Morphological Effects on the Heterogeneous Freezing of Droplets Containing Different Ice Nucleating Particles

Atmospheric aerosol particles can act as ice nucleating particles (INPs) and influence cloud formation, precipitation, and radiative forcing. However, ice nucleation remains poorly understood because atmospheric aerosols exhibit complex chemical compositions and morphologies. In particular, how uniformly mixed and phase-separated organic particles influence freezing behavior remains poorly constrained. We investigated immersion freezing in aqueous organic droplets with different morphologies and containing various insoluble INPs using a custom-built cooling stage with a high-resolution camera. The chemical composition of the different droplets was characterized by PTIR and Raman spectroscopy. In core-shell droplets containing diethyl sebacate (DS), the shell crystallized at warmer temperatures of -8 to -13 °C, whereas the core froze at a much lower temperature of -21 to -25 °C. Black carbon was placed in different locations within the core-shell diethyl sebacate droplets, and the largest difference between shell crystallization and core freezing temperatures was 13 °C when black carbon was present in both regions. Both K-feldspar and silver iodide promoted warmer freezing temperatures of the core. These results provide new insights into the role of complex organic aerosols in mixed-phase clouds and highlight the need to incorporate particle morphology into the parameterizations used in climate models.