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

Mineral dust particles are known to be efficient ice nuclei in the atmosphere. Previous work has probed heterogeneous ice nucleation on various laboratory dust samples including Arizona Test Dust, kaolinite, montmorillonite, and illite as atmospheric dust surrogates. However, it has recently been suggested that NX illite may be a better representation of atmospheric dust. Hiranuma et al. ( 2015 ) performed a laboratory comparison for immersion ice nucleation on NX illite, but here we focus on depositional ice nucleation because of its importance in low temperature cirrus cloud formation. A Raman microscope setup was used to examine the ice-nucleating efficiency of NX illite. Organic coatings on the NX illite particles were also investigated using a mixture of 5 dicarboxylic acids (M5). The ratio of NX illite to M5 was varied from 1:10 to 100:1. It was found that NX illite efficiently nucleates ice with S ice = 1.07 ± 0.01 at −47 °C, with S ice slightly increasing at lower temperatures. In contrast, pure M5 is a poorer ice nucleus with S ice = 1.30 ± 0.02 at −40 °C, relatively independent of temperature. Further, it was found that M5 coatings on the order of several monolayers thick hindered the ice nucleating ability of NX illite. Optical images suggest that at colder temperatures (< −50 °C) 1:1 NX illite:M5 particles and pure M5 particles nucleate ice depositionally, while at warmer temperatures (> −50 °C) subsaturated immersion ice nucleation dominates. These experiments suggest that mineral dust particles may become less active towards ice nucleation as they age in the atmosphere.