Six Principal Pollutants - Nitrogen Dioxide (NO₂)

Nature and Sources of the Pollutant: Nitrogen dioxide belongs to a family of highly reactive gases called nitrogen oxides (NO_x). These gases form when fuel is burned at high temperatures, and come principally from motor vehicle exhaust and stationary sources such as electric utilities and industrial boilers. A suffocating, brownish gas, nitrogen dioxide is a strong oxidizing agent that reacts in the air to form corrosive nitric acid, as well as toxic organic nitrates. It also plays a major role in the atmospheric reactions that produce ground-level ozone (or smog).

Health and Environmental Effects: Nitrogen dioxide can irritate the lungs and lower resistance to respiratory infections such as influenza. The effects of short-term exposure are still unclear, but continued or frequent exposure to concentrations that are typically much higher than those normally found in the ambient air may cause increased incidence of acute respiratory illness in children. EPA's health-based national air quality standard for NO₂ is 0.053 ppm (measured as an annual arithmetic mean concentration). Nitrogen oxides contribute to ozone formation and can have adverse effects on both terrestrial and aquatic ecosystems. Nitrogen oxides in the air can significantly contribute to a number of environmental effects such as acid rain and eutrophication in coastal waters like the Chesapeake Bay. Eutrophication occurs when a body of water suffers an increase in nutrients that leads to a reduction in the amount of oxygen in the water, producing an environment that is destructive to fish and other animal life.

Trends in Nitrogen Dioxide Levels: Nationally, annual NO₂ concentrations remained relatively constant throughout the 1980's, followed by decreasing concentrations in the 1990's. Average NO₂ concentrations in 1995 were 14 percent lower than the average concentrations recorded in 1986. The two primary sources of the NO_x emissions in 1995 were fuel combustion (46 percent) and transportation (49 percent). Between 1986 and 1995, emissions from fuel combustion decreased 6 percent, and emissions from highway vehicles decreased 2 percent. Overall, national total NO_x emissions decreased 3 percent. Additionally, 1995 is the fourth year in a row that all monitoring locations across the nation, including Los Angeles, met the Federal NO₂ air quality standard.

Nitrogen dioxide (NO₂) is a brownish, highly reactive gas that is present in all urban atmospheres. NO₂ can irritate the lungs, cause bronchitis and pneumonia, and lower resistance to respiratory infections. Nitrogen oxides are an important precursor both to ozone (O₃) and acid rain, and may affect both terrestrial and aquatic ecosystems. The major mechanism for the formation of NO₂ in the atmosphere is the oxidation of the primary air pollutant nitric oxide (NO). NO_x plays a major role, together with VOCs, in the atmospheric reactions that produce O₃. NOx forms when fuel is burned at high temperatures. The two major emissions sources are transportation and stationary fuel combustion sources such as electric utility and industrial boilers.

Title 40, Part 50 of the Code of the Federal Regulations lists the ambient air quality standard for nitrogen dioxide.

Nitrogen Dioxide - NO₂, is the lesser of the two emitted NO_x gases from high temperature combustion in air. It is an important species in the atmosphere. Since it absorbs in the visible wavelength region--creating the Brown Cloud see over Denver, LA, Mexico City, Beijing, etc.--and can be photolyzed and yield oxygen atoms that can react with molecular oxygen to create ozone, NO₂ and the NO/NO₂ ratio is important in tropospheric chemistry.
[Environmental Science and technology; v26; 74-79; 1992.]
[Science; v242; 555-558; 1988.]
Atmospheric Chemistry Glossary