Effects of Water Vapor and Inorganic Seed Particles on Secondary Organic Aerosol Water-Uptake

Volatile organic compounds (VOC) in the atmosphere such as -Pinene and -caryophyllene undergo aging processes, chemical, and photooxidation reactions to create secondary organic aerosols (SOA), which can influence the indirect effect of aerosols and the radiative budget, although the magnitude of this effect remains uncertain. Secondary Organic Aerosol formation can occur at high and low relative humidities. However, the impact on the water-uptake and CCN properties at of SOA formed in and without the presence of water-vapor is not well understood. Furthermore, water-uptake of ammonium sulfate seeded SOA formed under high and low humidity 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 -Pinene and -caryophyllene SOA formed under dark ozonolysis. Changes in water uptake of SOA in the presence of water vapor at ~70 % RH and ammonium sulfate seeds was observed. Measured hygroscopicity was parameterized by the single hygroscopicity parameter (). of -Pinene SOA was measured to be 0.04, and can increase up to 0.19 in the presence of water vapor and ammonium sulfate. -caryophyllene SOA exhibited non-hygroscopic properties with 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.