||The impacts of aerosols depend upon the particle size, composition and phase, which are controlled by the interaction with water. Hence, comprehensive studies of the phase and hygroscopicity of atmospheric aerosols are essential to understand their impacts on the climate and environment. This work is divided into two parts. The first part focuses on technical measurements. Optical microscopy and micro-Raman spectroscopy were applied to investigate simultaneously the hygroscopicity and phase transition of various particles. Raman spectra measured at different relative humidity (RH) were used to determine the hygroscopicity and characterize the phase of particles. The hygroscopicity is presented in terms of water-to-solute ratios, which are obtained as relative Raman peak area of the water band to a solute peak. Deliquescence and crystallization can be recognized by the abrupt changes of Raman shifting and the full-width-half-height of distinct solute peaks. The results generally show good agreements with the literature findings and model predictions, demonstrating the capability of micro-Raman spectroscopy in studying the hygroscopicity and phase transition. The second part of this work focuses on the application of techniques to extensive investigations of an atmospherically relevant system, ammonium sulfate and adipic acid (AS-AA), with AA weight percentage ranging from 3 to 90% were studied. At the highest RH of 94%, AA solids were formed from the solution droplets. Complete crystallization was recorded between 31% and 42% RH. When the RH was increased, partial deliquescence in all mixtures was observed at 80% RH. Full deliquescence was only observed in 3 wt% AA particles at 91% RH. The hygroscopic behavior of the AS fraction in the mixed particles to 50 wt% AA 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 AA wt% (AA < 30 wt%).