CCN and INP activity of middle eastern soil dust

Abstract

The term mineral dust encompasses a myriad of particle compositions from both fertile and arid regions. Due to this diversity, the quantitative understanding of mineral dust as Cloud Condensation Nuclei (CCN) and Ice Nucleating Particles (INPs) in the Earth’s atmosphere demand further investigation. This study characterizes the CCN and INP activity of mineral dust particles from samples collected from one of the Earth’s major arid regions, Saudi Arabia. Samples were size selected at particle diameters (Dp) of 300, 700, and 950 nm and introduced into a Cloud Condensation Nuclei Counter (CCNC) and a SPectrometer for Ice Nuclei (SPIN) chamber to investigate cloud nucleation activity. The chemical composition of the particles was analyzed with laser mass spectrometry and mineralogical information was provided by polarized light microscopy. Transmission electron microscopy was used to ascertain particle morphology. Each particle size was exposed to water supersaturations of 0.06–1.0% in the CCNC and to ice supersaturation ratios of 1.1 to 1.5 at temperatures from −25 to −42 °C in SPIN. The CCN activity ranged from hygroscopicity values (κ) of 0.001 to 0.01. This is towards the lower range of critical supersaturations found for other mineral dust samples from e.g. the Sahara, North Africa, China and Asia. The INP activity, defined by fractional activation and supersaturation at the onset of ice nucleation was in the range of other natural mineral dusts (e.g., the Sahara, Canary Islands), and somewhat lower than industrially processed Arizona Test Dust. This study highlights the importance of considering size-resolved compositional data when interpreting the cloud-nucleation activity of natural mineral and soil dusts.