Applying GC-TCD to investigate the hygroscopic characteristics of mixed aerosols

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