1. Weak acid formed in solution by neutralisation of the nitrite ion. Also formed when nitrogen dioxide, NO₂, dissolves in water. 2. Gaseous compound, formula HONO, that builds up in polluted, typically urban air, containing nitrogen oxides. The only known gas-phase reaction leading to its formation, that between hydroxyl radicals (OH) and nitric oxide (NO), is not rapid enough to account for observed levels, and the formation is thus thought to involve particles. Nitrous acid photodissociates to give OH + NO, and can thus serve as an OH source, particularly in the early morning.