Ice nucleation properties of mineral dust particles: determination of onset RHi, IN active fraction, nucleation time-lag, and the effect of active sites on contact angles

摘要

A newly developed ice nucleation experimental set up was used to investigatethe heterogeneous ice nucleation properties of three Saharan and one Spanishdust particle samples. It was observed that the spread in the onset relativehumidities with respect to ice (RH) for Saharan dust particles variedfrom 104% to 110%, whereas for the Spanish dust from 106% to110%. The elemental composition analysis shows a prominent Ca feature inthe Spanish dust sample which could potentially explain the differences innucleation threshold. Although the spread in the onset RH for thethree Saharan dust samples were in agreement, the active fractions andnucleation time-lags calculated at various temperature and RHconditions were found to differ. This could be due to the subtle variationin the elemental composition of the dust samples, and surface irregularitieslike steps, cracks, cavities etc. A combination of classical nucleationtheory and active site theory is used to understand the importance of thesesurface irregularities on the nucleability parameter, contact angle that iswidely used in ice cloud modeling. These calculations show that the surfaceirregularities can reduce the contact angle by approximately 10 degrees.