Ice nucleation rates near ∼225 K

We have measured the ice nucleation rates, J_ice, in supercooled nano-droplets with radii ranging from 6.6 nm to 10 nm and droplet temperatures, T_d, ranging from 225 K to 204 K. The initial temperature of the 10 nm water droplets is ∼250 K, i.e., well above the homogeneous nucleation temperature for micron sized water droplets, T_H ∼235 K. The nucleation rates increase systematically from ∼10²¹ cm^-3 s^-1 to ∼10²² cm^-3 s^-1 in this temperature range, overlap with the nucleation rates of Manka et al. [Phys. Chem. Chem. Phys. 14, 4505 (2012)], and suggest that experiments with larger droplets would extrapolate smoothly the rates of Hagen et al. [J. Atmos. Sci. 38, 1236 (1981)]. The sharp corner in the rate data as temperature drops is, however, difficult to match with available theory even if we correct classical nucleation theory and the physical properties of water for the high internal pressure of the nanodroplets.