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Ice nucleating particles (INPs) in the atmosphere are responsible forglaciating cloud droplets between 237 and 273 K. Different mechanisms ofheterogeneous ice nucleation can compete under mixed-phase cloud conditions.Contact freezing is considered relevant because higher ice nucleationtemperatures than for immersion freezing for the same INPs were observed. Ithas limitations because its efficiency depends on the number of collisionsbetween cloud droplets and INPs. To date, direct comparisons of contact andimmersion freezing with the same INP, for similar residence times andconcentrations, are lacking. This study compares immersion and contactfreezing efficiencies of three different INPs. The contact freezing data wereobtained with the ETH CoLlision Ice Nucleation CHamber (CLINCH) using80 µm diameter droplets, which can interact with INPs for residencetimes of 2 and 4 s in the chamber. The contact freezing efficiency wascalculated by estimating the number of collisions between droplets andparticles. Theoretical formulations of collision efficiencies gave too highfreezing efficiencies for all investigated INPs, namely AgI particles with200 nm electrical mobility diameter, 400 and 800 nm diameter Arizona TestDust (ATD) and kaolinite particles. Comparison of freezing efficiencies bycontact and immersion freezing is therefore limited by the accuracy ofcollision efficiencies. The concentration of particles was1000 cm−3 for ATD and kaolinite and 500, 1000, 2000 and5000 cm−3 for AgI. For concentrations  <  5000 cm−3,the droplets collect only one particle on average during their time in thechamber. For ATD and kaolinite particles, contact freezing efficiencies at2 s residence time were smaller than at 4 s, which is in disagreement witha collisional contact freezing process but in accordance with immersionfreezing or adhesion freezing. With “adhesion freezing”, we refer to acontact nucleation process that is enhanced compared to immersion freezingdue to the position of the INP on the droplet, and we discriminate it fromcollisional contact freezing, which assumes an enhancement due to thecollision of the particle with the droplet. For best comparison with contactfreezing results, immersion freezing experiments of the same INPs wereperformed with the continuous flow diffusion chamber Immersion Mode CoolingchAmber–Zurich Ice Nucleation Chamber (IMCA–ZINC) for a 3 s residencetime. In IMCA–ZINC, each INP is activated into a droplet in IMCA and provides its surface for ice nucleation inthe ZINC chamber. The comparison of contact and immersion freezing resultsdid not confirm a general enhancement of freezing efficiency for contactcompared with immersion freezing experiments. For AgI particles the onset ofheterogeneous freezing in CLINCH was even shifted to lower temperaturescompared with IMCA–ZINC. For ATD, freezing efficiencies for contact andimmersion freezing experiments were similar. For kaolinite particles, contactfreezing became detectable at higher temperatures than immersion freezing.Using contact angle information between water and the INP, it is discussedhow the position of the INP in or on the droplets may influence its icenucleation activity

Comparing contact and immersion freezing from continuous flow diffusion chambers