Oxides of nitrogen are among the major environmental pollutants that have a significant detrimental effect on the atmosphere. Fossil-fuel combustion is responsible for over half the total global emissions of nitrogen oxides, and up to two-thirds of the emissions from human activities. Although the contribution from coal combustion alone is not known, it is evident that oil combustion in the transport sector commonly contributes over 50% of emissions from human activities in individual countries. Other sources from human activities include biomass burning and the use of ertilisers. During coal combustion, nitrogen present in both the coal and the combustion air is converted to nitric oxide (NO) and nitrogen dioxide (NO2) (together commonly referred to as NOx). These are the two most important oxides of nitrogen with respect to air pollution, and while nitrous oxide (N2O) is also an oxide of nitrogen, its role in the environment is distinct from that of NO and NO2. NOx emissions are of concern because they are associated with the increased acidity of aerosol particles, cloud water and precipitation (acid rain), causing damage to aquatic and terrestrial ecosystems, as well as being responsible for the corrosion of building materials and affecting human health. NOx can act as a fertiliser in soils but, in excess quantities, leads to eutrophication (an overabundance in nutrients leading to oxygen depletion) in many environments. Furthermore, NOx emissions are of crucial importance in the formation of photochemical smog and oxidants, especially ozone (O3), in the lower atmosphere.

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