Minimal cooling-rate dependence of ice nuclei activity in the immersion mode

Atmospheric ice nuclei play an important role in initiating the ice-phase in clouds at temperatures warmer than -38 deg C. In principle, ice nucleation is a stochastic process that is described by a time-dependent nucleation rate. The development of accurate parametrizations for ice nucleation in cloud models will depend on correct descriptions of the relative sensitivities of the nucleation rate to time and supercooling temperature. Here we present new experiments and simulations that aim to better constrain theoretical models fitted to laboratory data. Ice nucleation data are collected using a droplet freezing assay setup that allows for the cooling rates to be changed between 10 and 0.01 K min-1, thus simulating in-cloud residence times up to 50 hr. Data are presented for a broad range of active sites spanning a characteristic temperature range from -5 to -35 deg C and spanning multiple mechanisms by which heterogeneous surfaces induce freezing. Substances for which data are available include minerals/ash (montmorillonite, kaolinite, Arizona Test Dust, soil, volcanic ash), biological particles (freeze dried Pseudomonas Syringae), graphitic particles (flame soot, carbon black), and alcohol monolayers (nonadecanol). Furthermore, ice nuclei present in rainwater samples are analyzed to test whether laboratory proxy substances capture the behavior of ambient ice nuclei. Discrete event simulations based on a variant of the multiple-component stochastic model of heterogeneous freezing nucleation were used to show how droplet freezing assays can be used to quantify nucleation rates in spite of the methods' limitations to precisely control for the number of particle inclusions in an individual droplet. The experimental data demonstrate that the time-dependence for all materials is minimal. Slow cooling rates over a 50 hr time interval change median freezing temperatures of a population by a maximum of 3K compared to faster cooling rates over a 20 min interval. This suggests that the modified singular approximation is approximately valid over the range of times and temperatures encountered for mixed-phase clouds. We anticipate that these laboratory findings increase the confidence in parametrization of ice nucleation that are based on chemical composition, size or surface area, and temperature, but are independent of in-cloud residence time.