Ice nucleation by kaolinite: Morphology shapes the size dependence of ice nucleation active sites on dust particles.

Mineral dusts are the most important type of ice-nucleating particles and play a key role in cloud formation and climate. Their activity is commonly assumed to scale with particle surface area, yet whether this assumption holds across particle sizes remains unverified. Here, we quantify size-dependent ice nucleation activity using compositionally pure, size-segregated kaolinite (KGa-1b). Ice nucleation active site (INAS) densities were determined for size classes from submicron to 48 μm, revealing two distinct ice-nucleation regimes. At low temperatures (∼240 K), INAS density per surface area is independent of particle size. In contrast, at warm temperatures (∼260 K), particles of 6 to 12 μm exhibit the highest INAS densities, up to two orders of magnitude greater than smaller particles and nonsize-selected bulk samples. Linking these results to particle morphology indicates that well-aligned, multilayered platelet edges provide the continuous hydroxylated surfaces required for efficient ice nucleation. Our findings demonstrate that morphology can modulate particle size-dependent ice nucleation, highlighting limitations of simple surface area-based parameterizations.