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Direkte Beobachtung von Elementarprozessen bei der heterogenen Eis- Nukleation durch nichtlineare optische Spektroskopie: Die Rolle von Hydroxyl-Gruppen an den Oberflächen von mineralischen Aerosolpartikeln

Antragsteller Ahmed Abdelmonem, Ph.D.
Fachliche Zuordnung Physik und Chemie der Atmosphäre
Physikalische Chemie von Molekülen, Flüssigkeiten und Grenzflächen, Biophysikalische Chemie
Förderung Förderung von 2014 bis 2021
Projektkennung Deutsche Forschungsgemeinschaft (DFG) - Projektnummer 261509307
 
Erstellungsjahr 2021

Zusammenfassung der Projektergebnisse

Water- and ice-mineral interactions play vital roles in the atmosphere. The chemical and morphological properties of aerosol surfaces play direct and indirect roles in the climate system. Understanding the role of mineral surface properties and their potential variations under atmospheric conditions remains challenging. Six decades of atmospheric research on ice nucleation have led to significant knowledge about the ice nucleation abilities of different aerosol particles. However, we still struggle to explain in a fundamental way why one aerosol nucleates ice more effectively than another, motivating my first DFG proposal in 2014. I realized that there is a pressing need to understand atmospheric processes in general and ice nucleation in particular on the molecular level. I have then decided to employ my dual knowledge in atmospheric science (from the postdoc period) and non-linear optical (NLO) spectroscopy (from my PhD period) to fill the gap between ice nucleation ability and the molecular level understanding of the process. I proposed, in 2014, to build and use supercooled second-harmonic generation (SHG) and sum-frequency generation (SFG) setups to probe the molecular arrangement of water and ice on the surface of relevant atmospheric minerals before, during and after the ice nucleation process. SHG and SFG are powerful tools to investigate molecular interactions at surfaces and interfaces. Indeed, the setups have been built and immersion and deposition freezing next to sapphire and mica basal planes as two analogue surfaces for mineral dusts were investigated using supercooled. These studies indicated the importance of further understanding the surface chemistry of such mineral aerosols and the rule of hydroxyl groups in water freezing next to mineral oxides. These two aspects were tackled afterwards within the second phase of the project. I proposed an experimental plan that aims to reproduce heterogeneous ice nucleation on the surface of common atmospheric ice nucleating substances (e.g. silica and aluminum oxide different surface cuts). The aim was to observe the freezing process on the molecular level under different environmental conditions (temperature, acidity and basicity, salt content, and aging). The work plan had two approaches, which were complementary: 1) SHG experiments, which relate to the overall arrangements of the water molecules at the interface and provide insight into the interfacial molecular properties at the air, water, and ice / ice-nucleating agent interface; and 2) SFG experiments, which additionally deliver chemical information about different species at the interface, particularly the OH vibrations of the surface hydroxyl groups. Other surface techniques (e.g. streaming potential, optical microscopy, XPS, and SEM) were additionally employed to characterize the surface properties. In a series of temperature-dependent SHG measurements on sapphire and mica, in immersion freezing mode, I observed two distinctly different pathways for the evolution of water structuring on the surfaces of sapphire 0001 and mica. The effect of aging of mica, under atmospheric conditions, on its ice nucleating properties was then discussed. Afterwards, I studied the effect of surface charge and dissolved ions on the structuring of water on the surface of mineral oxide (a-alumina) and hence the ice nucleation efficiency of the surface using SFG spectroscopy. Deposition freezing on mica and a-alumina was then investigated using supercooled SHG. A transient signal has been observed at the phase change, and fully discussed in two papers. Several mineral surfaces, including mica and sapphire, were investigated. In parallel, I have built a novel set-up for simultaneous measurement of SHG and streaming potential to correlate the spectroscopic and electrochemistry data. A detailed SHG study on cloud history effect on water-ice-surface interactions of oxide mineral aerosols (e.g. Silica) has been done. I showed and explained how a mineral aerosol particle may change its ice nucleation properties during its residence in the cloud. I have reported an unexpected behavior of sodium sulfate observed in immersion freezing and correlated that to similar behavior in corrosion studies. Finally, I carried out a direct molecular level comparison between immersion and deposition freezing on the sapphire (1120) surface using SFG spectroscopy. I showed that deposition freezing leads to a highly coordinated water structure compared to immersion freezing. The two phases of the project led to ten peer reviewed relevant publications in high rank scientific journals and fourteen international conference contributions. I have successfully set out a new research line, Atmospheric Surface-Science (A.S.S.) at KIT.

Projektbezogene Publikationen (Auswahl)

 
 

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