Mineral dust particles are known to be efficient ice nuclei (IN) in the atmosphere. Previous work has probed heterogeneous ice nucleation on laboratory dust samples, mostly using Arizona Test Dust as an atmospheric dust surrogate. However, it has recently been suggested that dusts containing illite are a better representation of atmospheric dust. We have used a Raman microscope setup to examine the ice-nucleating efficiency of NX illite. We also investigated the effects of an organic coating on the NX illite particles using a mixture of 5 dicarboxylic acids (M5). The ratio of NX illite to M5 was varied in this work from 1:10 to 10:1. Ice nucleation on pure M5 was also studied. We find that NX illite is a good ice nucleus with an Sice value of 1.07 ± 0.01 at -47°C, with a slight temperature dependence. In contrast, pure M5 is a poor ice nuclei with an Sice value of 1.30 ± 0.02 at -40°C and shows two different ice nucleation regimes. In addition, we find that 1:1 NX illite:M5 is the threshold composition for hindering the ice nucleating ability of NX illite. Optical images reveal that at colder temperatures (lower than -50°C) 1:1 NX illite:M5 mixtures (and pure M5) nucleate depositionally, while at warmer temperatures (higher than -50°C) immersion/homogeneous ice nucleation occurs. By knowing more about laboratory surrogates of atmospheric dust, the formation of ice clouds and their effect on the climate can be better understood.

Depositional ice nucleation on NX illite and mixtures of NX Iilite with organic acids

249th National Meeting (2015)

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