Does a lower size limit for mineral dust ice nuclei in the immersion mode exist?

There is previous empirical evidence suggesting that atmospheric aerosol particles that can act as ice nuclei (IN) are larger than approximately 100nm in diameter (e.g. Fletcher, 1959). However, recently observations of IN active macromolecules have been proposed to be responsible for the enhanced ice formation in the washing water of pollen and indicate no such size limit (Augustin et al., 2012). To investigate the size dependence of mineral dust species on their ability to serve as IN, the size dependent frozen fraction of droplets containing single immersed mineral dust is investigated with the IMCA-ZINC experimental setup (Lüönd et. al., 2010). Special consideration was given to produce monodisperse particles in the lower size range, by using a two-stage size selection setup including a differential mobility analyser and a centrifugal particle mass analyser. The results indicate no discontinuous decrease in the ice nucleation ability of 100nm kaolinite particles. Whether the available surface area itself catalyses ice nucleation or rather specific active sites which are more likely to be present on a larger surface, is addressed by comparing the experimental findings to model predictions following the classical nucleation theory and the active site approach. Measurements with different dust species corroborate the general findings from kaolinite. Compared to the active site model, the experimental data exhibit a lower cut off (below 100nm) before homogenous freezing conditions are reached compared to the model prediction that shows a cut off between 200 – 400 nm. These results can be useful for modelling studies that investigate the influence of dust emissions from different sources, which undergo long-range transport over varying distances and therefore exhibit different size distributions. Additionally, the experimental results support the applicability of the ice nucleation active surface site density (INAS) approach (e.g. Murray et al., 2012) to compare different measurements based on the surface area parameterization as a function of temperature. References Augustin, S. Hartmann, S.,Pummer, B., Grothe, H., Niedermeier, D., Clauss, T., Voigtländer, L., Wex, H. And Strarmann, F.: Immersion freezing of birch pollen washing water Atmos. Chem. Phys. Discuss., 12, 32911-32943, 2012.

Fletcher, N.H.: On Ice-Crystal Production by Aerosol Particles, J. Meteo., 16, 173-180, 1959.

Lüönd, F., Stetzer, O., Welti, A., and Lohmann, U.: Experimental study on the ice nucleation ability of size selected kaolinite particles in the immersion mode, J. Geophys. Res., 115, D14201, 2010.

Murray, B. J., O'Sullivan, D., Atkinson, J. D., and Webb, M. E.: Ice nucleation by particles immersed in supercooled cloud droplets, Chem. Soc. Rev., 41, 6519-6554, 2012.