The chemical reactions of water droplets containing dissolved salts and of solid salt particles with ammonia gas have been studied. The salts used were MgCl2*6H2O, KNO3, and MgSO4. In contrast to the irreversible formation of a solid ammonium chloride shell when MgCl2*6H2O dissolved in water droplets is used, we observed formation of solid microcrystals when using KNO3 together with ammonia, whereas no reaction was observed in the case of MgSO4. All three inorganic salts act similarly as a sink for ammonia gas, resulting in solid or dissolved NH4+ components. Only the system KNO3/NH4NO3 can act as source dependent on the concentration of ammonia gas.
J. Musick, W. Kiefer, and J. Popp, "Chemical Reactions of Single Levitated Inorganic Salt Particles with Ammonia Gas," Appl. Spectrosc. 54, 1136-1141 (2000)