12.4.3: Human Sources

Properties

• Lifetime in Atmosphere: 114 years
• Global Warming Potential (100-year): 265¹

In 2021, nitrous oxide (N₂O) accounted for 6% of all U.S. greenhouse gas emissions from human activities. Human activities such as agriculture, fuel combustion, wastewater management, and industrial processes are increasing the amount of N₂O in the atmosphere. Nitrous oxide is also naturally present in the atmosphere as part of the Earth's nitrogen cycle and has a variety of natural sources. Nitrous oxide molecules stay in the atmosphere for an average of 121 years before being removed by a sink or destroyed through chemical reactions. The impact of 1 pound of N₂O on warming the atmosphere is 265 times that of 1 pound of carbon dioxide.¹

Globally, 40% of total N₂O emissions come from human activities.² Nitrous oxide is emitted from agriculture, land use, transportation, industry, and other activities, described below.

Figure \(\PageIndex{1}\): Note: All emission estimates from the Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990–2021.

• Agriculture. Nitrous oxide can result from various agricultural soil management activities, such as application of synthetic and organic fertilizers and other cropping practices, the management of manure, or burning of agricultural residues. Agricultural soil management is the largest source of N₂O emissions in the United States, accounting for 75% of total U.S. N₂O emissions in 2021. While not shown in the figure and less significant, emissions of N₂O also occur as a result of land use and land management activities in the Land Use, Land-Use Change, and Forestry sector (e.g. forest and grassland fires, application of synthetic nitrogen fertilizers to urban soils (e.g., lawns, golf courses) and forest lands, etc.).
• Fuel Combustion. Nitrous oxide is emitted when fuels are burned. The amount of N₂O emitted from burning fuels depends on the type of fuel and combustion technology, maintenance, and operating practices.
• Industry. Nitrous oxide is generated as a byproduct during the production of chemicals such as nitric acid, which is used to make synthetic commercial fertilizer, and in the production of adipic acid, which is used to make fibers, like nylon, and other synthetic products. Nitrous oxide is also emitted from use in other applications such as anesthesia and semiconductor manufacturing.
• Waste. Nitrous oxide is also generated from treatment of domestic wastewater during nitrification and denitrification of the nitrogen present, usually in the form of urea, ammonia, and proteins.

Nitrous oxide emissions occur naturally through many sources associated with the nitrogen cycle, which is the natural circulation of nitrogen among the atmosphere, plants, animals, and microorganisms that live in soil and water. Nitrogen takes on a variety of chemical forms throughout the nitrogen cycle, including N₂O. Natural emissions of N₂O are mainly from bacteria breaking down nitrogen in soils and the oceans. Nitrous oxide is removed from the atmosphere when it is absorbed by certain types of bacteria or destroyed by ultraviolet radiation or chemical reactions.

Trends

Nitrous oxide emissions in the United States decreased by 3% between 1990 and 2021. During this time, nitrous oxide emissions from mobile combustion decreased by 56% as a result of criteria pollutant emission standards for on-road vehicles. Nitrous oxide emissions from agricultural soils have varied during this period and were about the same in 2021 as in 1990.

Reducing Nitrous Oxide Emissions

There are a number of ways to reduce emissions of N₂O, discussed below.

Examples of Reduction Opportunities for Nitrous Oxide Emissions

Emissions Source	Examples of How Emissions Can be Reduced
Agriculture	The application of nitrogen fertilizers accounts for the majority of N2O emissions in the United States. Emissions can be reduced by reducing nitrogen-based fertilizer applications and applying these fertilizers more efficiently,^3 as well as modifying a farm's manure management practices.
Fuel Combustion	Nitrous oxide is a byproduct of fuel combustion, so reducing fuel consumption in motor vehicles and secondary sources can reduce emissions. Additionally, the introduction of pollution control technologies (e.g., catalytic converters to reduce exhaust pollutants from passenger cars) can also reduce emissions of N2O.
Industry	Nitrous oxide is generally emitted from industry through fossil fuel combustion, so technological upgrades and fuel switching are effective ways to reduce industry emissions of N2O. Production of nitric acid and adipic acid result in N2O emissions that can be reduced through technological upgrades and use of abatement equipment.

References

1. IPCC (2013). Climate Change 2013: The Physical Science Basis. Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change. [Stocker, T.F., D. Qin, G.K. Plattner, M. Tignor, S.K. Allen, J. Boschung, A. Nauels, Y. Xia, V. Bex and P.M. Midgley (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA, 1535 pp.
2. IPCC (2021). Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change [Masson-Delmotte, V., P. Zhai, A. Pirani, S.L. Connors, C. Péan, S. Berger, N. Caud, Y. Chen, L. Goldfarb, M.I. Gomis, M. Huang, K. Leitzell, E. Lonnoy, J.B.R. Matthews, T.K. Maycock, T. Waterfield, O. Yelekçi, R. Yu, and B. Zhou (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA, 2391 pp.
3. EPA (2005). Greenhouse Gas Mitigation Potential in U.S. Forestry and Agriculture. U.S. Environmental Protection Agency, Washington, DC, USA.

Excerpted from:

Overview of Greenhouse Gases, United States Environmental Protection Agency. Accessed December 2023, https://www.epa.gov/ghgemissions/overview-greenhouse-gases, Public Domain