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DISPLAYTITLE:NOx In atmospheric chemistry, 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. 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. gases are also produced naturally by lightning. 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. is defined as the sum of plus the compounds produced from the oxidation of which include nitric acid, nitrous acid (HONO), dinitrogen pentoxide (N2O5), peroxyacetyl nitrate (PAN), alkyl nitrates (RONO2), peroxyalkyl nitrates (ROONO2), the nitrate radical (NO3), and peroxynitric acid (HNO4). Because of energy limitations, oxygen and nitrogen do not react at ambient temperatures. But at high temperatures, they undergo an endothermic reaction producing various oxides of nitrogen. Such temperatures arise inside an internal combustion engine or a power station boiler, during the combustion of a mixture of air and fuel, and naturally in a lightning flash. In atmospheric chemistry, the term refers to the total concentration of NO and since the conversion between these two species is rapid in the stratosphere and troposphere. During daylight hours, these concentrations together with that of ozone are in steady state, also known as photostationary state (PSS); the ratio of NO to is determined by the intensity of sunshine (which converts to NO) and the concentration of ozone (which reacts with NO to again form ). In other words, the concentration of ozone in the atmosphere is determined by the ratio of these two species.

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