Light from the particle accelerator helps to understand ozone decomposition

In the experimental chamber, a very thin vertical jet of water can be seen, which flows downward in the middle of the picture from a small tube. During the experiment, the chamber contains a gas mixture including ozone, which reacts on the surface with bromide in the water and produces bromine. As an intermediate step in the process, a short-lived compound of bromide and ozone is made, which was detected for the first time ever with the help of X-ray light from SLS. For this proof, the X-ray light knocked electrons out of the compound, and these made their way to the detector through an opening in the cone (to the left in this photo). (Photo: Paul Scherrer Institute/Mahir Dzambegovic)

A new experimental chamber coupled to the Swiss Light Source (SLS), a large-scale research facility of the Paul Scherrer Institute PSI, allows researchers to recreate atmospheric processes in the laboratory through unprecedented precision analysis involving X-rays. In their first experiments, researchers detailed how bromine molecules are formed in the air. These play an essential role in the decomposition of ozone in the lower layers of the atmosphere. With their results, the researchers have also made an important contribution to models designed to explain and predict changes in climate and air composition. In the future, the experimental setup will be available to researchers in all scientific fields and those particularly concerned with the chemistry of the atmosphere or other topics in energy and environmental research.

Oxidation of bromide in aqueous environments initiates the formation of molecular halogen compounds, which is important for the global tropospheric ozone budget. In the aqueous bulk, oxidation of bromide by ozone involves a [Br•OOO−] complex as intermediate. Here we report liquid jet X-ray photoelectron spectroscopy measurements that provide direct experimental evidence for the ozonide and establish its propensity for the solution - vapour interface. Theoretical calculations support these findings, showing that water stabilizes the ozonide and lowers the energy of the transition state at neutral pH. Kinetic experiments confirm the dominance of the heterogeneous oxidation route established by this precursor at low, atmospherically relevant ozone concentrations. Taken together, our results provide a strong case of different reaction kinetics and mechanisms of reactions occurring at the aqueous phase - vapour interface compared with the bulk aqueous phase.