RESEARCH STARTER

Nitrogen oxides as pollutants

Nitrogen oxides (NOx) are compounds consisting of nitrogen and oxygen that serve as significant environmental pollutants, particularly linked to the energy and agricultural sectors. They contribute to various environmental issues, including global climate change, ozone depletion, acidic precipitation, eutrophication, and the formation of photochemical smog. Naturally occurring through both biological processes, such as nitrogen fixation by certain organisms, and abiotic processes like lightning, NOx levels have been greatly amplified by human activities, notably the combustion of fossil fuels and the use of nitrogen-based fertilizers.

Nitrous oxide (N2O), a potent greenhouse gas with a warming potential far exceeding that of carbon dioxide, has seen a rise in atmospheric concentrations, with agriculture as a primary contributor. This gas is also implicated in ozone layer depletion and has been recognized as a significant driver of climate change, leading to regulations under international agreements like the Kyoto Protocol. The combustion of fossil fuels generates nitric acid, contributing to acid rain, which poses environmental hazards especially in proximity to urban and industrial areas.

Additionally, nitrogen oxides can affect air quality, with elevated levels of nitrogen dioxide (NO2) linked to respiratory problems, particularly in vulnerable populations such as children with asthma. Despite reductions in average NO2 concentrations in the U.S. since 1980, indoor sources remain a public health concern, highlighting the complexity of managing nitrogen oxides as pollutants in both outdoor and indoor environments.

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DEFINITION: Compounds consisting of nitrogen and oxygen

Nitrogen oxides are extremely important environmental pollutants that are closely linked to the energy and agricultural sectors of the global economy. They are causally associated with global climatic change, ozone depletion, acidic precipitation, eutrophication, and photochemical smog formation.

Nitrogen oxides are naturally occurring, biologically active compounds that are produced through a variety of biotic and abiotic processes. Nitrogen-fixing organisms, including mutualists associated with leguminous plants, capture atmospheric nitrogen and produce compounds that are readily converted to nitrites and nitrates by nitrifying bacteria. The biological decomposition of organic matter also leads to the formation of nitrogen oxides. Intense heating causes atmospheric nitrogen and oxygen to react, and lightning is an important naturally occurring, abiotic source of nitrogen oxides in the Earth’s atmosphere.

The use of fossil fuels and nitrogen-based fertilizers and the cultivation of leguminous plants have substantially increased nitrogen oxide concentrations. Some fossil fuels (such as coal) contain significant amounts of nitrogen, which is oxidized during combustion. Fossil-fuel combustion also produces the heat required to oxidize atmospheric nitrogen.

Nitrous oxide is a powerful greenhouse gas with a warming potential 270 to 300 times greater than that of carbon dioxide. Agricultural practices are primarily responsible for increasing nitrous oxide concentrations in the Earth’s atmosphere, but combustion is also important. The Intergovernmental Panel on Climate Change has identified nitrous oxide as an important driver of human-caused global climate change, and nitrous oxide emissions are internationally regulated under the Paris Agreement (formerly regulated under the Kyoto Protocol). Between 1980 and 2020, nitrous oxide emissions increased by 40 percent, and human activities have increased the atmospheric nitrous oxide concentration by approximately 25 percent since the Industrial Revolution began.

Nitrous oxide is also responsible for ozone thinning. Research published in 2009 indicated that nitrous oxide was the most important ozone-depleting compound released by human activities. In the stratosphere, nitrous oxide is converted to nitric oxide, which catalyzes ozone-destroying reactions. According to a report by the UN Environment Programme, nitrous oxide was the most impactful ozone-depleting substance emitted by human activity in the mid-2020s.

Nitrogen oxides react with moisture in the Earth’s atmosphere to produce nitric acid, a primary component of acid rain. The most significant environmental damage attributable to nitric acid formation and acidic precipitation is linked to the combustion of fossil fuels and generally occurs downwind of large cities, power plants, and industrial centers.

Nitrogen oxides are important fertilizers, and their production fosters eutrophication, or the overenrichment of bodies of water with nutrients. Nitrogen-limited estuaries and coastal ecosystems may be particularly sensitive to nitrogen inputs, and atmospheric nitrogen loading is associated with harmful algal blooms and a host of other ecological impacts, including declines in species diversity.

The photodissociation of nitrogen oxides leads to the formation of ozone, and the release of nitrogen oxides from automobiles and stationary sources contributes to the formation of photochemical smog. Exposure to abnormally high ambient nitrogen dioxide (NO2) concentrations may worsen asthma symptoms, and some individuals may experience an increased risk of respiratory infection, heart failure, or complications during pregnancy. Average ambient NO2 concentrations decreased substantially in the United States since 1980.

Fuel-burning appliances, such as gas stoves, furnaces, fireplaces, and space heaters, produce NO2, and indoor NO2exposure is a recognized public health concern. Research indicates that indoor NO2 can produce respiratory symptoms among children with asthma, even at concentrations well below the US Environmental Protection Agency’s ambient air standard of 53 parts per billion.


Bibliography

"Basic Information about NO2 ." Environmental Protection Agency, 10 July 2025, www.epa.gov/no2-pollution/basic-information-about-no2. Accessed 17 Sept. 2025.

Jacobson, Mark Z. Atmospheric Pollution: History, Science, and Regulation. Cambridge UP, 2002.

"Nitrous Oxide Emissions Grew 40 Percent from 1980 to 2020, Accelerating Climate Change." National Oceanic and Atmospheric Administration, 13 June 2024, research.noaa.gov/nitrous-oxide-emissions-grew-40-percent-from-1980-to-2020-accelerating-climate-change. Accessed 17 Sept. 2025.

"Rise in Nitrous Oxide Emissions Endangers Pathway to 1.5°C, the Ozone Layer, and Human Health ." UN Environment Programme, 12 Nov. 2024, www.unep.org/news-and-stories/press-release/rise-nitrous-oxide-emissions-endangers-pathway-15degc-ozone-layer. Accessed 17 Sept. 2025.

Vallero, Daniel. Fundamentals of Air Pollution. 6th ed., Academic Press, 2025.

Full Article

DEFINITION: Compounds consisting of nitrogen and oxygen

Nitrogen oxides are extremely important environmental pollutants that are closely linked to the energy and agricultural sectors of the global economy. They are causally associated with global climatic change, ozone depletion, acidic precipitation, eutrophication, and photochemical smog formation.

Nitrogen oxides are naturally occurring, biologically active compounds that are produced through a variety of biotic and abiotic processes. Nitrogen-fixing organisms, including mutualists associated with leguminous plants, capture atmospheric nitrogen and produce compounds that are readily converted to nitrites and nitrates by nitrifying bacteria. The biological decomposition of organic matter also leads to the formation of nitrogen oxides. Intense heating causes atmospheric nitrogen and oxygen to react, and lightning is an important naturally occurring, abiotic source of nitrogen oxides in the Earth’s atmosphere.

The use of fossil fuels and nitrogen-based fertilizers and the cultivation of leguminous plants have substantially increased nitrogen oxide concentrations. Some fossil fuels (such as coal) contain significant amounts of nitrogen, which is oxidized during combustion. Fossil-fuel combustion also produces the heat required to oxidize atmospheric nitrogen.

Nitrous oxide is a powerful greenhouse gas with a warming potential 270 to 300 times greater than that of carbon dioxide. Agricultural practices are primarily responsible for increasing nitrous oxide concentrations in the Earth’s atmosphere, but combustion is also important. The Intergovernmental Panel on Climate Change has identified nitrous oxide as an important driver of human-caused global climate change, and nitrous oxide emissions are internationally regulated under the Paris Agreement (formerly regulated under the Kyoto Protocol). Between 1980 and 2020, nitrous oxide emissions increased by 40 percent, and human activities have increased the atmospheric nitrous oxide concentration by approximately 25 percent since the Industrial Revolution began.

Nitrous oxide is also responsible for ozone thinning. Research published in 2009 indicated that nitrous oxide was the most important ozone-depleting compound released by human activities. In the stratosphere, nitrous oxide is converted to nitric oxide, which catalyzes ozone-destroying reactions. According to a report by the UN Environment Programme, nitrous oxide was the most impactful ozone-depleting substance emitted by human activity in the mid-2020s.

Nitrogen oxides react with moisture in the Earth’s atmosphere to produce nitric acid, a primary component of acid rain. The most significant environmental damage attributable to nitric acid formation and acidic precipitation is linked to the combustion of fossil fuels and generally occurs downwind of large cities, power plants, and industrial centers.

Nitrogen oxides are important fertilizers, and their production fosters eutrophication, or the overenrichment of bodies of water with nutrients. Nitrogen-limited estuaries and coastal ecosystems may be particularly sensitive to nitrogen inputs, and atmospheric nitrogen loading is associated with harmful algal blooms and a host of other ecological impacts, including declines in species diversity.

The photodissociation of nitrogen oxides leads to the formation of ozone, and the release of nitrogen oxides from automobiles and stationary sources contributes to the formation of photochemical smog. Exposure to abnormally high ambient nitrogen dioxide (NO2) concentrations may worsen asthma symptoms, and some individuals may experience an increased risk of respiratory infection, heart failure, or complications during pregnancy. Average ambient NO2 concentrations decreased substantially in the United States since 1980.

Fuel-burning appliances, such as gas stoves, furnaces, fireplaces, and space heaters, produce NO2, and indoor NO2exposure is a recognized public health concern. Research indicates that indoor NO2 can produce respiratory symptoms among children with asthma, even at concentrations well below the US Environmental Protection Agency’s ambient air standard of 53 parts per billion.


Bibliography

"Basic Information about NO2 ." Environmental Protection Agency, 10 July 2025, www.epa.gov/no2-pollution/basic-information-about-no2. Accessed 17 Sept. 2025.

Jacobson, Mark Z. Atmospheric Pollution: History, Science, and Regulation. Cambridge UP, 2002.

"Nitrous Oxide Emissions Grew 40 Percent from 1980 to 2020, Accelerating Climate Change." National Oceanic and Atmospheric Administration, 13 June 2024, research.noaa.gov/nitrous-oxide-emissions-grew-40-percent-from-1980-to-2020-accelerating-climate-change. Accessed 17 Sept. 2025.

"Rise in Nitrous Oxide Emissions Endangers Pathway to 1.5°C, the Ozone Layer, and Human Health ." UN Environment Programme, 12 Nov. 2024, www.unep.org/news-and-stories/press-release/rise-nitrous-oxide-emissions-endangers-pathway-15degc-ozone-layer. Accessed 17 Sept. 2025.

Vallero, Daniel. Fundamentals of Air Pollution. 6th ed., Academic Press, 2025.

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