Morphological effects on the hygroscopic properties of sodium chloride–sodium sulfate aerosols

• IR absorption of water reveals different morphologies for NaCl and Na2SO4 aerosols.
• Internal mixtures of NaCl and Na2SO4 aerosols exhibit an intermediate morphology.
• A metastable solid phase, Na2SO4 (III), is generated as the aerosols are formed.
• NaCl–Na2SO4 is a potential prototype for studying effects of aerosol morphology.

The hygroscopic properties of sodium sulfate, Na2SO4; sodium chloride, NaCl; and their mixed aerosols are monitored using a flow-cell apparatus and Fourier-transform infrared spectroscopy. While Na2SO4 exhibits the typical water uptake for an inorganic salt, spectra of NaCl and mixed NaCl–Na2SO4 aerosols reveal water content below the deliquescence relative humidity. This is explained on the basis of two different morphologies for NaCl and Na2SO4. While Na2SO4 can be described by a simple crystalline solid, NaCl contains pockets that trap water. The NaCl–Na2SO4 mixtures contain a combination of these two structural motifs, weighted by the relative amounts of the NaCl and Na2SO4 components. This makes NaCl–Na2SO4 mixtures an interesting candidate for exploring the effects of morphology on atmospherically relevant properties in a systematic fashion. Additionally, the formation of metastable Na2SO4 (III) is confirmed using Raman spectroscopy, though its effects on the hygroscopicity of NaCl–Na2SO4 mixtures are believed to be minimal.