From Glossary of Meteorology
Inorganic salt, formula (NH4)2SO4.
A large constituent of tropospheric aerosol, due to neutralization of acidic sulfuric acid aerosol by ammonia. Much of the aerosol is partially neutralized, and the formula NH4HSO4, corresponding to ammonium bisulfate, is generally more appropriate. Ammonium sulfate is a solid, crystalline material that deliquesces at relative humidities greater than 75%, eventually forming liquid droplets.