Particle-Resolved Simulation of Immersion Freezing with Multi-Species Ice-Nucleating Particles

Immersion freezing, initiated by an ice-nucleating particle (INP) in a supercooled aqueous droplet, has been recognized to play an important role in the formation of ice crystals within clouds. The efficiency of the ice nucleation process depends strongly on the chemical composition of the INPs. Furthermore, INPs can exhibit various mixing states, ranging from external mixtures to internal mixtures, with diverse distributions of chemical species across the particle population. Here, we investigate the impact of the aerosol mixing state on immersion freezing using the stochastic particle-resolved aerosol model (PartMC). We have extended PartMC with a time-dependent representation of immersion freezing based on the water activity based immersion freezing model (ABIFM) and generalized the freezing model’s formulation to calculate freezing rates of INPs that contain multiple species. We analytically derived the limiting behavior for the fraction of droplets that undergo immersion freezing when the INPs are internally mixed compared to externally mixed. The impact on the presence of different species on the frozen droplet fraction is explored through simulation. Our findings indicate distinctly different immersion freezing rates when considering internal and external particle mixtures, which is exacerbated when species with different freezing efficiencies are present.