Cloud Condensation Nuclei and Immersion Freezing Abilities of Al2O3 and Fe2O3 Particles Measured with the Meteorological Research Institute's Cloud Simulation Chamber

Aluminum oxide (Al2O3) and iron oxide (Fe2O3) particles have been observed not only in industrial areas and their surroundings, but also in natural atmospheric environments. These types of aerosols can influence aerosol–cloud interactions. In this study, physicochemical properties such as size distribution and the ability to act as cloud condensation nuclei (CCN) as well as ice nucleating particles (INPs) of surrogates of ambient Al2O3 and Fe2O3 particles were investigated using a CCN counter, the Meteorological Research Institute's (MRI) cloud simulation chamber, the MRI's continuous-flow-diffusion-chamber-type ice nucleus counter (CFDC-type INC), and an array of aerosol instruments. The results indicated that their hygroscopicity parameter (κ-value) ranged from 0.01 to 0.03. This range is compatible with that of surrogates of mineral dust particles and is smaller than typical κ-values of atmospheric aerosols. On the other hand, based on their ice nucleation active surface site (INAS) densities, these materials may act as effective INPs via immersion freezing (i.e., ice nucleation triggered by particles immersed in water droplets). In the cloud chamber experiments, Al2O3 and Fe2O3 particles continuously nucleated ice crystals at temperatures below −14°C and −20°C, respectively. This result indicates that the Al2O3 particles were better INPs than the Fe2O3 particles were. Moreover, the INAS density of the Al2O3 particles was comparable to that of natural ambient dust.