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Volatile organic matter that is suspended in the atmosphere such as α-Pinene and β-caryophyllene undergoes aging processes, as well as chemical and photooxidation reactions to create secondary organic aerosol (SOA), which can influence the indirect effect of aerosol particles and the radiative budget. The presence and impact of water vapor and ammonium sulfate (ubiquitous species in the atmosphere) on the hygroscopicity and CCN activity of SOA has not been well characterized. In this research, three water-uptake measurement methods: cavity ring-down spectroscopy (CRD), humidified tandem differential mobility analysis (HTDMA), and cloud condensation nuclei counting (CCNC) were employed to study the hygroscopicity of α-pinene and β-caryophyllene SOA formed under dark ozonolysis. We observed the changes in water uptake of SOA in the absence and presence of water vapor at ~70 % RH and ammonium sulfate seeds. Measured hygroscopicity was represented by the single hygroscopicity parameter (κ). κ of α-pinene SOA was measured to be 0.04 and can increase up to 0.19 in the presence of water vapor and ammonium sulfate. β-caryophyllene SOA exhibited non-hygroscopic properties with κ values that were effectively 0. It is proposed that a difference in the viscosity and hydrophobicity of the SOA may be the primary factor that leads to changes in hygroscopicity.