Characterisation of ice particle residuals and aerosol particles in laboratory and field experiments by scanning electron microscopy during INUIT (Ice Nuclei research UnIT)

Ice nucleating particles are necessary for formation of ice crystals in mixed-phase clouds, but there are still uncertainties regarding physico-chemical properties, variability and concentration of these particles in the environment. The ice nuclei research unit (INUIT), funded by the German research foundation (DFG), has thoroughly investigated ice nucleation in laboratory, model and field studies. This thesis contains research from a field study (Jungfraujoch, Switzerland) and a laboratory experiment (of coal fly ash) as well as a methodological part to study the performance of single particle techniques applied. Scanning electron microscopy (SEM) with energy dispersive X-ray microanalysis (EDX) was used as main technique in this thesis. The capability of SEM-EDX in ice nucleation research is demonstrated in the following chapters and in the corresponding papers to this thesis. Single particle analysis by SEM-EDX can be performed operator controlled (opSEM) or computer controlled (ccSEM). In this thesis, it is shown that the technique of choice is dependent on the research question and the particle composition. A comparison of the techniques for different types of aerosol samples provided valuable information about the differences and was further used as a guide to the technique of choice in the remaining studies. Ice particle residuals were studied in mixed-phase clouds at Jungfraujoch in February 2017. The results show that different silicate groups were the most important ice nucleating particles, consistent with previous literature. However, in our study, soot and complex secondary particles were not found in the ice particle residual fraction which is inconsistent with previous results. Furthermore, an upgraded sampling procedure gave the opportunity to determine enrichment and depletion of particle groups in the ice particle residual fraction relative to total aerosol, as well as to identify sampling artefacts. The ice nucleation properties of coal fly ash particles were investigated in a laboratory study. In this study, SEM-EDX was applied to characterise the fly ash particles before and after suspension in water. Anhydrite and CaO, in one of the fly ash samples, reacted to gypsum, calcium hydroxide and calcite needles after suspension. This change led to an overestimation of the ice nucleation active surface site density, which was later corrected for because of the morphological information from SEM. Last, a newly developed method for phase determination by single particle mass spectrometry (SP-MS) was tested and compared to SEM-EDX. Both techniques show the same differences in chemical composition of mineral dust from North-Africa. SP-MS could characterise the phase composition of the particles. Still, SP-MS was limited by matrix effects which was seen from a higher abundance of Ca- and Mg-rich silicates obtained by SEM-EDX. A better understanding of these matrix effects is needed to improve the application of the former method to ice particle research