as the roadside, there are many vehicles and large emissions, but the ozone concentration is relatively low, because the NO emitted from the exhaust of the car "eats" O3 and becomes NO2, and these NO2 will be blown to the wind In the downwind direction, a chemical reaction with VOCs generates ozone. As a result, in suburbs with fewer vehicles, the ozone concentration is sometimes higher than in the urban center. "The prevention and control of ozone is a complex issue, and we need to consider the coordinated prevention and control of local and regional, urban and suburban, VOCs and NOx." Duan Yusen said.
This complex relationship of ozone generation makes the use of a word for ozone control: synergy. Lei Yu said that the drastic reduction of both nitrogen oxides and volatile organic compounds will definitely bring about a decline in ozone, but this nonlinear and repeated relationship will occur in certain intervals, so collaborative governance is very critical, how to go To grasp the ratio of these two pollutants to reduce emissions, scientific research is needed to give the answer.
Wan Wei said that Japan is a typical example. After 2001, Japan began to implement measures to reduce NOx emissions from motor vehicles. The effect is very good. The concentration of NOx and particulate matter has been declining, but the ozone index has not decreased but increased. "It is very similar to China's situation, also because there is no coordinated emission reduction." Japan revised its "Atmospheric Law" in 2004 to reduce the total amount of VOCs emissions by 30%, and the ozone concentration in 2009 only fell sharply.
In addition, when the concentration of PM2.5 in the atmosphere drops significantly, it will lead to enhanced optical radiation, which is conducive to ozone generation. The monitoring results also show that the interannual changes of PM2.5 and ozone in China are generally inversely correlated. Many studies have shown that a significant reduction in PM2.5 may bring the potential risk of increased ozone content.
After PM2.5, ozone treatment is on the agenda
Bo Qiuyong, Director of the Ecological Environment Monitoring Department of the Ministry of Ecology and Environment, concluded that the fundamental cause of ozone pollution in China at this stage is that ozone precursors such as VOCs and NOx are still maintained at a relatively high concentration level. In strong sunlight, high temperatures, and few clouds Under unfavorable weather conditions such as high volume, weak wind, and little rainfall, the photochemical reaction is accelerated, resulting in ozone concentration exceeding the standard.
"Since the "Twelfth Five-Year Plan", China has made some progress in reducing nitrogen oxides through total emission reduction. In contrast, VOCs control is a shortcoming." Lei Yu said that the emission characteristics of VOCs and other pollutants are not Too much the same, it has too many emission points, and is very scattered, the past regulatory measures and means are not fully applicable. This part is also where the governance gap between China and developed countries is relatively large. At the same time, China’s situation is also more complicated: there are many industrial categories, and the level of enterprise management is uneven. There are the most advanced enterprises and the most backward enterprises. .
Hao Jiming's team also pointed out that during the "Air Pollution Prevention Action Plan" from 2013 to 2017, national NOx emissions were reduced by 21%, but the reduction in VOCs was only 2%. Such a combination change is effective for controlling PM2.5 instead of ozone. The air data of these years also reflects this result.
For comparison, Wan Wei analyzed that between 1997 and 2004 in the United States, NOx emissions were reduced by 25% and VOCs emissions were reduced by 21%. Cooperative control and simultaneous emission reduction were basically achieved. She said that although ozone's atmospheric photochemical mechanism is more complicated, as long as the precursor pollutants are coordinated to reduce emissions, such as 1:1 reduction in equal proportions, there will definitely be improvement.
In recent years, China has issued several policy documents on the treatment of VOCs, such as the “Thirteenth Five-Year Plan for the Prevention and Control of Volatile Organic Compound Pollution” promulgated in 2017, and the “Comprehensive Management Scheme for Volatile Organic Compounds in Key Industries” issued in 2019 And the recently released "2020 Tackling Plan for Volatile Organic Compounds (VOCs)"... This short board is constantly being strengthened.
As far as technology is concerned, the treatment of ozone is not difficult. As early as 2008, a report on future ozone pollution written by the Royal Society wrote: "The technology to solve ground ozone pollution has been widely used. Among global environmental problems, ground ozone has a unique controllable Sex."
Over the past decade, the core of China's atmospheric governance has been around PM2.5. Lei Yu said that in the binding indicators of the “13th Five-Year Plan” from 2016 to 2020, one is PM2.5, and the other is a good number of days. Ozone is not managed as a separate indicator. But in the next stage, ozone will be on the stage of China's atmospheric governance. In many academic conferences held recently, Professor He Kebin, a professor at Tsinghua University and an academician of the Chinese Academy of Engineering, pointed out that the coordinated control of PM2.5 and ozone has become the key to continuous improvement of air quality.
In accordance with established practice, the "Fourteenth Five-Year Plan" (that is, 2021-2025) will be released during the next two sessions of the country next year, and various special plans will be introduced successively. The environmental planning institute where Lei Yu is also involved in the “Fourteenth Five-Year Plan”, he said that the focus of the “Thirteenth Five-Year Plan” is on PM2.5. “The “Fourteenth Five-Year Plan” will continue to require PM2.5 to fall, but ozone The synergy with particulate matter will definitely be a focus. Another possible focus is how to promote the transition of the economy to high-quality development through environmental protection."
At a press conference on May 15, Liu Bingjiang, director of the Atmospheric Environment Department of the Ministry of Ecology and Environment, revealed that in the preparation of the special plan for the prevention and control of atmospheric pollution during the “Fourteenth Five-Year Plan”, two precursors of ozone, VOCs, were specifically targeted. Nitrogen oxides have designed emission reduction targets.
"China News Weekly" No. 25, 2020
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