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 −20 • C and −55 • C. 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 re-spect to ice (RH i) 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 re-quired the lowest RH i to activate. Deposition nucleation be-low water saturation was found only below −30 • C or −35 • C dependent on particle size. Minimum RH i for 1% activa-tion were 105% for illite, kaolinite and montmorillonite at −40 • C, respectively 110% for ATD at −45 • C. In addition, a possible parameterisation for the measured activation spectra is proposed, which could be used in modeling studies.