Aerosol deliquescence plays a significant role in atmospheric phenomena such as acidic precipitation, formation ofclouds and fogs, and changes in visibility. Important factors including aerosol composition and atmospheric relativehumidity govern the aerosol deliquescence. However, recent work has revealed that certain aerosols form a metastablecomposition, which cannot be elucidated by measuring the solubility of bulk-phase solution, nor can it be predicted bytraditional thermodynamic theory. This composition, however, contributes to the difference between the predicted andmeasured deliquescence of atmospheric aerosol. This work applied the newly developed semi-automated GC-TCD (gaschromatograph equipped with thermal conductivity detector) aerosol water mass measurement system to measure thehygroscopic characteristics of aerosols of NaCl, Na2SO4,NH4NO3,NH4Cl and mixed aerosols of NaCl–Na2SO4andNH4NO3–NH4Cl in various proportions. The measurements are compared with those in the literature to validate theGC-TCD method and investigate the metastable solid form of the Na2SO4aerosol, which cannot be predicted bytraditional thermodynamic theory. The comparisons confirm that measurements of the characteristics of single- andmixed-salt aerosols, obtained by GC-TCD, are satisfactorily accurate. Furthermore, the Na2SO4aerosol is found toform an anhydrate other than the dehydrate predicted by traditional thermodynamic theory. This result may explainthe difference between the measured and modeled results at the DRH of NaCl–Na2SO4mixed aerosol.r2002 ElsevierScience Ltd. All rights reserved.