Nitrogen oxides are compounds of nitrogen and oxygen, two elements that do not normally react with each other but will do so during high temperature combustion – such as in a car engine. Examples include nitric oxide (NO) and nitrogen dioxide (NO2), which contribute to air pollution, and nitrous oxide (N2O), which is a major greenhouse gas. Although its warming effect is far less than CO2, it persists in the atmosphere for far longer, so measured over 100 years its impact is 298 times greater.