Measurement of nitrogen oxides
Gaseous air pollution

Nitrogen oxides consist of all chemical species in which nitrogen is bonded to at least one oxygen atom and whose oxidation number is at least 2. These are NO, N ₂O₃, NO₂, N₂O₅, the corresponding acids HONO and HNO ₃, and numerous organic nitrates, of which PAN (peroxyacetyl nitrate CH ₃C(O)OONO₂) is the most abundant. Nitrogen oxides are emitted during the high-temperature combustion of fossil fuels. Bacterial decomposition in soil is an important natural source of NO _x in the atmosphere. The predominant form in emissions is NO.

Chemically, NO reacts rapidly with O₃ to give NO ₂ (characteristic time < 1 min); NO₂ is photolyzed during the...