As sunlight penetrates into the stratosphere, high-energy UV photons react with oxygen gas molecules, splitting them into individual oxygen atoms. These highly reactive oxygen atoms are examples of free radicals; they quickly enter into chemical reactions that allow them to attain stable arrangements of electrons. In the stratosphere free radicals can combine with oxygen molecules to form ozone. A third molecule, typically nitrogen gas or atmospheric oxygen (represented by M in the equation), carries away excess energy from the reaction but remains unchanged.
Each ozone molecule formed in the stratosphere can absorb a UV photon with a wavelength of less than 320nm. This energy absorption prevents potentially harmful UV rays from reaching the earth’s surface. The energy also causes the ozone to decomposed, producing an oxygen molecule and an oxygen free radical. These products can then carry on the cycle by replacing ozone in the protective stratospheric layer.
CFC’s (chlorofluorocarbons) are highly stable molecules in the troposphere, however, high energy UV photons in the stratosphere split chlorine radicals from CFC’s by breaking their C-Cl bond. The freed chlorine radicals are very reactive and can participate in a series of reaction that destroy ozone by converting it to diatomic oxygen. Every chlorine radical that participates in the first reaction can later be regenerated . Thus each chlorine radical acts as a catalyst participating in not just one, but an average of 100,000 ozone –destroying reactions. In doing so, it speeds up ozone destruction but remains unchanged itself.
