Ten questions and answers about PM2.5

Core Tip: If it is the first contact, "PM2.5" this string of characters may make you see the clouds in the fog, incomprehensible. In fact, it has an easy-to-understand Chinese name - fine particles, is the general term for solid particles or droplets in the air with a diameter of less than or equal to 2.5 microns.

1. What is PM2.5?

If it is the first contact, the "PM2.5" string of characters may make you see the cloud in the fog, not knowing what to do. In fact, it has an easy-to-understand Chinese name - fine particles, is the general term for solid particles or droplets in the air with a diameter of less than or equal to 2.5 microns. The particles are so small that they are invisible to the naked eye. They can float in the air for several days. The slender hair of humans is about 70 microns in diameter, which is nearly 30 times larger than the maximum PM2.5.

PM is an acronym for English particulate matter.

The definition of accurate PM2.5 requires adding the modifier "aerodynamics" before "diameter". This is not a profound case. The particle in the air is not a regular sphere. How do you measure its diameter? In practice, if the aerodynamic characteristics exhibited by a particulate matter as it passes through a detection instrument are consistent with spherical particles having a diameter of less than or equal to 2.5 microns and a density of 1 g/cm3, it is referred to as PM2.5. This definition also determines the need to use aerodynamics to separate PM2.5 from larger particles in the determination of PM2.5 rather than using a 2.5 micron filter.

Knowing the definition of PM2.5, it is easy to draw the definition of PM10 - replace 2.5 in the definition with 10, PM10 is also known as respirable particulates. In PM10, particles with diameters between 2.5 and 10 microns are called coarse particles, as opposed to fine particles.

2. What are the ingredients of PM2.5?

Although natural processes also generate PM2.5, its main source is human emissions. Humans are not only directly discharging PM2.5, but also emit some gaseous pollutants and convert them into PM2.5 in the air. Direct emissions mainly come from the combustion process, such as the combustion of fossil fuels (coal, gasoline, diesel), combustion of biomass (straw, firewood), and waste incineration. The gaseous pollutants that are converted into PM2.5 in the air are mainly oxide dioxide, nitrogen oxides, ammonia, and volatile organic compounds. Other man-made sources include road dust, construction dust, industrial dust, and kitchen smoke. Natural sources include wind, dust, volcanic ash, forest fires, floating sea salt, pollen, fungal spores, and bacteria.

The source of PM2.5 is complex and the composition is naturally complex. The main components are elemental carbon, organic carbon compounds, sulfates, salts, and ammonium salts. Other common components include various metal elements, such as sodium, magnesium, calcium, aluminum, iron and other abundant elements in the earth's crust, but also lead, zinc, arsenic, cadmium, copper, and other heavy metals that mainly result from human pollution.

In 2000, researchers measured the sources of PM2.5 in Beijing: dust accounted for 20%; sulfates, salts, and ammonia salts converted from gaseous pollutants accounted for 17%, 10%, and 6%, respectively; coal combustion occurred. 7%; emissions from diesel and gasoline emissions contribute 7%; crops and other biomass contribute 6%; and plant debris contributes 1%. Interestingly, smoking also contributes 1%, but this is only a rough scientific estimate and is not necessarily accurate [1]. The composition of Beijing PM2.5 was also determined in this study: carbon-containing particulate matter, sulfate, and y-root, which together accounted for 69% of the weight. Similarly, 41.6% of the Shanghai PM2.5 measured in 1999 was ammonium sulfate, ***, and 41.4% were carbon-containing substances [2].

3. What are the hazards of PM2.5 to health?

PM2.5 mainly causes harm to the respiratory system and cardiovascular system, including respiratory tract irritation, cough, dyspnea, decreased lung function, increased asthma, chronic bronchitis, arrhythmia, non-fatal heart disease, and heart and lung disease Premature death [3]. The elderly, children and patients with heart and lung diseases are sensitive to PM2.5 contamination.

Routes of Contamination If the concentration of PM2.5 in the air is longer than 10 μg/m3, the risk of death begins to rise. For every 10 micrograms/cubic meter increase in concentration, the overall risk of death rose by 4%, the risk of death from heart and lung disease increased by 6%, and the risk of death from lung cancer increased by 8% [4-5]. How much risk does this mean? We can compare smoking. Smoking can increase the risk of male death from lung cancer by 21 times (i.e., 2100%), that of females by 11 times (1100%), and that of middle-aged people by 2 times (200%).[6] ]. Compared with smoking, the hazard of PM2.5 is very small. If smoking does not make you frightened, then you do not have to worry about the health impact of the current PM2.5 exceeded.

However, from the perspective of society as a whole, reducing these seemingly small risks has great benefits. The U.S. Environmental Protection Agency made an estimate in 2003: “If the PM2.5 reaches the standard, the entire United States can avoid tens of thousands of premature deaths each year, treat tens of thousands of people on medical visits, lose millions of times, and millions of children’s respiratory systems. Disease" [7]. Compared to the current China, the air quality in the United States was already quite good at that time, and there were only a few areas with a slight excess of standards [8]. If China's PM2.5 can achieve compliance, the social benefits will undoubtedly be huge.

The above data on the risk of death from PM2.5 originated from a paper published in the Journal of the American Medical Association in 2002 [4]. This paper analyzes the relationship between mortality and air pollution among participants in a long-term study and finds that increased mortality is associated with PM2.5 and CO2 pollution, but is not reliably linked to coarse particulate pollution. . The forward-looking study conducted in the United States began in 1982, when 1.2 million participants were recruited. The conclusion of the paper is based on up to 16 years of follow-up data and is currently the most reliable evidence that PM2.5 pollution increases the risk of death.

4. If there is no pollution, how high is the concentration of PM, and how high is it actually?

Even if there is no artificial contamination, there is a certain concentration of PM2.5 in the air. This concentration is called the background concentration. In the United States and Western Europe, the background concentration is approximately 3-5 μg/m 3 [5], and the background concentration in Australia is also approximately 5 μg/m 3 [9]. How high is the background concentration in China? There is no published data yet, but it should not be too different from other countries.

China has not yet carried out large-scale monitoring of PM2.5, and the published PM2.5 data are very limited. The South China Environmental Science Research Institute of the Ministry of Environmental Protection in Guangzhou has released PM2.5 monitoring values ​​on a daily basis since June 13 in 2011 [10]. As of November 20, the concentration range was between 0.6 and 99 μg/m3 ( Note: 0.6 this data should be due to instrument failure, the normal value will not be so low), the average value of 38 micrograms / cubic meter, this value exceeds the proposed annual average standard (35 micrograms / cubic meter) [11]. Of these 121 days, 6 days have exceeded the proposed daily average (75 μg/m3). From the scientific papers published in the past decade or so, the measured values ​​of PM2.5 at a certain stage in some large cities in China can be found. For example, the PM2.5 annual mean value measured at five monitoring sites in Beijing in 2000 was 101 μg/m3 [2]; during the 17 days of the 2008 Beijing Olympics, the PM2.5 measured at Peking University was a minimum of 28.2, with a maximum of 147.4. Microgram/m3, with an average of 64.7 μg/m3 [12]. In 1999, the average PM2.5 values ​​measured at two monitoring sites in Shanghai were 57.9 and 61.4 μg/m3 [2]. These annual averages are much higher than the proposed annual average (35 micrograms/cubic meter).

In addition to these sporadic data, we can also estimate PM2.5 concentration based on PM10 data. According to China's current air quality standards, PM10 is a routine monitoring indicator. National monitoring has been carried out for more than 10 years. From 2001 to 2009, the average value of PM10 in major cities across the country fell from 125 to 90 μg/m3 [13]. The ratio between PM2.5 and PM10 is usually between 0.5 and 0.8. We take an extreme estimate of 0.8. The average PM2.5 in major cities in China is 72 micrograms/cubic meter in 2009, which will be released soon. 2.1 times the standard (35 μg/m3). How much worse is this compared to the air quality in the United States? In 2009, the average annual PM2.5 in the United States was 9.9 μg/m3, and the annual average of more than 90% of the 724 monitoring sites was lower than 12.6 μg/m3 [8].

The annual average value of the country is only used to reflect the overall status of particulate pollution in China, and it is of little significance to evaluate the air quality of our cities. We need to pay more attention to the data from the monitoring points that are closest to our lives and work. Where is this data? If you live in Beijing and happen to be near the U.S. embassy, ​​you can refer to the real-time PM2.5 data published by the museum.

However, it is worth mentioning that although the U.S. embassy’s monitoring equipment is professional (see Q&A 9), the embassy is not an environmental monitoring department after all. There is no evidence that their staff have the relevant expertise and they measured PM2.5 values ​​are often higher than PM10 measured by environmental protection agencies and third parties. This is not normal. Therefore, the data of the U.S. Embassy can only be "only for reference".

However, more of us do not live in Beijing. Even if we are not in the vicinity of the U.S. embassy in Beijing, where should we look at the data? The real-time PM10 data of the major cities in the country can be found on the website of the environmental monitoring terminal. There are several monitoring points in each city. We can select the nearest point for reference. If you are optimistic, you can estimate PM2.5=PM10×0.5. If you are pessimistic, estimate PM2.5=PM10×0.8.



5. Are other countries implementing the PM2.5 standard? What is the standard value?

Since the United States took the lead in establishing air quality standards for PM2.5 in 1997, many countries have been following up on the inclusion of PM2.5 in monitoring indicators. If we simply proceed from the purpose of protecting human health, the national standards should be the same, because the standard is based on the same scientific research results. However, the formulation of standards also needs to take into account the pollution status and economic development level of each country. The development of extremely strict air quality standards in a developing country with serious air pollution can only become a magnificent display and has no practical significance. According to the findings of the American Cancer Society and Harvard University, the World Health Organization (WHO) established a guideline value for PM2.5 in 2005. Above this value, the risk of death rises significantly. At the same time, the WHO has set up three transitional target values, which provide phased targets for areas that are not yet available in one step. Among them, the target-1 is the most relaxed and the target-3 is the most rigorous [5].

The following table lists the standards of the WHO and several representative countries. The standards to be implemented in China are the same as those for the WHO transitional period-1. The United States and Japan have the same standards and are basically the same as Target-3. EU standards are slightly looser, consistent with target-2, Australia's standards are the most stringent, and annual averages are lower than WHO guidelines. The degree of strictness of the standard basically reflects the air quality situation of each country. The better the air quality is, the better the country can formulate and implement more stringent standards.

World Health Organization (WHO) and some countries' PM2.5 standards (Unit: micrograms per cubic meter)

6. Is China's PM2.5 standard worse than other countries?

China's PM2.5 standard is scheduled to take effect in 2016. Although it is less than a decade or two behind the United States, it is not too late compared to the EU's entry into force in 2015. If only from the standard value, China's upcoming new standards have been consistent with the WHO transitional goal -3, although lagging behind developed countries, but it can be regarded as the first step in three steps. However, even if the standard values ​​are the same and the way in which the judgement is met is different, the binding force is very different. For example, China's current air quality standards were established in 1996. The average daily PM10 standard is 150 micrograms/cubic meter, which is ostensibly as stringent as the current US standards. However, according to U.S. standards, an average of at most 1 day per year can exceed the limit. Otherwise, if it does not meet the standards, the over-qualified areas need to submit improvement plans and implement them. There are no similar regulations in the Chinese standard documents. When implementing standards, each region only calculates the “days of compliance” and “rate of compliance” of each year. The PM10 standard has been implemented for 15 years so far. An 86.2% compliance rate can also be used as a positive news report [14].

In the upcoming PM2.5 new standard, there is still no stipulation on how high the compliance rate is acceptable. How is WHO and other countries stipulated? WHO requires a maximum of 3 days per year (99% compliance rate), Australia up to 5 days, and the United States and Japan require a compliance rate of 98%. The backwardness of the Chinese PM2.5 standard is not only in the standard value, but more importantly in the binding force.

7. The new standard is about to be released. Why should it not be implemented until 2016?

For this issue, the standard setter responded by saying: “Considering that the implementation of the ambient air quality standard is a complex system engineering and the current status of the national environmental monitoring capability, combined with the experience of the current standard implementation process, to ensure accurate data Sex and comparability, the time for the national implementation of this standard was set as January 1, 2016, in order to allow sufficient preparation time for each region to strengthen the relevant supporting work [15].

Is it reasonable to say so? We may wish to refer to what other countries do. On the website of the US and Australian environmental protection agencies, there is a very detailed memorandum on the development process of the PM2.5 standard. We will use these two countries as examples.

As early as 1994, the United States announced that it would increase the PM2.5 target. During 1994-1996, several seminars were held and drafts were released at the end of 1996. During the consultation period, 14,000 calls were received, 4,000 e-mails were received, 50,000 written or oral comments were received, and opinions were heard through hearings, conferences, and television programs. After this sincere and earnest opinion solicitation, the PM2.5 standard was finally released on September 16, 1997. However, at that time, no national monitoring of PM2.5 had been carried out, and it was not until 1999 that the states began to perform routine monitoring of PM2.5 in 2000 [16].

Australia began to consider it in 2001, and in 2003 it established non-mandatory standards for PM2.5. The purpose of this standard is to collect data in order to review the reasonableness of this standard and to prepare to consider the establishment of mandatory standards in 2005. In the process of soliciting opinions, there are opponents who believe that the compulsory standards should be established directly. Otherwise, they lack the binding force and the significance is not significant. The Australian Environmental Protection Commission (NEPC) believed that there was not enough PM2.5 monitoring data at the time and it was not able to assess how the non-compliance would have an impact and adhered to the original practice [9]. Until this year (2011), Australia's PM2.5 is still not a mandatory indicator [17], but during this period has been doing a lot of monitoring and basic research work [18].

China's PM2.5 mandatory standards are being solicited for comments and are planned to be implemented in 2016. The meaning of “implementation” should refer to conducting routine inspections and publishing results. It took the United States two to three years from the standard issued in 1997 to the national monitoring and regularization in 2000. Australia issued non-mandatory standards in 2003, followed by national monitoring. Considering China's national conditions, it is reasonable to postpone the "implementation" in a few years. But is the period of four or five years too long?

8, how to determine PM2.5?

There are various sizes of particulates floating in the air. PM2.5 is the smaller one (as defined in Q1). It is easy to think that determining the concentration of PM2.5 requires two steps: (1) separation of PM2.5 from larger particles, and (2) measurement of the weight of separated PM2.5. At present, there are three methods for measuring PM2.5 widely used by environmental protection agencies in various countries: gravimetric method, β-ray absorption method, and micro-oscillation balance method. The first step of these three methods is the same, the difference lies in the second step.

PM2.5 is trapped directly on the filter and then weighed using a balance. This is the gravimetric method. It is worth mentioning that the filter does not collect all the PM2.5, and some very fine particles can still wear the filter. As long as the filter has a rejection efficiency of more than 99% for particles larger than 0.3 microns, it is acceptable [19]. The loss of very fine particles has little effect on the results because that part of the particles contributes little to the weight of PM2.5.


Determination of PM2.5

Gravimetric method is the most direct and reliable method. It is a benchmark to verify the accuracy of other methods. However, the weight method requires manual weighing, and the procedure is cumbersome and time-consuming. If you want to achieve automatic monitoring, you need to use the other two methods.

Beta-ray absorption method: PM2.5 was collected on a filter paper and then irradiated with a beam of beta rays. The radiation penetrated the filter paper and particles and was attenuated by scattering. The degree of attenuation was proportional to the weight of PM2.5. The weight of PM2.5 can be calculated based on the attenuation of the radiation [20]. The well-known instrument of the American Embassy is based on this principle.

Micro-oscillation balance method: a thick hollow glass tube with a coarse head fixed and a thin head fitted with a filter element. Air from the coarse head into the fine head out, PM2.5 was trapped in the filter element. Under the action of the electric field, the fine head oscillates at a certain frequency, which is inversely proportional to the square root of the weight of the fine head. Thus, based on the change in the oscillation frequency, the weight of the collected PM2.5 can be calculated [20].

How does the cutter that separates PM2.5 work? Under the action of the suction pump, when air flows through the cutter at a constant flow rate, those larger particles are trapped on the oiled part because of their large inertia, and are trapped. PM2.5 with smaller inertia can be retained. Most of it passed smoothly with the air. Perhaps you have noticed that this is very similar to what happens in our respiratory tract: large particles are easily trapped by the nose, throat, and trachea, while fine particles are more easily reached deeper into the lungs, creating greater health risks .

For a PM2.5 cutter, 2.5 microns is a size step on the edge. Particles that are exactly 2.5 microns in diameter have a 50% probability of passing through the cutter. Particles larger than 2.5 microns are not all retained, and particles smaller than 2.5 microns are not all passable. For example, according to the requirements of the Measurement of Gravity Method for Ambient Air PM10 and PM2.5, the pass rate of particles above 3.0 μm needs to be less than 16%, while the pass rate of particles below 2.1 μm is more than 84% [21].

The special structure together with the specific air flow rate determines the effect of the cutter on the separation of the particles. A slight change in the two will have a great influence on the determination and make the results unreliable. Therefore, when the United States Environmental Protection Agency established the world's first PM2.5 standard in 1997, it also specified the specific structure of the cutter [16]. Thus, although PM2.5 measuring instruments have many brands, their appearance is very similar.

9. Some mobile phone-sized instruments on the market are known to measure PM2.5.

Compared with the standard methods adopted by the environmental protection agencies, PM2.5 measurement using non-specialized instruments is obviously unreliable, but it is difficult to say how accurate it is. Only by comparison with standard methods can we know. The measured data may be able to explain some problems, such as whether someone in the room smokes or whether they have just swept away, but your nose can do it.

Non-professional instruments on the market use the principle of light scattering to determine the concentration of particulate matter. This method has not been adopted as a standard method by environmental protection agencies in various countries. However, there are professional instruments made on the basis of this principle, which are also used in scientific research. The higher the concentration of particles in the air, the stronger the scattering of light. The scattering of light is relatively easy to measure, and it can be measured. Theoretically, the particle concentration can be calculated. However, in practice, things are not so simple. The relationship between light scattering and particle concentration is very uncertain and is influenced by many factors, such as the chemical composition, shape, specific gravity, and particle size distribution of the particles. These depend on the composition of the pollution source. This means that the conversion formula between light scattering and particulate matter concentration may change at any time and place, requiring the instrument user to constantly use standard methods for calibration, and amateurs without scientific training are unlikely to be able to do so. Some researchers have done theoretical calculations: the use of light scattering measurement of PM2.5, at least 30-40% of the uncertainty [22]. This kind of uncertainty is intrinsic to this type of instrument, and the quality of reliable professional instruments is still the case. What's more, the quality of instruments on the market is not always ideal.

As ordinary people, instead of spending their energy and money on monitoring air quality, it is better to call on environmental protection agencies to monitor PM2.5 and disclose data. The new Ambient Air Quality Standard is now being solicited for public comment and is intended to be implemented in 2016 [11], and the public’s voice may be able to make this time much earlier. As for the claims that they have started hands-on monitoring, they can supervise the environmental protection department and prevent them from forging data. This is actually unreasonable. The results obtained by non-professionals operating non-professional or low-quality professional instruments are unreliable and do not have the ability to challenge the results of the environmental protection department. Such supervision can be said to be ineffective.

10. Is Ash Day caused by PM2.5?

Although the particles in the air are invisible to the naked eye, the particles can reduce the visibility of the air, so that the blue sky disappears, and the sky becomes a gray one. This kind of weather is ashy. According to the “2010 Ashes Pilot Monitoring Report”, the concentration of PM2.5 was significantly higher than usual on the hazy days, and the higher the concentration of PM2.5, the lower the visibility [23].

Although particles of different sizes in the air can reduce the visibility, the finer PM2.5 is more capable of reducing visibility than coarse particles. The decrease in visibility is essentially a hindrance to the transmission of visible light. When the diameter of the particles and the wavelength of visible light are close to each other, the scattering of light by the particles is the strongest. The wavelength of visible light is between 0.4-0.7 microns, and the particle size near this size is the main component of PM2.5. The theoretically calculated data also clearly shows this: the extinction coefficient of coarse particles is about 0.6 m 2 /g, while the extinction coefficient of PM 2.5 is much larger, between 1.25-10 m 2 /g, of which PM 2 is. The extinction coefficients of the main components of ammonium sulphate, *** and organic particles are all around 3, which is 5 times that of coarse particles [24]. Therefore, PM2.5 is the main reason for the reduced visibility in hazy days.

It is worth mentioning that ash days are caused by particulate pollution, and fog days are natural weather phenomena, and there is no inevitable connection with human pollution. The main difference between the two is the air humidity, which is usually referred to as fog when the humidity is greater than 90%. The humidity is less than 80% and is referred to as helium. The humidity is between 80-90%, which is a mixture of smog [25]. . (Source: Shouju Energy)