Deliquescence and efflorescence behavior of individual NaCl and KCl mixture aerosol particles

The hygroscopic behavior of laboratory-generated, micrometer sized NaCl and KCl mixture aerosol particles in nine mixing ratios (mole fractions of KCl (XKCl) = 0.1-0.9) was investigated systematically to observe their deliquescence and efflorescence behavior, obtain experimental phase diagrams for their deliquescence and efflorescence, and understand the efflorescence mechanism. Aerosol particles with a eutonic composition of XKCl = 0.3 showed only one phase transition during the humidifying process at the mutual deliquescence relative humidity (MDRH) of 72.3(±0.5)%, whereas the aerosol particles with other mixing ratios showed two distinct deliquescence transitions. First, the eutonic component dissolved at the MDRH and second, the remainder in the solid phase dissolved completely at various DRHs, resulting in a phase diagram composed of four different phases. During the dehydration process, droplets at all mixing ratios showed a single transition at their efflorescence RHs (ERHs), resulting in a phase diagram composed of two different phases. The minimum ERH of 43.7-43.2% was observed for XKCl = 0.4. This suggests that short-range ionic interactions and the common-ion effect may substantially affect the mutual solubility of NaCl and KCl at high ionic-strengths, and hence the supersaturation of the salts. Elemental X-ray mapping of the effloresced NaCl-KCl mixture particles at all mixing ratios suggests that more supersaturated salt nucleated homogeneously to crystallize in the center, and the other salt underwent the heterogeneous crystallization on the former.