[1] Changes in the ice nucleation properties of mineral dust particles due to soluble coatings are still not well understood. Here we show that the reactivity with soluble materials deposited on the surfaces of kaolinite particles is an important factor affecting the ice nucleation properties of the particles. Using kaolinite particles treated with levoglucosan or H 2 SO 4 (i.e., non-reactive and reactive materials, respectively), we investigated the fraction of particles capable of nucleating ice at temperatures ranging from À34C to À26 C. Below water saturation, both the levoglucosan and H 2 SO 4 coatings similarly reduced the ice nucleating ability of kaolinite particles. Above water saturation, however, only the H 2 SO 4 coatings reduced the ice nucleating ability of the particles, particularly at warmer temperatures. We suggest that the absence or presence of surface chemical reactions plays an important role in determining the number concentrations of ice crystals formed from mineral dust ice nuclei under mixed-phase cloud conditions. Citation: Tobo, Y., P. J. DeMott, M. Raddatz, D. Niedermeier, S. Hartmann, S. M. Kreidenweis, F. Stratmann, and H. Wex (2012), Impacts of chemical reactivity on ice nucleation of kaolinite particles: A case study of levoglucosan and sulfuric acid,

[1] Water and carbon dioxide ice clouds have been observed in the Martian atmosphere where they are dynamic parts of that planet's water and carbon cycles. Many Martian atmospheric models struggle to correctly predict clouds and, with insufficient data, some use untested simplifications that cloud formation occurs exactly at the saturation point of the condensed phase or at the same conditions as terrestrial cirrus clouds. To address the lack of data, we have utilized an 84 m 3 cloud chamber built for studies of high altitude cirrus and polar stratospheric ice clouds in the Earth's atmosphere and adapted to Martian conditions. Using this chamber, we have been able to produce water ice clouds from aerosol in an inert and low pressure atmosphere mimicking that of Mars. At temperatures between 189 and 215 K, we investigated cloud formation by mineral dust particulates of a similar composition and size to those found on Mars. We show that these surrogate materials nucleate effectively at the higher temperatures, with minor temperature dependence at saturations ratios with respect to the ice phase of~1.1, similar to what has been found for terrestrial cirrus. At the lower end of the temperature range, this saturation rises to~1.9, a result consistent with previous studies.

The immersion mode ice nucleation efficiency of clay minerals and biological aerosols has been investigated using the AIDA (Aerosol Interaction and Dynamics in the Atmosphere) cloud chamber. Both monodisperse and polydisperse populations of (1) various clay dust samples as well as (2) Snomax ® (a proxy for bacterial ice nucleators) and (3) hematite are examined in the temperature range between-4 ÛC and-35 ÛC. The temperature dependence of ice formation inferred by the INAS (Ice Nucleation Active Surface-Site) density is investigated and discussed as a function of cooling rate and by comparing to predicted nucleation rates (i.e., classical nucleation theory with ș-probability density function nucleation scheme). To date, we observe that maintaining constant AIDA temperature does not trigger any new ice formation during the immersion freezing experiments with clay dust samples and Snomax ® , implying strong temperature dependency (and weak time dependency) within our time scales and conditions of experiments. Ice residuals collected through a newly developed PCVI (Pumped Counter-flow Virtual Impactor) with the 50% cut size diameter of 10 to 20 ȝm have also been examined by electron microscope analyses to seek the chemical and physical identity of ice nuclei in clay minerals. In addition to the AIDA results, complementary measurements with mobile ice nucleation counters are also presented.

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