Produced by: Popular Science China
Production: Institute of Atmospheric Physics, Chinese Academy of Sciences Sun Yang
Produced by: Computer Network Information Center of Chinese Academy of Sciences
According to data from the China Meteorological Administration , the national high temperature range in May this year was the highest record in nearly 50 years, with the highest number of high temperatures. In general, the high temperature weather this year came early and widely distributed. Moreover, according to the 50-year data, the trend of high temperatures has basically formed. Here, there are both major background factors that promote global warming due to the emission of greenhouse gases, as well as natural factors.
The harm of high temperature is obvious. It is reported that as of mid-May, multiple patients with heat stroke and heat stroke had appeared in Zhejiang, Jiangxi, Hunan and other places. This reflects the high temperature, and on the other hand, the high temperature is inadequately prepared.
Another hazard of high temperature heat wave is ozone (O3) pollution. According to monitoring data released by the ecological and environmental protection department, due to the continuous high temperature in May and June last year, there were more ozone pollution in Beijing, Tianjin and Hebei. During May this year, the average maximum 8-hour sliding value of a single station in Beijing was 279 micrograms per cubic meter, and the maximum hourly average value was 337 micrograms per cubic meter, and all of them exceeded the standard many times. Some stations even exceeded the standard by one to two-thirds of the number of days in a month. Among the 367 cities monitoring ozone nationwide, 252 cities experienced an average sliding concentration of eight hours exceeded the standard, with an exceeding standard rate of 68.7%. Among the 338 prefecture-level and above cities in 2016, 86% (288) of the whole year experienced an exceeding standard. Beijing-Tianjin-Hebei-Shandong-Henan, , Yangtze River Delta, Pearl River Delta and Chengdu-Chongqing are areas with more concentrated exceeding the standard. These regions have been on an upward trend year by year in the past decade, and the Beijing-Tianjin-Hebei region has the highest growth rate.
Figure 1: Distribution map of the 8-hour sliding average days exceeding the standard in 367 ozone monitoring cities across the country in May 2018 (data comes from the Ministry of Ecology and Environment) How did ozone pollution come about?
90% of the ozone in the atmosphere is distributed at altitudes of 20 km to 50 km from the ground. The stratosphere ozone generation is composed of ultraviolet rays from the sun, which divides an oxygen molecule into two oxygen atoms, and then each oxygen atom and the non-divided oxygen molecule are combined to form an ozone molecule composed of three oxygen atoms. The unstable ozone molecules will also decompose and merge to form a dynamically balanced ozone layer. ozone has a strong absorption effect on the ultraviolet rays of solar radiation. Therefore, this layer of ozone protects the earth's organisms from excessive ultraviolet rays at high altitudes, and is the protective umbrella of our earth's organisms (but some sensitive people will also feel the stimulation of ozone when flying).
, but another 10% of the ozone is present in troposphere , where we live. will have some downward delivery of ozone from the stratosphere in the northern hemisphere in spring, but more tropospheric ozone comes from photochemical reactions. This part of tropospheric ozone is both a greenhouse gas that causes global warming and a harmful pollutant gas that damages the earth's organisms. What we currently call ozone pollution is mainly harmful ozone in the troposphere closer to humans.
In general, the amount of near-scale ozone emitted by human activities is insignificant. It usually comes from laser printers used in daily life, electrostatic air purifiers, some disinfection devices that use ozone generators, etc., which only pose certain health hazards to nearby users. However, catering oil fume filtering devices and industrial volatile organic compounds emission reduction devices that have been widely used in recent years will produce excessive local ozone emissions.
A large amount of near-formed ozone is derived from indirect emissions from human activities . The exhaust gas emitted by humans and the combustion of fossil fuels contains a large amount of nitrogen oxides and volatile organic matter. Under specific meteorological conditions, such as strong sunlight, windless or breeze, these substances produce photochemical contaminants, mainly containing ozone, aldehydes and a variety of peroxyacyl nitrates, of which the ozone content accounts for 90%.
Therefore, just like fine particulate matter (PM2.5) is the main pollutant for haze pollution, ozone is the primary pollutant for photochemical pollution, also known as tracer. means that ozone exceeds the standard, indicating that other photochemical pollution products are produced along with each other, especially some organic aerosols, which is also one of the main components in fine particulate matter. Simply put, the three main causes of tropospheric ozone: nitrogen oxides, volatile organic matters and sunlight.
What are the dangers of ozone?
High concentration of ozone has a great impact on human health. can lead to damage to the central nervous system, lung function and thyroid function, tissue hypoxia, decreased vision and visual sensitivity, causing chest tightness, cough, sore throat, and aggravated the worsening of respiratory diseases. A large number of epidemiological studies have shown that long-term inhalation of ozone can increase the incidence and mortality of asthma, chronic obstructive pulmonary disease. The combined effect of ozone and other air pollutants, such as fine particulate matter and nitrogen oxide, on the cardiovascular system is more obvious.
Ozone is very unstable and has strong oxidation properties. It is harmful in itself and will participate in many chemical reactions in the air. can react with many organic and inorganic substances, harm indoor and outdoor materials, decorative paints and cultural relics, etc. At the same time, these reactions will produce secondary pollution. The secondary pollutants contain a large number of aldehyde compounds, so the secondary pollutants are more irritating and more serious harm to the human body and materials.
As outdoor atmospheric ozone exceeds the standard, window ventilation or ventilation facilities will be actively ventilated, ozone will be transmitted into the room. Since indoor walls, furniture, daily necessities, etc. have more surfaces, ozone generally has a short lifespan in the room. If there is no continuous source, it will basically react on its own within a few minutes to dozens of minutes. However, if there is a continuous external source, it will cause the indoor ozone to continue to exceed the standard. The more complex and harmful problem is that some volatile organic compounds can react quickly with ozone in the indoor environment. Research shows that some indoor formaldehyde is the main product of ozone reaction with most indoor unsaturated hydrocarbons. These reactants also include propionaldehyde, hexanal, propionic acid and hexanoic acid, as well as a small amount of acetaldehyde, valeraldehyde and nonanal.
Figure 2 The hazards of ozone (cited from: What are the hazards of near-Earth ozone and how to deal with it? Source: "Science and Life" Weekly
How to protect ozone pollution?
In the summer when the sun is strong, we should pay more attention to environmental monitoring data released by the environmental protection department, and pay attention to personal protection when ozone exceeds the standard. Because ozone itself has strong chemical activity, it will undergo decomposition and oxidation reactions with most surfaces, but it is just decomposition. The rate depends on temperature, humidity, contact material and residence time. Therefore, when outdoor ozone exceeds the standard, it can reduce outdoor ventilation and turn on the air purifier. air purifier can increase the number and time of contact between ozone and various surfaces. Especially, many purifiers are currently equipped with activated carbon filters, which reduces the concentration of ozone and is more efficient in reducing ozone concentration. However, it should also be noted that some high-voltage electrostatic purifiers themselves will release ozone. It should be ensured that they have an ozone filter and will not be used for several hours. It is saturated and penetrated during use.
Because even masks that protect fine particles do not intercept particles by smaller pore sizes, but are removed by the electrostatic fiber web through the principle of electrode adsorption, it will naturally have a certain adsorption and surface decomposition of ozone molecules. Of course, activated carbon materials with larger surface areas have higher efficiency. In other words, is effective for sensitive people to wear masks when outdoor activities with ozone exceeding the standard, but different materials, device type coordination, etc. Causes different efficiency. How to control ozone pollution?
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Near-formal ozone pollution problem is also a global problem. In the famous chemical smog incident, Los Angeles , it has been at the forefront of the US ozone concentration rankings over the years.
According to local research, Los Angeles residents are expected to have ozone-polluted air due to long-term breathing of ozone-polluted air, and it is expected that there will be One million adults and 300,000 children suffer from asthma, causing about $2.6 billion in economic losses.Ozone pollution is also the most important air pollution problem in EU member states.
It is reported that due to ozone pollution, the average life expectancy of EU residents has been reduced by 8 months. The number of deaths in the EU due to ozone pollution problems each year exceeds 2,000, and 60% of people have been exposed in environments where ozone concentration exceeds the standard. Pollution among the countries that the EU monitors ozone is mainly concentrated in France, Italy, Portugal and Spain. The high level of ozone pollution in this region is directly related to the high temperature and heat waves in summer in the region and motor vehicle emissions. The population, economy and consumption in global hotspots continue to grow. More energy consumption will emit more ozone precursors, and ozone pollution will continue to intensify.
The Obama administration in the United States once relaxed the ozone concentration restrictions in order to protect economic growth, but the standards of developed countries in Europe and the United States are still stricter than the current standards in my country. Compared with the EU's maximum 8-hour average of 120 micrograms/cubic meter and the World Health Organization's maximum 8-hour average of 100 micrograms/cubic meter, my country is 160 micrograms/cubic meter.
Near-stratigraphic ozone generally has obvious seasonal changes and daily changes in major cities in my country, that is, peaks appear in summer and autumn, but the precipitation increases after summer, which is manifested as valley values in mid-summer, bi-peak shapes in early summer and autumn. The daily changes are generally relatively regular sine wave shapes, and peaks appear in the afternoon. However, peak tailings or even bi-peak shapes will appear in remote suburbs due to transmission, pollution source distribution and boundary layer changes.
Because ozone is an indirect product, treatment can only be the raw material to reduce it. The difficulty is that our current atmospheric environment is in a very sufficient and continuous source of ozone production materials. As long as the meteorological conditions are suitable, it will immediately reach a pollution state. Recently, the widespread ozone pollution in northern regions is that high temperatures arrive early, meeting the conditions for producing high concentrations of ozone. Because ozone is the product of a nonlinear reaction, the reduction of raw materials does not directly correspond to the decrease in the concentration of pollutants. Sometimes, emission reduction leads to an increase in the concentration of pollutants.
The raw materials of ozone, such as the aforementioned nitrogen oxides and volatile organic matters, also known as precursors. These two precursors are very widely sourced. The nitrogen oxides in cities mainly come from motor vehicle exhaust gas, fossil fuel combustion, and industrial production processes. The main sources of volatile organic matter include automobile spraying, printing plant ink volatilization, gas station oil and gas volatilization, chemical plant refining process oil and gas volatilization, solvent use, etc. In addition to artificial sources, nitrogen oxides have natural sources from soil ecosystems, and volatile organic matters have natural sources from forest vegetation, etc. Since the implementation of the "Ten Atmospheric Measures", the national nitrogen oxide emission reduction has reached 5.69 million tons, but the prevention and control of volatile organic compounds is still in its infancy, with problems such as unclear emission base, unfavorable governance measures, insufficient monitoring capabilities, lack of standards and lagging regulations. At present, the national volatile organic compounds emissions are still on the rise. Especially at present, there are more emission reductions for nitrogen oxides, but there are fewer emission reductions for volatile organic compounds and no natural source emission reductions are involved. This just forms a ratio that is more likely to be generated by ozone, resulting in an increase in ozone concentration. Because the amount of ozone generation and the rate of generation are not only related to the concentration of nitrogen oxides and volatile organic compounds, but also have a strong nonlinear relationship with their ratio. Under low nitrogen oxide conditions, i.e., the nitrogen oxide control area, the ozone concentration increases with the increase of nitrogen oxides, while the change in the concentration of volatile organic matter has little effect on ozone; under high nitrogen oxide conditions, the volatile organic matter control area, the decrease of nitrogen oxides will cause the increase of ozone concentration, and the increase of volatile organic matter will increase the concentration of ozone. At present, most areas in my country are in the volatile organic matter control area.
Currently, my country's volatile organic compounds control level and technology are not yet mature, which is manifested as high emission reduction efficiency combustion facilities are very costly, and if you are not careful, it will increase nitrogen oxide emissions; low-cost methods and devices are often very low-efficiency, and even directly emit ozone, as mentioned above.It is necessary to focus on it. Studies have shown that olefins and aromatic hydrocarbons in volatile organic compounds are more active and critical types for ozone generation. Therefore, emission reduction should be given priority for highly active volatile organic compounds, and technical methods with high emission reduction efficiency and no secondary pollutants should be selected.
Figure 3 Principle of ozone production by artificial high voltage electrical devices
(Picture source: http://www.11ozone.com/jishuwenxian/7.html)
also needs to remind everyone that research results show that even a small amount of ozone that does not exceed the air quality standard still has potential harm to human health. , especially for people who work outdoors for several hours a day, even if the ozone concentration is lower than the stricter ambient air quality standards, it will have a negative impact on lung function. Therefore, when economic and technical conditions allow, reducing the ozone concentration in the air as much as possible should be the atmospheric environment optimization goal we pursue.
(all pictures marked from the source in this article have been authorized)
