||The phase transitions of atmospheric aerosols are important in determining their effects on visibility, global climate, and other environmental impacts. Multi-component aerosols, which are ubiquitous in the atmosphere, exhibit complicated phase transition behaviors, including 1) the formation of double salts and 2) partial crystallization and partial deliquescence spanning over a range of relative humidity (RH). In this study, single particle Raman spectroscopy was applied to study 1) the double salt formation in ammonium nitrate/ ammonium sulfate (AN/AS) particles, and 2) the partial crystallization/ deliquescence of mixtures of AS with malonic acid (MA), glutaric acid (GA) and succinic acid (SA). Equimolar AN/AS particles crystallized to form metastable double salt, 3AN·AS, which is not predicted from thermodynamics. The metastable 3AN·AS gradually transformed into stable 2AN·AS and the transformation rate increased with increasing RH. This study illustrates the possible occurrence of metastable salts in the atmosphere. In equimolar AS/organic mixtures, AS/MA particles exhibited partial crystallization at 16% RH, while AS/GA and AS/SA particles crystallized completely at 31-36% RH and about 30% RH, respectively. Deliquescence spanned over RH of <10% to 79%, 70% to 80% and 80% to >90% for the AS/MA, AS/GA and AS/SA particles, respectively. Solid AS and MA were observed in the partially crystallized AS/MA particles, suggesting the heterogeneous nucleation of MA by solid AS. AS first deliquesced at 76% RH in the AS/GA system, followed by GA at 78% RH. In the AS/SA system, AS dissolved at 80% RH, while SA remained as solid for RH as high as 90%. Metastable salts formed in the freshly crystallized AS/GA and AS/SA particles. Transformation of the metastable crystals to the stable forms was observed before water uptake for AS/GA particles, while SA in AS/SA particles transformed in the presence of water during partial deliquescence. The results of this work improves our understandings to the phase transition behaviors of multi-component aerosol particles and are useful for refining models in predicting the behaviors of atmospheric aerosols with mixed phases.