pH is one of the most basic chemical properties of aqueous solution, but its measurement in nanoscale aerosol particles presents many challenges. The pH of aerosol particles is of growing interest in the atmospheric chemistry community because of its demonstrated effects on heterogeneous chemistry and human health, as well as potential effects on climate. The authors have shown that phase transitions of aerosol particles are sensitive to pH, focusing on systems that undergo liquid−liquid phase separation. Currently, aerosol pH is calculated indirectly from knowledge of species present in the gas and aerosol phases through the use of thermodynamic models. From these models, ambient aerosol is expected to be highly acidic (pH ∼ 0−3). Direct measurements have focused on model systems due to the difficulty of this measurement. This area is one in which physical chemists should be encouraged to contribute because of the potential consequences for aerosol processes in the environment.

Determining the factors that constitute an efficient ice nucleus is an ongoing area of research in the atmospheric community. In particular, surface characteristics such as functional groups and surface defects impact the ice nucleation efficiency. Crystal structure has been proposed to be a possible factor that can dictate ice nucleation activity through the templating of water molecules on the surface of the aerosol particle. If the crystal structure of the surface matches that of the crystal structure of ice, it has been shown to increase ice nucleation activity. In this study, alumina was chosen as a model system because crystal structure and crystallinity can be tuned, and the effect on immersion freezing was explored. The nine alumina samples include polymorphs of AlOOH, Al(OH) 3 , and Al 2 O 3 , which have a range of crystal structures and crystallinities. The samples were characterized with X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive spectroscopy (EDS), and Brunauer−Emmett−Teller (BET) analysis. From the immersion freezing experiments, corundum [α-Al 2 O 3 ] was shown to have the highest ice nucleation activity likely because of its high lattice match and high degree of crystallinity. Crystal structure alone did not show a strong correlation with ice nucleation activity, but a combination of a hexagonal crystal structure and a highly crystalline surface was seen to nucleate ice at warmer temperatures than the other alumina samples. This study provides experimental results in the study of ice nucleation of a range of alumina samples, which have possible implications for alumina-based mineral dust particles. Our findings suggest that crystallinity and crystal structure are important to consider when evaluating the ice nucleation efficiency of aerosol particles in laboratory and modeling studies.

FeO has enhanced ice nucleation activity due to functional groups that are exposed upon mechanical processing.