German scientists have discovered a solution to a 20 year old mystery in atmospheric physics - why does atmospheric ozone contain so many 'heavy' isotopes of oxygen. Ozone molecules contain three oxygen atoms. Some ozone molecules are asymmetric and contain different isotopes of oxygen. Others are symmetric and contain three atoms of the same isotope. A large fraction of ozone molecules in the atmosphere contain isotopes of oxygen-17 and oxygen-18, as opposed to 'light' oxygen 16.
In theory, the creation of ozone – caused by the breakdown of oxygen in the atmosphere by ultraviolet light – is a mass-independent process. However, experiments by Klaus Mauersberger and colleagues at the Max-Planck institute for nuclear physics in Heidelberg indicate that the collisions between oxygen atoms and molecules that form ozone are not mass-independent (Science 283 370).
The German researchers studied the six different atom-molecule reactions that lead to the formation of ozone. By creating ozone in a series of different gas mixtures, each containing different amounts of the three oxygen isotopes, the German group was able to study the reaction rates for each isotope. It found that the reaction rate for creating heavy molecules of ozone was up to 50% higher than that for creating lighter versions.
