||The hygroscopicity, the water uptake property, of atmospheric aerosols describes the interaction between water vapor and aerosols. It affects the phase, size, and chemical composition of aerosols, and thus influences many atmospheric processes such as cloud activation activity, light scattering, and chemical reactions. Recent studies have revealed that organic species derived from biomass burning, amino acids, and macromolecular polyacids contribute to a substantial portion of water-soluble organic compounds (WSOC) and the aerosol mass. This thesis focuses on studying the hygroscopicity of particles of these three major groups of WSOC using the single particle levitation technique in an electrodynamic balance (EDB). The hygroscopic measurements were performed by equilibrating singly levitated particles at different relative humidity (RH) in the EDB for in-situ mass determination. Levoglucosan, marnosan, and galactosan are commonly detected in the biomass burning aerosols, derived from the combustion of cellulose. The particles of these species exist as highly concentrated solutions at RH as low as 5%. Our results suggest that biomass burning aerosols, which contain these non-crystallizing organic species, may not be completely dry and absorb water at low RH. Our results support the reports in the literature that the light scattering coefficient of biomass burning aerosols starts to increase with increasing RH, even at low RH. Crystallization was observed in some amino acid particles (glycine, alanine, serine, glutamine, and threonine) upon evaporation of water while no phase transition was observed in some others (arginine and asparagine), even at 5%RH. Best fittings of the literature and experimental data with the model estimates yield a new set of UNIFAC interaction parameters that give predictions to within 15% of the measurements up to the supersaturation regime for atmospheric application. Water-soluble macromolecular polyacids have molecular structures similar to natural fulvic acids (FA) and are referred to as humic-like substances (HULIS). The role of HULIS on the hygroscopicity of atmospheric aerosols was studied using their model compounds: two natural FA: Nordic Aquatic Fulvic Acid (NAFA) and Suwannee River Fulvic Acid (SRFA). The NAFA and SRFA particles absorbed and desorbed water reversibly without crystallization and retained water at RH < 10%. Our measurements of the crystallization characteristics of these natural FA particles are different from the literature data of another natural FA. The differences are attributed to the difference in the composition of the natural FA, which depends the isolation methods. A standardization of the isolation methods of natural FA and HULIS in atmospheric aerosols is needed for a more systematic understanding their hygroscopicity and their role on the properties of atmospheric aerosols.