NOx is a generic term for the mono-nitrogen oxides NO and NO
2 (
nitric oxide and
nitrogen dioxide). They are produced from the reaction among
nitrogen,
oxygen and even hydrocarbons (during
combustion), especially at high temperatures. In areas of high motor vehicle traffic, such as in large cities, the amount of nitrogen oxides emitted into the atmosphere as
air pollution can be significant. NO
x gases are formed whenever combustion occurs in the presence of nitrogen – as in an air-breathing engine; they also are produced naturally by
lightning. In
atmospheric chemistry, the term means the total concentration of NO and NO
2. NO
x gases react to form
smog and
acid rain as well as being central to the formation of
tropospheric ozone.