oxidation of nitrogen to nitrogen oxide in the channel of a lightning:
N₂ + O₂ = 2NO
oxidation of natural nitrogen oxide and of nitrogen oxide of combustion gases to nitrogen dioxide:
2NO + O₂ = 2NO₂
nitrogen dioxide reaction with moisture of air or rain water:
4NO₂ + O₂ + 2H₂O = 4HNO₃