Nitrogen Dioxide Air Pollutant and Sources

A white cloud with the chemical formula for nitrogen dioxide, which is NO2

Nitrogen dioxide, or NO₂, is a gas composed of one nitrogen atom and two oxygen atoms in each molecule. When prevalent in the air, NO₂ is a foul-smelling gas that appears as a reddish-brown haze.

Nitrogen dioxide is one of a group of highly reactive gases known as nitrogen oxides, or NO_x. This group includes nitric oxide (NO), nitrous oxide, nitrous acid and nitric acid. Nitrogen oxides react with other chemicals in the air to form other pollutants, known as secondary pollutants. These secondary pollutants include ozone, particulate matter, acid rain and other toxic chemicals.

Nitrogen oxides are formed primarily by burning fossil fuels — coal, oil and gas — at high heat. NO is produced during the fuel combustion processes from nitrogen and oxygen in the atmosphere. NO is then converted to NO₂, either within the exhaust system of the combustion device or in the atmosphere. It is released in the emissions from cars, trucks other vehicles or machinery that burn fuel, and from power plants and other equipment involving fuel combustion. NO₂ combines with volatile organic compounds (VOCs) in the presence of sunlight and heat to create ozone. More information is available on EPA's Nitrogen Dioxide (NO2) Pollution webpage.

Missouri monitors NO₂ concentrations in the air at locations across the state. Three of these sites are near roadways with heavy traffic, two in the St. Louis area and one in Kansas City. Click on the Air Monitoring Sites tab to learn more about these sites. Click on the Monitoring Data tab to learn more about the data collected from this sites.

For information about other ambient air pollutants that the department monitors, visit Air Pollutants and Sources.

Health Effects

Breathing air with a high concentration of nitrogen dioxide, NO₂, can irritate airways in the human respiratory system. Such exposures over short periods can aggravate respiratory diseases, particularly asthma, leading to respiratory symptoms such as coughing, wheezing or difficulty breathing. Longer exposures to elevated concentrations of NO₂ may contribute to developing asthma and potentially increase susceptibility to respiratory infections. People with asthma, as well as children and the elderly, are generally at greater risk for the health effects of NO₂.

NO₂ along with other nitrogen oxides, NO_x, react with other chemicals in the air to form both particulate matter and ozone. Both of these are also harmful when inhaled due to effects on the respiratory system.

Monitoring Data

Nitrogen Dioxide

In 2010, EPA revised the one-hour NO₂ standard. The primary National Ambient Air Quality Standard (NAAQS), based on health effects, for NO₂ is 100 parts per billion (ppb) averaged over one hour. The one-hour standard is based on a three-year design value, which is calculated by taking the 98th percentile of the one-hour daily maximum concentrations recorded each year, and averaging three years together. The annual standard of 0.053 parts per million (ppm) – or 53 ppb – is based on the annual arithmetic mean and is not to be exceeded. An area is in compliance if the design value is at or below the standard.

Air Quality Analysis for Nitrogen Dioxide - Weekly Monitor Data