NOx

In atmospheric chemistry, NOx is shorthand for nitric oxide (NO) and nitrogen dioxide (NO2), the nitrogen oxides that are most relevant for air pollution. These gases contribute to the formation of smog and acid rain, as well as affecting tropospheric ozone. NOx gases are usually produced from the reaction between nitrogen and oxygen during combustion of fuels, such as hydrocarbons, in air; especially at high temperatures, such as in car engines. In areas of high motor vehicle traffic, such as in large cities, the nitrogen oxides emitted can be a significant source of air pollution. NOx gases are also produced naturally by lightning. NOx does not include nitrous oxide (N2O), a fairly inert oxide of nitrogen that contributes less severely to air pollution, notwithstanding its involvement in ozone depletion and high global warming potential. NOy is the class of compounds comprising NOx and the NOz compounds produced from the oxidation of NOx which include nitric acid (HNO3), nitrous acid (HONO), dinitrogen pentoxide (N2O5), peroxyacetyl nitrate (PAN), alkyl nitrates (RONO2), peroxyalkyl nitrates (ROONO2), the nitrate radical (NO3), and peroxynitric acid (HNO4).