Observation of the crystallization and supersaturation of mixed component NaNO₃-Na₂SO₄ droplets by FTIR-ATR and Raman spectroscopy

We present here a study of the phase behavior of mixed component NaNO ₃-Na₂SO₄ (SNS) droplets with NaNO₃ to Na₂SO₄ molar ratios of 1:1, 3:1, and 10:1, comparing observations with thermodynamic predictions. Measurements are made by Fourier transform infrared attenuated total reflection and micro-Raman spectroscopy for SNS droplets deposited on ZnSe and quartz substrates, respectively. The conventional deliquescence/efflorescence hysteresis in phase behavior is observed. On drying, heterogeneous crystallization leads to phase behavior that is consistent with bulk solution thermodynamics, with the formation of the mixed salt NaNO₃·Na₂SO₄·H ₂O, Na₂SO₄ (s), and NaNO₃ (s) all observed to form at relative humidities that coincide with predictions by the aerosol inorganics model. However, conditioning of the droplet at high relative humidity prior to drying is observed to lead to quantitative differences between the fractions of different salts formed. When substrate effects do not influence the crystallization process, supersaturated solutions are formed, and this leads to the observation of contact ion pairs. Such measurements of the phase behavior of mixed component droplets are important for testing the reliability of thermodynamic models.