Five Questions and Five Answers on VOCs Management

What are VOCs?

According to the WHO definition, volatile organic compounds (VOCs) refer to various organic compounds with a boiling point ranging from 50°C to 260°C at normal temperature. VOCs can be further classified based on their chemical structure: alkanes, aromatics, esters, aldehydes, and others. Currently, more than 300 types have been identified. The most common ones include benzene, toluene, xylene, styrene, trichloroethylene, trichloromethane, trichloroethane, diisocyanate (TDI), diisocyanomethane, etc.

Where do VOCs come from in our production and daily life?

VOCs, as a type of volatile organic compound, have a wide range of sources in daily production and life.

First, industrial sources. Industries such as petrochemicals, chemicals, coal burning, and coking will emit a large amount of VOCs. Painting spraying will also emit VOCs, such as home decoration and car painting.

Second, sources from daily life. Vehicles are a major source of VOCs. Not only vehicle exhaust, but also the oil used by vehicles, its volatilization during transportation, storage, and at gas stations is also a major source of VOCs. Additionally, dining, barbecuing, and biomass burning will also volatilize a large amount of organic substances.

Third, natural sources. Plant volatile organic compounds, especially in favorable vegetation conditions during summer.

Overall, VOCs emissions are the most diverse type of pollution. Every aspect of our production and life will produce some VOCs emissions.

What impact do VOCs have on air quality?

First, since VOCs are a type of volatile organic compound, they include many types and many substances, totaling thousands. VOCs themselves include many substances harmful to human health, such as polycyclic aromatic hydrocarbons.

Second, VOCs are the most important substance foundation in the photochemical reaction chain of the atmospheric environment. VOCs participating in photochemical reactions will lead to two aspects of impacts. On the one hand, in summer under suitable light and temperature conditions, VOCs participate in photochemical reactions triggered by nitrogen oxides, leading to an increase in ozone concentration.

Third, when VOCs participate in a series of photochemical reactions, they will convert from gaseous to solid state, producing secondary organic aerosols, becoming part of the particulate matter, and exacerbating PM2.5 pollution. In addition, due to the increase in ozone caused by VOCs, the oxidizing power of the atmosphere is enhanced, triggering a series of other reactions. For example, NO2 and SO2 in the atmosphere, under the oxidation of ozone, will generate nitrate and sulfate, indirectly increasing the concentration of PM2.5.

Why is the control of VOCs difficult?

First, due to the complex types of VOCs, their emission sources also vary greatly. Taking coal burning as an example that we are familiar with, the emission substances from different enterprises are SO2, NO2, etc., but the VOCs emissions from different enterprises will have various types due to the differences in raw materials and production processes. Some enterprises may emit VOCs mainly composed of phenol and di-phenol, while others may emit VOCs mainly composed of ketones. This will cause a more difficult situation in VOCs control and governance compared to single pollutants.

Second, from the perspective of China's air pollution control process, the issue of VOCs has only received widespread attention in recent years. Therefore, whether from the perspective of VOCs' control technology, related standards, or monitoring technology, our current work is still in the initial stage, and the various systems are not yet complete.

Third, the emission sources of many VOCs are unorganized emissions (in simple terms, they are not directly emitted through chimneys or other means). It is not convenient to collect and treat them centrally. This is also an aspect where the control of VOCs is more difficult compared to other pollutants.

What should we strive for?

Among ozone, VOCs and NO2, there exists a complex atmospheric photochemical reaction equilibrium. Therefore, the treatment cannot be generalized. In the treatment of ozone, we must first determine whether the ozone in this city is within the control zone of VOCs or NO2. Then, we should establish a targeted VOCs source list and adopt different control measures for different emission sources to achieve twice the result with half the effort.