Cold-stage system for Impact-initiated Contact freezing Experiments (C-ICE)

Abstract. Ice nucleation by atmospheric aerosols plays a central role in cloud microphysical processes, exerting strong influences on Earth's radiation balance and precipitation formation. Among heterogeneous ice nucleation pathways, contact freezing induced by aerosol-droplet collisions remains one of the least quantitatively constrained, largely due to the scarcity of experimental approaches capable of directly probing impact-initiated freezing under controlled and reproducible conditions. Here we present the Cold-stage system for Impact-initiated Contact freezing Experiments (C-ICE), a laboratory system developed to enable controlled, repeatable, and size-resolved investigation of contact freezing triggered by particle-droplet collisions. C-ICE combines a fixed supercooled droplet with a precisely conditioned aerosol jet, allowing particle size, number concentration, collision geometry, and thermodynamic state to be independently characterized and experimentally constrained. At the single-droplet level, freezing onset is identified using complementary optical criteria based on breath figure formation and grayscale analysis. Application of C-ICE to silver iodide aerosols reveals sharp frozen fraction transitions over narrow temperature intervals (≈2 °C), with median freezing temperatures shifting systematically toward higher values from -12.6 °C for 200 nm particles to -12.0 °C for 800 nm particles, and to -10.1 °C for the polydisperse aerosol. By integrating a theoretical collision model with experimentally determined collision efficiencies, we constrain the collision efficiency of the C-ICE system, enabling measured aerosol concentrations to be converted into effective particle collision rates onto the droplet. This novel C-ICE system provides a robust method for quantifying impact-initiated contact freezing and advances process-level understanding of contact freezing in clouds.