Newsportal - Ruhr-Universität Bochum
How ice particles promote the formation of radicals
The production of chlorofluorocarbons, which damage the ozone layer, has been banned as far as possible. However, other substances can also tear holes in the ozone layer in combination with ice particles, such as those found in clouds. Researchers at Ruhr-Universität Bochum, the University of Duisburg-Essen and Friedrich-Alexander-Universität Erlangen-Nürnberg have discovered a possible mechanism for this. They describe it in the journal Physical Review Letters on 13 November 2018.
The work was part of a long-standing cooperation between the teams from Bochum, Duisburg-Essen, and Erlangen-Nuremberg led by Professor Karina Morgenstern, Dr. Cord Bertram, Professor Uwe Bovensiepen and Professor Michel Bockstedte, which is currently being continued within the framework of the cluster of excellence Ruhr Explores Solvation, or Resolv for short.
Organic molecules are deposited on ice particles
Chemical processes can significantly influence the weather, the climate and the composition of the atmosphere. Cosmic rays or UV light provide the energy to split chemical compounds. In the case of bromine, chlorine or fluorine compounds, radicals, i.e. particularly reactive molecules, are formed. These attack the ozone molecules and can trigger chain reactions in the ozone layer. An earlier laboratory study had shown that ice particles with a silver core can promote such reactions. The team investigated the mechanism behind this effect in the current study.
In the laboratory, the scientists produced tiny ice particles and analysed how certain compounds containing chlorine or bromine interacted with them. They condensed the ice particles onto copper. In nature, mineral dust particles, among other things, form condensation nuclei for the ice particles.
Using microscopic and spectroscopic methods, they observed that the molecules preferentially attached themselves to defects in the ice structure. The surrounding water molecules of the ice structure then reoriented themselves and hydrogenated the molecules. This, in turn, made it easier to ionise the molecules in the experiment.
UV radiation generates radicals
The researchers irradiated the ice crystals with the attached molecules using UV light, which excited electrons in the ice particles in the vicinity of the molecules. These excited electrons ionised the chlorine and bromobenzene molecules. Through ionisation, the molecules disintegrated into organic residues and highly reactive chlorine and bromine radicals.
“The mechanism could explain what happens when UV light hits mineral-contaminated ice,” says Cord Bertram. “Our results could thus help to understand the fundamental processes behind phenomena such as ozone holes.”
Since 2007, the three Ruhr universities have been working strategically together under the umbrella of the University Alliance Ruhr (UA Ruhr). By joining forces, the services of the partner universities are systematically expanded. Under the motto “better together”, there are now more than 100 collaborations in research, teaching and administration. With more than 120,000 students and almost 1,300 professors, the UA Ruhr is one of Germany’s largest and most efficient science locations.
The work was supported by the German Research Foundation as part of the cluster of excellence Resolv (EXC1069), and the projects BO1841/3-1, BO1823/5-1 and MO960/18-1. Computer simulations were carried out at Research Center Jülich and at the HPC cluster at the regional computer centre Erlangen at Friedrich-Alexander-Universität Erlangen-Nürnberg.
Philipp Auburger, Ishita Kemeny, Cord Bertram, Manuel Ligges, Michel Bockstedte, Uwe Bovensiepen, Karina Morgenstern: Microscopic insight into electron-induced dissociation of aromatic molecules on ice, in: Physical Review Letters, 2018, DOI: 10.1103/PhysRevLett.121.206001
Prof. Dr. Karina Morgenstern
Department of Physical Chemistry I
Faculty of Chemistry and Biochemistry
Phone: +49 234 32 25283
Dr. Cord Bertram
Faculty of Physics
University of Duisburg-Essen
Phone: +49 203 379 4568
7 December 2018