Phase Transition and Hygroscopic Properties of Internally Mixed Ammonium Sulfate and Adipic Acid (AS-AA) Particles by Optical Microscopic Imaging and Raman Spectroscopy

We studied the phase transition and hygroscopicity of particles of a mixture of two atmospherically relevant species, ammonium sulfate and adipic acid. We conducted extensive investigations on ammonium sulfate-adipic acid (AS-AA) particles deposited on a hydrophobic substrate with the weight percentage (wt%) of AA ranging from 3 to 90%. Crystallization and deliquescence were observed through an optical microscope, which enabled multiple particles to be examined simultaneously. At an initial relative humidity (RH) of 94%, which was the highest RH setting, AA solids were formed from the deposited solution droplets, leading to the presence of mixed-phase particles prior to the complete crystallization recorded between 31% and 42% RH of all mixtures. The complete crystallization RH values were close to that of pure AS, indicating that the AA solids did not promote effective heterogeneous nucleation of AS. When the RH was increased, partial deliquescence in all mixtures was observed at 80% RH, which was attributed to the dissolution of the AS fractions. Full deliquescence was only observed in 3 wt% AA particles at 91% RH. We also used Micro-Raman Spectroscopy to determine the hygroscopicity of mixtures with up to 50 wt% AA. The hygroscopic behavior of the AS fraction in the mixed particles was found to resemble that of pure AS particles. However, there was observable water uptake in the solid particles at about 70% RH in mixtures with up to 70 wt% AA. This early water uptake was more pronounced in mixtures with lower weight percentages of AA (AA less than 30 wt%).