Nitrogen Oxides (NOx)

The many chemical species of the oxides of nitrogen are collectively termed as nitrogen oxides (NOx). This group of gases usually enters the air as a result of combustion processes which involve high temperatures, such as those produced by power plants and vehicular engines.

Nitric oxide (NO)
Nitric oxide is the main NOx emitted during combustion and it can be converted into nitrogen dioxide.

Nitrogen dioxide (NO2)
Nitrogen dioxide is a corrosive and highly oxidising light brown gas which has a characteristic pungent smell at high concentrations. It is the reaction of nitrogen dioxide with reactive organic substances, such as Volatile Organic Compounds (VOCs), in the presence of sunlight that produces ozone (see information on 'Ozone and Photochemical Oxidants)'. Nitrogen dioxide is, therefore, an important part of urban haze or photochemical smog.