Heterogeneous ice nucleation on dust particles sourced from nine deserts worldwide – Part 2: Deposition nucleation and condensation freezing

摘要

Mineral dust particles from deserts are amongst the most commonice nucleating particles in the atmosphere. The mineralogy of desert dustdiffers depending on the source region and can further fractionate during thedust emission processes. Mineralogy to a large extent explains the icenucleation behavior of desert aerosol, but not entirely. Apart from puremineral dust, desert aerosol particles often exhibit a coating or are mixed with small amounts ofbiological material. Aging on the ground orduring atmospheric transport can deactivate nucleation sites, thus strongice nucleating minerals may not exhibit their full potential. In the partnerpaper of this work, it was shown that mineralogy determines most but notall of the ice nucleation behavior in the immersion mode found for desert dust.In this study, the influence of semi-volatile organic compounds and thepresence of crystal water on the ice nucleation behavior of desert aerosol isinvestigated. This work focuses on the deposition and condensation icenucleation modes at temperatures between 238 and 242 K of 18 dust samplessourced from nine deserts worldwide. Chemical imaging of the particles' surfaceis used to determine the cause of the observed differences in ice nucleation.It is found that, while the ice nucleation ability of the majority of the dustsamples is dominated by their quartz and feldspar content, in onecarbonaceous sample it is mostly caused by organic matter, potentiallycellulose and/or proteins. In contrast, the ice nucleation ability ofan airborne Saharan sample is found to be diminished, likely by semi-volatilespecies covering ice nucleation active sites of the minerals. This studyshows that in addition to mineralogy, other factors such as organics andcrystal water content can alter the ice nucleation behavior of desert aerosolduring atmospheric transport in various ways.