WHAT ARE THE PRINCIPAL GREENHOUSE GASES IN GREENHOUSE GAS INVENTORY?

1. Overview of the Greenhouse Effect and Greenhouse Gases
1.1. What is the Greenhouse Effect?
According to the UNFCCC, the greenhouse effect refers to the process by which heat from the Earth's surface is trapped by gases such as water vapor, carbon dioxide, ozone and several other gases in the atmosphere, leading to an increase in the Earth's average atmospheric temperature. If the atmospheric concentrations of these greenhouse gases rise, the average temperature of the atmosphere will gradually increase, contributing to climate change [1].

1.2. What are Greenhouse Gases?
Greenhouse gases (GHGs) are gases that trap heat in the atmosphere and maintain a temperature conducive to life on Earth. Under the Kyoto Protocol (Doha Amendments) [2] and Article 91 of Vietnam’s Environmental Protection Law 2020 [3], the seven main GHGs causing the greenhouse effect are:

• Carbon dioxide (CO₂)
• Methane (CH₄)
• Nitrous oxide (N₂O)
• Hydrofluorocarbons (HFCs)
• Perfluorocarbons (PFCs)
• Sulfur hexafluoride (SF₆)
• Nitrogen trifluoride (NF₃)

2. 07 main greenhouse gases and their key emission sources
2.1. Carbon Dioxide (CO₂)
Carbon Dioxide (CO₂) is the largest contributor to global warming, accounting for the majority of global GHG emissions. It can stay in the atmosphere for hundreds of years or longer. CO₂ is emitted through both natural processes (e.g., respiration by humans, animals, and plants; volcanic eruptions) and human activities (e.g., industrial production, fossil fuel combustion). Key sources of CO₂ emissions include:

• Electricity generation: Most electricity is generated by burning fossil fuels, primarily coal and natural gas.
• Transportation: CO₂ emissions come from the combustion of petroleum-based fuels (e.g., gasoline and diesel) to power vehicles such as cars, trucks, ships, trains, and airplanes.
• Industry: CO₂ is released during fossil fuel combustion for energy production and chemical reactions in manufacturing processes, such as cement production.
• Commercial and residential sectors: CO₂ is released through fossil fuel combustion for heating, use of CO₂-containing products (e.g., fire extinguishers), and waste management.
• Land use and forestry: Land and forests can act as carbon sinks, absorbing CO₂ from the atmosphere. However, when land use changes, such as deforestation or converting forests to agricultural land, the stored CO₂ is released into the atmosphere. Conversely, sustainable land management practices, such as afforestation and reducing deforestation, can enhance carbon sequestration.

2.2. Methane (CH₄)
Methane (CH₄) is the second-largest contributor to global warming after CO₂. According to IPCC AR6 [4], over a 100-year period, 1 kilogram of fossil-origin CH₄ is approximately 29.8 times more potent as a greenhouse gas than CO₂, while non-fossil CH₄ is about 27 times more potent. However, CH₄ has a shorter atmospheric lifespan (about 7–12 years) [5]. CH₄ naturally exists as the main component of natural gas, presents in pond sediments, swamps, biogas chambers, etc., and is also generated by human activities such as agricultural practices, livestock farming, coal mining, waste management, etc. Key sources of CH₄ emissions include:

• Livestock farming: CH₄ is emitted the fermentation process in the digestive systems of livestock (cattle, buffalo, and pigs,...) and manure management.
• Rice cultivation: Flooded rice fields create an ideal environment for microorganisms to produce CH₄ through a process called “methanogenesis”.
• Biomass burning: CH₄ is released during the incomplete combustion of biomass from forests, grasslands, and agricultural waste.
• Waste management: Decomposition of organic waste in landfills generates CH₄.
• Fossil fuel production: CH₄ can escape during the extraction of oil and natural gas.

2.3. Nitrous Oxide (N₂O)
Nitrous Oxide or Dinitrogen monoxide (N₂O) is the third-largest contributor to global warming. According to IPCC AR6, 1 kilogram of N₂O has a global warming potential 273 times greater than CO₂ over 100 years. N₂O persists in the atmosphere for approximately 125 years [6]. Key sources of N₂O emissions include:

• Agriculture and Land Use, Land-Use Change, and Forestry (LULUCF): N₂O is emitted from agricultural land management activities such as the use of synthetic and organic fertilizers, crop cultivation, manure management, or the burning of agricultural residues (e.g., rice straw, rice husks, corn stalks, etc.). N₂O is also released from land use and land management activities (e.g., forest and grassland fires, the application of nitrogen-based synthetic fertilizers in urban soils such as lawns, golf courses, and forests,...).
• Fuel combustion: N₂O emissions depend on the type of fuel, combustion technology, and equipment maintenance. For instance, burning charcoal at the household level, especially with traditional stoves, generates significant N₂O emissions due to suboptimal combustion. In contrast, advanced cooking methods (e.g., induction, electric, or infrared stoves) not only minimize N₂O emissions but also enhance energy efficiency.
• Industry: N₂O is released as a by-product during the production of chemicals like nitric acid (used in manufacturing synthetic fertilizers) and adipic acid (used to produce nylon fibers and other synthetic materials). Additionally, N₂O is emitted in applications such as anesthesia in healthcare and semiconductor production.
• Wastewater treatment: N₂O is released during the treatment of municipal wastewater, particularly in the nitrification and denitrification stages, which involve nitrogen compounds in wastewater, such as urea, ammonia, and proteins.

2.4. Hydrofluorocarbons (HFCs)
Hydrofluorocarbons (HFCs) are synthetic gases with a high global warming potential (GWP), ranging from hundreds to thousands of times greater than CO₂ by weight. For example, over 100 years, 1 kilogram of HFC-32 can cause 771 times more warming than 1 kilogram of CO₂, while HFC-125 causes 3,740 times more. HFCs remain in the atmosphere for about 15 years. HFCs are entirely human-made, primarily used in refrigeration and air conditioning; foam insulation production; small-scale solvent applications and firefighting equipment. Most HFC emissions result from damage, improper maintenance, and leaks when equipment expires. [7]

2.5. Perfluorocarbons (PFCs)
Perfluorocarbons (PFCs) are synthetic fluorinated compounds with an extremely high GWP. Their impact on global warming can be thousands of times greater than CO₂ by weight. For example, 1 kilogram of PFC-116 causes 12,400 times more warming than CO₂ over 100 years. Known as "eternal chemicals," PFCs persist in the environment for millennia. Key sources of PFC emissions include [8]:

• Semiconductor manufacturing: PFCs are widely used in the semiconductor industry, particularly in processes of plasma etching and chemical vapor deposition which help create microelectronics by etching patterns on silicon wafers. These compounds can be released into the atmosphere during use.
• Cooling and air conditioning: PFCs are used as refrigerants in air conditioning and cooling systems. When these devices leak or are improperly disposed of, PFCs can escape into the environment.
• Electrical equipment: PFCs are used as insulating fluids in high-voltage equipment and may be emitted if devices malfunction or are improperly disposed of.

2.6. Sulfur Hexafluoride (SF₆)
Sulfur Hexafluoride (SF₆) is a synthetic fluorine compound and the most potent greenhouse gas ever known. 1 kilogram of SF₆ causes 24,300 times more global warming than 1 kilogram of CO₂ over 100 years. SF₆ can remain in the atmosphere for over 1,000 years. Key sources of SF₆ emissions include [9]:

• Electricity sector: SF6 is used in electrical equipment due to its excellent insulating properties and stability at high temperatures, but it can leak when the equipment malfunctions or is not properly maintained.
• Magnesium industry: SF6 is used to protect magnesium from oxidation during casting, but some SF6 may be emitted into the air during production.
• Electronics industry: SF6 is commonly used in semiconductor manufacturing (wafer chips) for plasma etching or as an etchant before chemical vapor deposition (CVD), and can be released during production.

2.7. Nitrogen Trifluoride (NF₃)
Nitrogen Trifluoride (NF₃) is a synthetic inorganic compound. 1 kilogram of NF₃ causes 17,400 times more global warming than 1 kilogram of CO₂ over 100 years. SF₆ can remain in the atmosphere for about 550 years. Key sources of NF₃ emissions include:

• Electronics and semiconductor manufacturing: NF3 is primarily used for etching microprocessors and manufacturing LCD panels, releasing emissions during production.
• Agriculture and livestock: NF3 is emitted during waste management and fertilizer production.
• Fuel combustion: NF3 emissions can be emitted from burning fossil fuels, especially in industrial processes involving fluorine-containing compounds or chemical reactions that produce NF3 as a byproduct.

References: 
[1] UNFCCC Glossary 
https://unfccc.int/resource/cd_roms/na1/ghg_inventories/english/8_glossary/Glossary.htm
[2] The Doha Amendment 
https://unfccc.int/process/the-kyoto-protocol/the-doha-amendment
[3] Law No. 72/2020/QH14 on Environmental Protection in Vietnam
https://datafiles.chinhphu.vn/cpp/files/vbpq/2021/02/72.signed.pdf
[4] IPCC Sixth Assessment Report (AR6) 
https://ghgprotocol.org/sites/default/files/2024-08/Global-Warming-Potential-Values%20%28August%202024%29.pdf
[5] NASA: Methane
https://climate.nasa.gov/vital-signs/methane/?intent=121#:~:text=The%20largest%20sources%20of%20methane,about%20the%20Global%20Methane%20Budget
[6] Advancing Earth and space science (AGU), Journal of Geophysical Research: Atmospheres: Measuring and modeling the lifetime of nitrous oxide including its variability
https://acd-ext.gsfc.nasa.gov/People/Jackman/Prather_2015.pdf
[7] Climate & Clean Air Coalition, UNEP:  Hydrofluorocarbons
https://www.ccacoalition.org/short-lived-climate-pollutants/hydrofluorocarbons-hfcs
[8] European Geosciences Union, Article (Volume 20, issue 8): Trends and emissions of six perfluorocarbons in the Northern Hemisphere and Southern Hemisphere
https://acp.copernicus.org/articles/20/4787/2020/