1. Characteristics of nitrogen oxide molecules

The term NO _x is the generic term for a series of compounds containing nitrogen and oxygen at different oxidation stages. These include nitrous oxide N ₂ O, nitric oxide NO, nitrogen dioxide NO ₂ , nitrogen tetraoxide N ₂ O ₄ and nitrogen trioxide N ₂ O ₃ (sometimes also N ₂ O ₅ ), but also, in the presence of water vapor, the nitrous and nitric acids HNO ₂ and HNO ₃ respectively.

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