Journal Article

Role of stacking disorder in ice nucleation

Lupi, Laura, Grünwald, Michael, Molinero, Valeria, Peters, Baron, Hudait, Arpa, Gotchy Mullen, Ryan, Nguyen, Andrew H.

2017

Overview

Stacking-disordered ice crystallites are shown to have an ice nucleation rate much higher than predicted by classical nucleation theory, which needs to be taken into account in cloud modelling. Ice born from disorder in the stack Water freezing is crucial to Earth's climate, and accurate weather and climate forecasts depend on reliable predictions of ice nucleation rates. Such predictions are typically based on classical nucleation theory, which assumes that the structure of the initially formed ice crystallites corresponds to that of thermodynamically stable hexagonal ice. Laura Lupi et al . now report simulations and energy calculations with a simple water model to show that, for emerging crystallites, ice with disordered stacking is more stable than hexagonal ice and results in ice nucleation rates more than three orders of magnitude higher than predicted by classical nucleation theory. This effect must be accounted for in cloud models and when interpreting ice nucleation rates measured in laboratory conditions and extrapolating them to temperatures important to clouds. The freezing of water affects the processes that determine Earth’s climate. Therefore, accurate weather and climate forecasts hinge on good predictions of ice nucleation rates 1 . Such rate predictions are based on extrapolations using classical nucleation theory 1 , 2 , which assumes that the structure of nanometre-sized ice crystallites corresponds to that of hexagonal ice, the thermodynamically stable form of bulk ice. However, simulations with various water models find that ice nucleated and grown under atmospheric temperatures is at all sizes stacking-disordered, consisting of random sequences of cubic and hexagonal ice layers 3 , 4 , 5 , 6 , 7 , 8 . This implies that stacking-disordered ice crystallites either are more stable than hexagonal ice crystallites or form because of non-equilibrium dynamical effects. Both scenarios challenge central tenets of classical nucleation theory. Here we use rare-event sampling 9 , 10 , 11 and free energy calculations 12 with the mW water model 13 to show that the entropy of mixing cubic and hexagonal layers makes stacking-disordered ice the stable phase for crystallites up to a size of at least 100,000 molecules. We find that stacking-disordered critical crystallites at 230 kelvin are about 14 kilojoules per mole of crystallite more stable than hexagonal crystallites, making their ice nucleation rates more than three orders of magnitude higher than predicted by classical nucleation theory. This effect on nucleation rates is temperature dependent, being the most pronounced at the warmest conditions, and should affect the modelling of cloud formation and ice particle numbers, which are very sensitive to the temperature dependence of ice nucleation rates 1 . We conclude that classical nucleation theory needs to be corrected to include the dependence of the crystallization driving force on the size of the ice crystallite when interpreting and extrapolating ice nucleation rates from experimental laboratory conditions to the temperatures that occur in clouds.

• Publisher: Nature Publishing Group UK,Nature Publishing Group
• Subject: 119/118 / 639/638/440/950 / 639/638/440/951 / 639/766/119/2795 / 704/106/35/824 / Atmospheric nucleation / Atmospheric temperature / Chemical properties / Chemical research / Climate / Climate prediction / Cloud formation / Clouds / Crystallites / Crystallization / Crystals / Entropy / Fluid dynamics / Free energy / Freezing / Humanities and Social Sciences / Ice / Ice crystals / Ice formation / Ice nucleation / letter / multidisciplinary / Natural history / Nucleation / Nucleation (Physics) / Physical simulation / Science / Simulation / Stacking / Temperature / Temperature dependence / Weather forecasting