nitrification the process whereby ammonia, typically in wastewater, is oxidized to nitrite and then to nitrate by bacterial or chemical reactions denitrification the biological reduction of nitrate or nitrite to nitrogen gas, typically by bacteria in soil stratosphere the portion of the atmosphere ten to twenty-five miles above the earth's surface
NOx is a common term for the more reactive nitrogen oxides and includes nitric oxide (NO) and nitrogen dioxide (NO2), but excludes, for example, nitrous oxide (N2O). NO2 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 (N2) to yield the majority of NO found in the environment. Natural sources of NO2 are soil microbial processes. In the soil the nitrification and denitrification processes pass through compounds that can break down and release NO and N2O into the atmosphere. This is a natural process that is enhanced when nitrogen fertilizers are used to improve crop yields.
Short-term exposure to NO2 at concentrations found in the United States can increase respiratory illness in children. There is evidence that long-term exposure to NO2 may lead to increased susceptibility to respiratory infection. The least reactive nitrogen oxide is N2O, 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 N2O would decrease the ozone layer. The lifetime of N2O is more than sixty years. Because it can absorb infrared radiation, the excess production of N2O can contribute to global warming.
NO and NO2 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 NOx 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 NOx, restrictions on their emissions are a common approach to air quality management even though only NO2 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 NOx. In the United States the national ambient air quality standard (NAAQS) for NO2 is 0.053 parts per million (ppm), and from 1988 to 1997, the average NO2 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 NOx SIP rule of 1998 is aimed at reducing summertime NOx 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 Acid Rain; Coal; Electric Power; Global WArming; Ozone; Petroleum; Smog; Vehicular Pollution.
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