Hygroscopicity is an important physicochemical property of aerosol that describes the ability of a particle to uptake water. The hygroscopic properties of uranyl fluoride (UO2F2) aerosol generated from a UF6 hydrolysis reactor was investigated for the first time using a custom-built Humidified Tandem Differential Mobility Analyzer (HTDMA). The HTDMA is capable of measuring UO2F2 nanoparticle growth determined by mobility size over a wide range of atmospheric humidity from dry conditions at <10% relative humidity (RH) to 85% RH. The hygroscopic properties were determined for nanoparticles as small as 3.5 nm in this study. Although the largest size of UO2F2 nanoparticles was 80 nm, monodisperse aerosol with a mobility diameter of up to approximately 500 nm can be investigated using the HTDMA. Anhydrous UO2F2 nanoparticles with a mobility diameter of 3.5 nm were shown to be highly hygroscopic with a deliquescence relative humidity (DRH) of 10%. Hydrates with a larger mobility diameter from 10 to 80 nm were non-hygroscopic with no observable DRH and limited water uptake up to 85% RH. These results demonstrate the hygroscopic properties of UO2F2 nanoparticles are highly variable and based on both the mobility size and hydration state. Hygroscopicity affects the physicochemical properties of UO2F2 nanoparticles, including the aerosol phase state and viscosity, with impacts on aerosol growth, coagulation, and deposition that is critical for understanding the fate and transport of UO2F2 nanoparticles in the atmosphere.
