Pay attention: The control of VOCs is first and foremost a scientific issue!

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In recent years, the prevention and control of air pollution has received increasing attention, and the management of VOCs has caught the interest of various industries. Volatile organic compounds (VOCs) are a class of compounds commonly found in the atmosphere; they refer to volatile organic substances with boiling points ranging from 50°C to 260°C and a saturated vapor pressure at room temperature exceeding 133.3 Pa. Their main components include hydrocarbons, sulfides, ammonia, etc. VOCs can form photochemical smog in the atmosphere, causing severe harm to the environment and human health. Whether it is the control of PM2.5 or ozone pollution, it is first and foremost a scientific issue that requires the study of the physical and chemical mechanisms underlying the reactions of various pollutants. Scientific research is very useful for governance technologies and prevention policies. For example, in the formation of ozone, there is no linear relationship between VOCs and NOx; rather, it depends on the ratio between the two. If only VOCs are controlled, or only NOx is controlled, or if the ratio is not properly adjusted, the effectiveness cannot be guaranteed. Some experts point out that VOC and NOx need to be controlled in conjunction. Current status of VOCs control and policy trends: Compared with dust removal, desulfurization, and denitrification, VOCs come in many types, are emitted by a wide range of industries, have dispersed emission sources, and require complex control technologies. Currently, domestic VOCs treatment enterprises are generally small and scattered in structure. The \"Action Plan for Reducing Volatile Organic Compounds in Key Industries\" issued recently by the Ministry of Industry and Information Technology and the Ministry of Finance states that by 2018, industrial emissions of VOCs are to be reduced by more than 3.3 million tons compared to 2015 levels. The plan emphasizes the need to focus on reduction at the source and process control, while also taking into account end-of-pipe treatment as part of a comprehensive prevention and control strategy. It devotes considerable space to outlining measures for reducing emissions at the source and during production processes, and it also lists some of the commonly used end-of-pipe treatment technologies in order to standardize and guide efforts to reduce VOCs emissions in China. “The Action Plan places greater emphasis on source reduction and process control among the three approaches of source reduction, process control, and end-of-pipe treatment; this represents the development trend for VOCs management in China in the future, and it is also the same approach adopted internationally. ”Luan Zhiqiang, deputy secretary-general of the Waste Gas Purification Committee of the China Environmental Protection Industry Association, said that this will increase the market demand for clean raw materials and process control technology and equipment, thereby promoting the development of these related industries. “However, efforts to treat VOCs at the end-of-pipe stage must not be relaxed at present. ” Types of VOCs VOCs include various organic compounds that can evaporate at room temperature; there are over 100 types of VOCs in typical indoor environments. The main components include benzene derivatives, organic chlorides, Freon compounds, organic hydrocarbons, amines, alcohols, ethers, esters, acids, and petroleum hydrocarbon compounds. Common VOCs include Toluene, Xylene, para-dichlorobenzene, Ethylbenzene, Styrene, Formaldehyde, Acetaldehyde, and others. Benzene, toluene, halogenated olefins (trichloroethylene, dichloroethylene), etc., have been suspected or identified as carcinogenic substances. The sources of volatile organic compounds are mainly chemicals, chemical solvents, vehicle exhaust, and combustion emissions. Chemicals are mainly found in production and sales facilities such as oil industries, chemical plants, and gas stations, while chemical solvents are closely related to everyone’s daily life; paints, interior and vehicle decorations, as well as electronic and electrical devices can all contain volatile organic compounds. There are many sources of VOCs, such as inks and organic solvents used in the tobacco industry, adhesives used in the textile industry, the fragrances in scented toys, coatings, paints, and adhesives used in furniture decoration materials, car accessories and paint, as well as detergents, cleaners, and fabric softeners used in the dry cleaning industry. Since it is a volatile gas, its difference from PM2.5 is that VOC can be smelled directly. VOC types ⇒ Pollution types: Different types of VOCs lead to different types of pollution. VOCs not only produce ozone but are also “precursors” to PM2.5. Experts say that organic compounds such as benzene and toluene undergo oxidation and polymerization to form volatile organic compounds, which then turn into secondary organic particles through processes like condensation, and eventually become PM2.5. The processes by which VOCs lead to the formation of PM2.5 and ozone are completely different. The formation of ozone is relatively simple; it mainly results from oxidation reactions of VOCs and nitrogen oxides (NOx) under sunlight. In cities, the emission of VOCs mainly comes from motor vehicles; industrial sources such as refineries and chemical plants are another source. The contribution of plant emissions to VOCs cannot be ignored either. The contribution of VOCs to PM2.5 is mainly related to the formation of secondary organic aerosols, which is a very complex atmospheric chemistry process. There may be thousands of VOCs, and the mechanisms by which they form secondary organic aerosols, including physical and chemical processes, are not yet fully understood by the academic community. This is true not only in China but also around the world. In some cities, the main pollutant is PM2.5, while in other cities it is ozone; this is related to atmospheric chemical processes, as different types of VOCs lead to different types of pollution. The main component of VOCs is organic carbon, which has only two fates: one part turns into carbon dioxide and can remain in the atmosphere for a long time; the other part forms secondary organic aerosols, which are eventually removed from the air. Different VOCs have very different reaction rates, and thus they contribute differently to air pollution. For example, methane can turn into carbon dioxide or secondary organic aerosols, but this transformation process may take several years. There are also some VOCs whose aerosol conversion takes several months. For these VOCs with slower reaction rates, they are carried to other areas as air moves, thereby affecting global air quality and climate. Some VOCs with fast reaction rates in the atmosphere can cause the rapid formation of local secondary organic aerosols. For example, aromatic hydrocarbons emitted by motor vehicles react very quickly. Numerous experiments and observations have shown that motor vehicle emissions contribute significantly to urban secondary organic aerosols. The VOCs produced by refineries are mainly olefins composed of relatively short carbon double bonds; they react rapidly and thus ozone is formed quickly as well, contributing significantly to ozone formation. However, their contribution to secondary organic aerosols on a local scale is not significant. For example, there are more refineries in the Houston area of the United States than in Beijing, yet the PM2.5 pollution there is not severe. At present, it is urgent to control VOCs that convert into aerosols at a relatively fast rate in the atmosphere. Therefore, VOC research needs to be more detailed, with control implemented on a categorized basis. If no distinctions are made, it is impossible to understand the contribution of different VOCs to pollution, which makes control efforts highly arbitrary.