What are the solid pollutants in the air?
Air pollutants refer to those substances that are discharged into the atmosphere due to human activities or natural processes and have harmful effects on any environment or are converted into secondary pollutants. According to their causes, pollutants can be divided into primary pollutants and secondary pollutants. Primary pollutants refer to the original pollutants discharged directly from the pollution source into the atmosphere; secondary pollutants refer to new pollutants with different properties from primary pollutants generated by a series of chemical or photochemical reactions between primary pollutants and existing components or several primary pollutants in the atmosphere.
According to their existing state, atmospheric pollutants can be classified into two categories: particulate pollutants and gaseous pollutants.
(I) Particulate pollutants
In air pollution, particulate pollutants refer to solid particles, liquid particles or their suspended systems in gaseous media with negligible sedimentation rates. From the perspective of air pollution control, according to their sources and physical properties, they can be divided into the following categories:
1. Dust (dust)
Dust refers to small solid particles suspended in a gas medium that can settle under the action of gravity, but can remain suspended for a period of time. It is usually formed due to mechanical processes such as crushing, grinding, grading, and transportation of solid materials, or natural processes such as weathering of soil and rocks. The size range i of particles is generally about 1-200 μm. There are many types of atmospheric pollutants that belong to the dust category, such as clay dust, quartz dust, coal dust, cement dust, and various metal dusts.
2. Smoke (fume)
Smoke generally refers to the aerosol of solid particles formed by the metallurgical process. It is the condensate of gaseous substances generated after the evaporation of molten substances, and is always accompanied by chemical reactions such as oxidation during the generation process. The size of smoke particles is very small, generally about 0.01-1 μm. Smoke is a relatively common phenomenon, such as lead oxide smoke and zinc oxide smoke produced in the process of non-ferrous metal smelting.
3. Fly ash
Fly ash refers to the finely dispersed ash that is expelled from the flue gas produced by fuel combustion.
4. Black smoke (smoke)
Black smoke generally refers to the visible aerosol produced by the combustion of fuels.
5. Fog (fog)
Fog is a general term for droplet suspensions in gases. In meteorology, it refers to small droplet suspensions that cause visibility less than 1km. In engineering, fog generally refers to small liquid particle suspensions, which may be formed due to the condensation of liquid vapor, the atomization of liquid, and chemical reactions, such as water mist, acid mist, alkali mist, oil mist, etc.
In China's Ambient Air Quality Standards, particulate matter is classified into total suspended partidclea (suspended particles) and inhalable particles (inhalable particles) according to their diameter. The former refers to particles suspended in the air with an aerodynamic equivalent diameter of ≤ 100 μm. The latter refers to particles suspended in the air with an aerodynamic equivalent diameter of ≤ 10 μm.
(Ii) Gaseous pollutants
Gaseous pollutants are pollutants that exist in molecular form. There are many types of gaseous pollutants, which can be generally classified as shown in Table 5-2-3.
Table 5-2-3 Classification of gaseous pollutants
Pollutant Primary Pollutant Secondary Pollutant
Sulfur-containing chemicals SO2, H2SSO3, H2SO4, MSO4
Nitrogen compounds NO, NH3NO3, HNO3, MNO3
Carbon oxides CO, CO2 None
Organic compounds C1-C10 compounds Aldol, ketone, peroxyacetyl nitrate, O3
Halogen compounds HF, HCl None
Note: MSO4 and MNO3 are sulfate and nitrate, respectively.
1. Sulfur oxides
Sulfur oxides mainly refer to SO2, which mainly comes from the combustion process of fossil fuels, as well as the roasting and smelting process of sulfide ores. Thermal power plants, non-ferrous metal smelters, sulfuric acid plants, oil refineries and all industrial furnaces that burn coal or oil emit SO2 flue gas.
2. Nitrogen oxides
The compounds of nitrogen and oxygen are N2O, NO, NO2, N2O3, N2O4 and N2O5, which are collectively represented by nitrogen oxides (NOx). Among them, NO and NO2 are the main ones that pollute the atmosphere. NO is not very toxic, but it can be slowly oxidized to NO2 after entering the atmosphere. When there are strong oxidants such as O3 in the atmosphere, or under the action of catalysts, the gas oxidation rate will be accelerated. The toxicity of NO2 is about 5 times that of NO. When NO2 participates in the photochemical reaction of the atmosphere and forms photochemical smog, its toxicity is even stronger. NOx produced by human activities mainly comes from the exhaust of various industrial furnaces, excited cars and diesel engines, followed by nitric acid production, nitrification process, explosive production and metal surface treatment. Among them, NOx produced by fuel combustion accounts for more than 90%.
3. Carbon oxides
CO and CO2 are the most common types of air pollutants, mainly from fuel combustion and motor vehicle exhaust. CO is an asphyxiating gas. When it enters the atmosphere, it generally does not cause harm due to atmospheric dilution and oxidation. However, during the winter heating season in cities or at busy traffic intersections, when the weather conditions are not conducive to exhaust expansion and dilution, the concentration of CO may reach a level that endangers human health. CO2 is a non-toxic gas, but when its concentration in the atmosphere is too high, the oxygen content is relatively reduced, which will have adverse effects on people. The increase in CO2 concentration on the earth can produce a "greenhouse effect", forcing governments to implement controls.
4. Organic compounds
There are many kinds of organic compounds, from methane to hydrocarbons of long-chain polymers. Atmospheric volatile organic compounds (VOCs), typically C1-C10 compounds, are not exactly the same as hydrocarbons in the strict sense because they often contain oxygen, nitrogen, and sulfur atoms in addition to carbon and hydrogen atoms. Methane is considered an inactive hydrocarbon, and hydrocarbon concentrations in the environment are often reported in the form of non-total methane hydrocarbons (NMHCs). Benzo [α] pyrene (B [α] P) in polycyclic aromatic hydrocarbons (PAHs) is a strong carcinogen. VOCs are precursors to the photochemical oxidants ozone and peroxyacetyl nitrate (PAN), and are one of the contributors to the room temperature effect. VOCs mainly come from motor vehicle and fuel combustion exhaust, as well as petroleum refining and organic chemical production.
5. Sulfuric acid fumes
Sulfuric acid fumes are sulfur oxides such as SO2 in the atmosphere. In the presence of water mist, suspended particles containing heavy metals or nitrogen oxides, a series of chemical or photochemical reactions occur to produce sulfuric acid mist or sulfate aerosol. The irritation and physiological reactions caused by sulfuric acid fog are much greater than those caused by SO2 gas.
6. Photochemical smog
Photochemical smog is a blue smog (sometimes purple and yellow-brown) produced by a series of photochemical reactions between nitrogen oxides, hydrocarbons and oxidants in the atmosphere under sunlight. Its main components are ozone, peroxyacetyl nitrate, ketones and alters. Photochemical smog is much more irritating and harmful than a single pollutant.
