Deliquescence behaviour of single levitated ternary salt/carboxylic acid/water microdroplets

The deliquescence relative humidities (DRH) of ammonium sulfate as well as ammonium sulfate/dicarboxylic acid (glutaric, maleic and tartaric) mixtures as a function of temperature and relative composition have been studied using an electrodynamic balance (EDB) in connection with optical microscopy and Mie scattering. The absolute DRH values for pure ammonium sulfate as well as their temperature dependence are consistent with literature data and with the AIM model of Clegg et al. The addition of either glutaric or maleic acid to ammonium sulfate leads to a decrease of the DRH value, with the temperature dependence either remaining constant (glutaric acid) or increasing (maleic acid) with increasing acid concentration. This difference is attributed to the higher acidity of maleic acid, which generates stronger ionic interactions with the ammonium sulfate system. In the case of tartaric acid, the deliquescence behaviour of ammonium sulfate is substantially influenced by the formation of insoluble ammonium tartrate.