The chemical reactions of water droplets containing dissolved salts and of solid salt particles with ammonia gas have been studied. The salts used were MgCl<sub>2</sub>*6H<sub>2</sub>O, KNO<sub>3</sub>, and MgSO<sub>4</sub>. In contrast to the irreversible formation of a solid ammonium chloride shell when MgCl<sub>2</sub>*6H<sub>2</sub>O dissolved in water droplets is used, we observed formation of solid microcrystals when using KNO<sub>3</sub> together with ammonia, whereas no reaction was observed in the case of MgSO<sub>4</sub>. All three inorganic salts act similarly as a sink for ammonia gas, resulting in solid or dissolved NH<sub>4</sub><sup>+</sup> components. Only the system KNO<sub>3</sub>/NH<sub>4</sub>NO<sub>3</sub> 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)