What is nitrogen dioxide?

Nitrogen dioxide ((NO_2)) is a highly reactive and poisonous gas, part of the nitrogen oxides ((NO_x)) group, and a dangerous air pollutant from burning fossil fuels. It appears as a reddish-brown gas or yellowish-brown liquid when cooled or compressed. As a criteria air pollutant, nitrogen dioxide causes inflammation and irritation of breathing passages and pulmonary edema. It also reacts to form nitric acid, contributing to acid rain, and plays a key role in forming photochemical smog in urban areas. Nitrogen dioxide is commercially prepared through the oxidation of (NO) with air. It can also be created in labs by heating heavy metal nitrate or adding copper metal to concentrated nitric acid.

What does nitrogen dioxide look and smell like?

Nitrogen dioxide ((NO_2)) is a highly reactive and poisonous gas. It has a pungent smell and typically appears as a reddish-brown gas but can also be a yellowish-brown liquid when cooled or compressed. It is a dangerous air pollutant associated with fossil fuel combustion. Nitrogen dioxide is one of several nitrogen oxides ((NO_x)) and is often used as an indicator for the larger group of nitrogen oxides.

How is nitrogen dioxide formed?

Nitrogen dioxide ((\text{NO}_2)), a poisonous gas and significant air pollutant, forms when molecular nitrogen in the air reacts with oxygen during high-temperature combustion. Major sources include coal - and gas-fired power plants and gasoline engines. (\text{NO}_2) causes inflammation and irritation of the respiratory system and contributes to acid rain and photochemical smog. In the atmosphere, nitrogen dioxide reacts to form nitric acid, which contributes to acid rain. It also participates in sunlight-driven reactions, leading to ozone and other irritant compounds that create reddish-brown smog in urban areas.

Where is nitrogen dioxide commonly found in the environment?

Nitrogen dioxide ((NO_2)), a toxic and highly reactive gas, is commonly found in environments with high combustion temperatures, where molecular nitrogen reacts with oxygen. Major contributors include stationary sources like coal- and gas-fired power plants, along with mobile sources such as gasoline engines. In urban areas, nitrogen dioxide plays a key role in forming photochemical smog, a reddish-brown haze resulting from sunlight-driven reactions in the lower atmosphere. Additionally, nitrogen dioxide pollution leads to the creation of nitric acid, contributing to acid rain.

Why is nitrogen dioxide considered harmful?

Nitrogen dioxide is considered harmful because it causes inflammation and irritation of breathing passages. Exposure can even lead to pulmonary edema, which is an accumulation of fluid in the lungs. Nitrogen dioxide also contributes to acid rain and the formation of photochemical smog , a reddish-brown haze common in urban areas. This smog can cause eye and chest irritation, coughing, and asthma exacerbation. The U.S. Environmental Protection Agency monitors and regulates nitrogen dioxide as one of six criteria air pollutants because of these health and environmental concerns.

How can people and cities reduce nitrogen dioxide pollution?

Nitrogen dioxide pollution, a significant health and environmental concern, can be reduced through various measures implemented by individuals and cities. Cities can install flue gas treatment systems in power plants and industrial facilities to remove nitrogen oxides before they are released into the atmosphere. These systems use processes like electrostatic precipitators and denitrification units. Cities can also encourage the use of cleaner energy sources, such as renewable energy, to reduce emissions from power plants. Individuals can contribute by using public transportation, carpooling, or switching to electric or hybrid vehicles to lower automobile emissions . They can also support policies and initiatives aimed at reducing air pollution and promoting cleaner energy.

nitrogen dioxide

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Images, Videos & Interactives

Thick brown layer of smog obscures the Los Angeles skyline, with city buildings and neighborhoods visible beneath a blue sky.

New York City skyline tinted orange from wildfire smoke.

Side-by-side world maps show global nitrogen dioxide emissions for 2005 and 2018, with color intensity indicating higher pollution levels, based on NASA satellite data.

Learn how pollution from industrial emissions and car exhaust is trapped against mountains to cause smog

Quick Facts

Key People:: Joseph Priestley

Related Topics:: air pollution; Acid rain; What causes acid rain?; criteria air pollutant; dinitrogen tetroxide

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nitrogen dioxide

chemical compound

Written and fact-checked by

Britannica Editors

Last updated

Oct. 10, 2026 •History

Top Questions

What is nitrogen dioxide?
Nitrogen dioxide ((NO_2)) is a highly reactive and poisonous gas, part of the nitrogen oxides ((NO_x)) group, and a dangerous air pollutant from burning fossil fuels. It appears as a reddish-brown gas or yellowish-brown liquid when cooled or compressed.

As a criteria air pollutant, nitrogen dioxide causes inflammation and irritation of breathing passages and pulmonary edema. It also reacts to form nitric acid, contributing to acid rain, and plays a key role in forming photochemical smog in urban areas.

Nitrogen dioxide is commercially prepared through the oxidation of (NO) with air. It can also be created in labs by heating heavy metal nitrate or adding copper metal to concentrated nitric acid.
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What does nitrogen dioxide look and smell like?
Nitrogen dioxide ((NO_2)) is a highly reactive and poisonous gas. It has a pungent smell and typically appears as a reddish-brown gas but can also be a yellowish-brown liquid when cooled or compressed. It is a dangerous air pollutant associated with fossil fuel combustion. Nitrogen dioxide is one of several nitrogen oxides ((NO_x)) and is often used as an indicator for the larger group of nitrogen oxides.
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1. nitrogen dioxide
2. air pollution: Nitrogen dioxide
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How is nitrogen dioxide formed?
Nitrogen dioxide ((\text{NO}_2)), a poisonous gas and significant air pollutant, forms when molecular nitrogen in the air reacts with oxygen during high-temperature combustion. Major sources include coal- and gas-fired power plants and gasoline engines. (\text{NO}_2) causes inflammation and irritation of the respiratory system and contributes to acid rain and photochemical smog. In the atmosphere, nitrogen dioxide reacts to form nitric acid, which contributes to acid rain. It also participates in sunlight-driven reactions, leading to ozone and other irritant compounds that create reddish-brown smog in urban areas.
5 Britannica Sources
Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.
Where is nitrogen dioxide commonly found in the environment?
Nitrogen dioxide ((NO_2)), a toxic and highly reactive gas, is commonly found in environments with high combustion temperatures, where molecular nitrogen reacts with oxygen.

Major contributors include stationary sources like coal- and gas-fired power plants, along with mobile sources such as gasoline engines. In urban areas, nitrogen dioxide plays a key role in forming photochemical smog, a reddish-brown haze resulting from sunlight-driven reactions in the lower atmosphere. Additionally, nitrogen dioxide pollution leads to the creation of nitric acid, contributing to acid rain.
3 Britannica Sources
Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.
Why is nitrogen dioxide considered harmful?
Nitrogen dioxide is considered harmful because it causes inflammation and irritation of breathing passages. Exposure can even lead to pulmonary edema, which is an accumulation of fluid in the lungs.

Nitrogen dioxide also contributes to acid rain and the formation of photochemical smog, a reddish-brown haze common in urban areas. This smog can cause eye and chest irritation, coughing, and asthma exacerbation. The U.S. Environmental Protection Agency monitors and regulates nitrogen dioxide as one of six criteria air pollutants because of these health and environmental concerns.
4 Britannica Sources
Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.
How can people and cities reduce nitrogen dioxide pollution?
Nitrogen dioxide pollution, a significant health and environmental concern, can be reduced through various measures implemented by individuals and cities.

Cities can install flue gas treatment systems in power plants and industrial facilities to remove nitrogen oxides before they are released into the atmosphere. These systems use processes like electrostatic precipitators and denitrification units. Cities can also encourage the use of cleaner energy sources, such as renewable energy, to reduce emissions from power plants.

Individuals can contribute by using public transportation, carpooling, or switching to electric or hybrid vehicles to lower automobile emissions. They can also support policies and initiatives aimed at reducing air pollution and promoting cleaner energy.
3 Britannica Sources
Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.

nitrogen dioxide, inorganic compound, a highly reactive and poisonous gas. Nitrogen dioxide is one of several nitrogen oxides (NO_x), including nitrous acid and nitric acid, and is often used as an indicator for the larger group of nitrogen oxides. Pungent and extremely volatile, the gas is a dangerous air pollutant associated with fossil fuel combustion. In industry, nitrogen dioxide is an intermediate in the manufacture of nitric acid and a powerful oxidizing agent utilized in chemical processes and rocket fuels.

Air pollution

Nitrogen oxides, including nitrogen dioxide, are formed when combustion temperatures are high enough to cause molecular nitrogen in the air to react with oxygen. Stationary sources such as coal- and gas-fired power plants are major contributors of this pollutant, although gasoline engines and other mobile sources are also significant. Nitrogen dioxide pollution carries significant health and environmental concerns and is one of the six criteria air pollutants monitored and regulated by the U.S. Environmental Protection Agency and other regulatory agencies worldwide. It is known to cause inflammation and irritation of breathing passages and even pulmonary edema, an accumulation of excessive fluid in the lungs. Nitrogen dioxide also reacts in the atmosphere to form nitric acid, contributing to the problem of acid rain.

Nitrogen dioxide also plays a key role in the formation of photochemical smog, a reddish brown haze that often is seen in many urban areas and that is created by sunlight-promoted reactions in the lower atmosphere. Photochemical smog was first characterized in Los Angeles in 1952. The large number of automobiles in that city, together with the bright sunlight and frequently stagnant air, leads to the formation of this form of air pollution. The process begins with the emission of nitrogen oxide (usually during the morning commuting hour), followed by the formation of nitrogen dioxide by oxygenation, and finally, through a complex series of reactions in the presence of hydrocarbons and sunlight, leads to the formation of ozone and peroxyacetyl nitrite and other irritant compounds. Eye irritation, chest irritation with cough, and the exacerbation of asthma can occur as a result.

Dense smog and orange haze blanket the skyline of Mexico City at sunset, with buildings in the Polanco neighborhood partially obscured by air pollution.

Chemistry

Nitrogen dioxide typically appears as a reddish brown gas; when cooled or compressed, it forms a yellowish brown liquid. The chemical is toxic if inhaled or absorbed through the skin.

Chemical properties of NO₂

Molecular weight/molar mass: 46.006 g/mol
Density: 1.880 g/dm³
Boiling point: 21.15 °C (70.07 °F)
Melting point: −9.3 °C (15.3 °F)

Nitrogen dioxide is prepared commercially by oxidizing NO with air, but it can be prepared in the laboratory by heating the nitrate of a heavy metal, as in the following equation, 2Pb(NO₃)₂ + heat → 2PbO + 4NO₂ + O₂, or by adding copper metal to concentrated nitric acid. Like nitric oxide, the nitrogen dioxide molecule is paramagnetic. Its unpaired electron is responsible for its color and its dimerization. At low pressures or at high temperatures, NO₂ has a deep brown color, but at low temperatures the color almost completely disappears as NO₂ dimerizes to form dinitrogen tetroxide, N₂O₄. At room temperature an equilibrium between the two molecules exists. 2NO₂ ⇌ N₂O₄

Nitrogen dioxide reacts with water in one of two ways. In cold water NO₂ disproportionates to form a mixture of HNO₂ and HNO₃, whereas at higher temperatures HNO₃ and NO are formed. In their chemical activity, the nitrogen oxides undergo extensive oxidation-reduction reactions. All the nitrogen oxides are, in fact, good oxidizing agents.

The Editors of Encyclopaedia BritannicaThis article was most recently revised and updated by John P. Rafferty.

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