⇒ Our Session in the EGU General Assembly 2018 (EGU 2018): AS3.2 - Atmospheric Surface Science Link
⇒ Open PhD position in Atmospheric surface spectroscopy Link
Atmospheric Surface Science is a new research line has been set out in IMKAAF after three successful grants:
- Start-Up-Budget (STUB 2012) in 2012 from the competence field Earth and Environment, KIT for “building a supercooled SHG system and performing a feasibility study on water/ice-nucleation-agent interface”.
- Research grant (DFG, AB 604/1-1) in 2014 from the German Research Foundation (DFG) to study “Elementary processes of heterogeneous ice nucleation observed by nonlinear optical spectroscopy”.
- Research grant (DFG, AB 604/1-2) in 2017 from DFG to proceed with advanced studies on “Elementary processes of heterogeneous ice nucleation observed by nonlinear optical spectroscopy: The role of hydroxyl groups on the surfaces of mineral aerosol particles”
The aim of this research is to investigate atmospheric interactions on the molecular level.
Studying the ice nucleation mechanism, particularly heterogeneous IN, is important for understanding the criteria of formation of different types of clouds, which in turn has its influence on our climate system. Despite all investigations on particle size and surface properties of IN agents upon nucleation efficiency, no clear statement is given about ice nucleation process. Technical and environmental issues require a good understanding of ice freezing on the surfaces of ice-nucleating agents, particularly on the molecular level.
On a molecular level, ice nucleation by a surface remains poorly understood. The microscopic mechanisms of heterogeneous ice nucleation and the characteristics that make some surfaces more effective for ice nucleation are still unclear. It is believed that good ice-nucleating surfaces are those which can reduce the free energy barrier for nucleation and/or template water in ice-like structure. However, some experiments showed that it is not always the case.
The scientific goal is to draw a sharp line between water and ice molecular properties during homogeneous and heterogeneous ice nucleation processes. Crystalline ice has a low density of OH-groups whereas amorphous ice or water has abundant dangling OH groups which can be characterized spectroscopically. Questions to be answered: What type of bonding exists between different OH groups and ice-nucleating agents? Why are some ice-nucleating agents more effective at higher temperatures than others? To which extent do dangling OH-groups contribute to IN process? This requires information about the abundance and degree of order of water molecules at the water and ice surfaces at different supersaturation conditions. A complete picture about the world wide puzzling IN process requires an access to a wide range of molecular vibrations. Probing the individual vibrational bands of water-like and ice-like vibrations as well as the molecular vibrations of ice-nucleating agents is a challenge. In a future step, this setup will be converted to TR-SFG to describe this system physically and chemically simultaneously.
Research and methodology
Bulk techniques, like FTIR, cannot recognize the contribution of the surface molecules due to their infinitesimal contribution to the overall detectable signal. However, second order nonlinear optical techniques can do. Second Harmonic Generation (SHG), as well as Sum Frequency Generation (SFG), is highly sensitive optical probe of surfaces and interfaces. A brand new Femtosecond laser system (Solstice, 800nm, 3.5mJ, 80fs) is available at the IMK-AAF and is utilized in this work. A supercooled SHG setup is build in total-internal-reflection (TIR) geometry to probe the abundance and degree of order of water molecules on the surface of ice-nucleating agents before, during and after the phase change. A precise temperature-controlled bidirectional optically-accessible cold stage is coupled to the system for this purpose. A complete picture about the world wide puzzling ice nucleation processes requires an access to a wide range of molecular vibrations. Probing the individual vibrational bands of water-like and ice-like vibrations as well as the molecular vibrations of ice-nucleating agents is a challenge. In a future step, the supercooled SHG setup will be upgraded to supercooled SFG to describe this system physically and chemically simultaneously.
For further information please contact Dr. Ahmed Abdelmonem
Abdelmonem, A.: Direct molecular-level characterization of different heterogeneous freezing modes on mica – Part 1, Atmos. Chem. Phys., 17, 10733-10741, doi: 10.5194/acp-17-10733-2017, 2017.
Abdelmonem, A., Backus, E. H. G., Hoffmann, N., Sánchez, M. A., Cyran, J. D., Kiselev, A., and Bonn, M.: Surface-charge-induced orientation of interfacial water suppresses heterogeneous ice nucleation on α-alumina (0001), Atmos. Chem. Phys., 17, 7827-7837, doi: 10.5194/acp-17-7827-2017, 2017.
Preocanin, T., Abdelmonem, A., Montavon, G., and Luetzenkirchen, J.: Charging Behavior of Clays and Clay Minerals in Aqueous Electrolyte Solutions — Experimental Methods for Measuring the Charge and Interpreting the Results, in: Clays, Clay Minerals and Ceramic Materials Based on Clay Minerals, edited by: Nascimento, G. M. D., InTech, 51-88, 2016.
Abdelmonem, A., Lützenkirchen, J., and Leisner, T.: Probing ice-nucleation processes on the molecular level using second harmonic generation spectroscopy, Atmos. Meas. Tech., 8, 3519-3526, doi: 10.5194/amt-8-3519-2015, 2015.