Importance of Inorganic and Organic Compounds in Desert Aerosol for Ice Nucleation

For the formation of ice crystals at temperatures warmer than -38 °C, ice nucleating particles (INPs) are required. Various particle types have been identified to be efficient INPs, amongst them certain bacteria and feldspar minerals. Mineral dust particles are amongst the most abundant INPs in the atmosphere. Their largest emission sources are deserts. The mineralogy of desert dust differs depending on the source region and can be further influenced by the dust emission processes. Mineralogy to a large extent explains the ice nucleation behavior of desert aerosol, but not entirely. Apart from pure mineral dust, desert aerosol particles often are internally or externally mixed with small amounts of biological material or particles exhibit a coating. Chemical and mechanical aging on the ground or during atmospheric transport can deactivate nucleation sites and strong ice nucleating minerals may not exhibit their full potential. Carboxylic acids, for example, are well known to hinder ice nucleation to a certain extent and other compounds may simply be able to shield, or react with the surface in a way that antagonizes the nucleation active sites.

In this study, we investigate how the ice nucleation behavior of desert aerosol is influenced by mineralogy, the presence of crystal water and heat labile organic compounds. We collected our samples from the surface of various deserts worldwide as well as directly from the air after atmospheric transport. With the Portable Ice Nucleation Chamber, PINC, we measured the ice active surface site density (n_s) in the deposition and condensation mode. Five samples were measured before and after heating them to 300 °C. In addition we used thermogravimetric analysis (TGA), X-ray diffraction (XRD), and Raman- and infrared- spectroscopy to search for the cause of observed differences in n_s. TGA, XRD and infrared spectroscopy were carried out on the bulk material and Raman spectroscopy was carried out on single particles by means of micro-Raman mapping of impacted dust.

Indications for heat labile organic compounds were found in two samples as well as indications for soot. While the n_s of one sample increased after heating, in a second sample the release of heat labile organic compounds led to suppression of the ice nucleation ability. In addition, the inorganic composition in the form of crystal water – and the subsequent change in mineralogical composition due to the loss of the crystal water – appears to play a role for one sample. Our study shows that in addition to mineralogy, other factors such as organics and crystal water can alter the ice nucleation behavior of desert aerosol during atmospheric transport in various ways.