Influence of particle size on the ice nucleating ability of mineral dusts

The recently developed Zurich Ice Nucleation Chamber (ZINC) was used to explore ice nucleation of size-selected mineral dust particles at temperatures between −20C and −55C. Four different mineral dust species have been tested: montmorillonite, kaolinite, illite and Arizona test dust (ATD). The selected particle diameters are 100 nm, 200 nm, 400 nm and 800 nm. Relative humidities with respect to ice (RHi) required to activate 1% of the dust particles as ice nuclei (IN) are reported as a function of temperature. An explicit size dependence of the ice formation efficiency has been observed for all dust types. 800 nm particles required the lowest RHi to activate. Deposition nucleation below water saturation was found only below −30C or −35C dependent on particle size. Minimum RHi for 1% activation were 105% for illite, kaolinite and montmorillonite at −40C, respectively 110% for ATD at −45C. In addition, a possible parameterisation for the measured activation spectra is proposed, which could be used in modeling studies.