ammonium nitrate
From Glossary of Meteorology
Inorganic salt, formula NH4NO3, formed from the neutralization of nitric acid by ammonia.
Ammonium nitrate is a deliquescent solid with a low vapor pressure, which consequently forms in the atmosphere when high concentrations of ammonia and nitric acid occur, for example, in polluted urban areas. It is thought to sometimes make up a considerable fraction of the urban aerosol present in smog episodes.
See deliquescence.