Ammonium chloride

From Glossary of Meteorology

ammonium chloride

Inorganic salt, formula NH4Cl, formed from the neutralization of hydrochloric acid by ammonia.

Ammonium chloride is a low vapor pressure solid, which deliquesces when the relative humidity exceeds 80%. It is thought to be present in the urban aerosol present in smog episodes.
See deliquescence.