NO _x (Nitrogen Oxides)

NO _x is a common term for the more reactive nitrogen oxides and includes nitric oxide (NO) and nitrogen dioxide (NO ₂ ), but excludes, for example, nitrous oxide (N ₂ O). NO ₂ is a reddish brown, highly reactive gas that is formed in the air by the oxidation of NO. Anthropogenic emissions from the high-temperature combustion of coal, oil, gas, and gasoline can oxidize atmospheric nitrogen (N ₂ ) to yield the majority of NO found in the environment. Natural sources of NO ₂ are soil microbial processes. In the soil the nitrification and denitrification processes pass through compounds that can break down and release NO and N ₂ O into the atmosphere. This is a natural process that is enhanced when nitrogen fertilizers are used to improve crop yields.

Short-term exposure to NO ₂ at concentrations found in the United States can increase respiratory illness in children. There is evidence that long-term exposure to NO ₂ may lead to increased susceptibility to respiratory infection. The least reactive nitrogen oxide is N ₂ O, but it can affect both the ozone layer and global warming. Once in the atmosphere, it slowly diffuses into the stratosphere where it is destroyed by the shorter-wavelength UV radiation. The NO produced by this photodissociation is critical in establishing the amount of ozone in the stratosphere, so any increase in N ₂ O would decrease the ozone layer. The lifetime of N ₂ O is more than sixty years. Because it can absorb infrared radiation, the excess production of N ₂ O can contribute to global warming.

NO and NO ₂ react with sunlight and unburned gasoline in a matter of hours to days to produce ozone that is critical in the development of photochemical smog. Atmospheric NO _x also reacts to produce nitric acid. While it is stable in dry air, nitric acid is very soluble and, along with sulphuric acid, significantly contributes to acid rain. Because acid rain and smog involve the reactions of NO _x , restrictions on their emissions are a common approach to air quality management even though only NO ₂ is classed as a criteria pollutant.

In most countries, smog control focuses on reducing ozone concentrations to the air-quality standard by controlling emissions of the precursors, including NO _x . In the United States the national ambient air quality standard (NAAQS) for NO ₂ is 0.053 parts per million (ppm), and from 1988 to 1997, the average NO ₂ concentration dropped 14 percent to 0.018 ppm. Each state prepares a state implementation plan (SIP) that describes how it will reduce pollutant levels, and presents that plan to the EPA for approval. The EPA, in turn, then supports state plans. The NO _x SIP rule of 1998 is aimed at reducing summertime NO _x emissions in order to cut down on the transport of ozone from power plants in the Midwest to eastern states. Other countries use similar approaches but rely on government and public pressure rather than statutory requirements to meet standards.

SEE ALSO A CID R AIN ; C OAL ; E LECTRIC P OWER ; G LOBAL W ARMING ; O ZONE ; P ETROLEUM ; S MOG ; V EHICULAR P OLLUTION .

Bibliography

Brimblecombe, Peter. (1996). Air Composition and Chemistry, 2nd edition. New York: Cambridge University Press.

Finlayson-Pitts, Barbara J., and Pitts, James N. (2000). Chemistry of the Upper and Lower Atmosphere. San Diego, CA: Academic Press.

Graedel, Thomas E., and Crutzen, Paul J. (1995). Atmosphere Climate and Change. New York: Scientific American Library (distributed by W.H. Freeman).

Turco, Richard. (1997). Earth under Siege. Oxford: Oxford University Press.

Donald R. Hastie