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Regner in red. philip madsen wrote: Ozone, O3, has an absorption peak at 254nm of 1e-17/[1/cm^2] with a width of 40nm (see, e.g., Parisse, Brion & Malicet, 1996, Chem. Phys. Letters, 248, 31-36). It is actually quite close to the Gaussian: 1.14*exp(-((lambda-254.)/24.)^2). This is a photo-detachment continuum, meaning that the molecule dissociates into an O2 molecule and a free oxygen atom. These can combine with others to form O3 again. O2, on the other hand, has a 3e-17/[1/cm^2] peak around 60nm and a 1.5e-17/[1/cm^2] peak around 140nm - same magnitude as for O3 but at much short wavelength in the vacuum UV (VUV). There are hardly any Solar photons there. There is also a 7e-24/[1/cm^2] peak at 200nm - this is a photo-detachment continuum causing dissociation into two free oxygen atoms when absorbing this light. These free atoms join with O2 molecules to form new O3 molecules (as opposed to the recycling described above). Due to the very low cross-section and the fewer Solar photons available at such short wavelengths, this is a fairly slow process. Note that this O2 absorption is almost 7 orders of magnitude weaker than the absorption by O3. Most of O2's UV absorption happens in the UV-C band and beyond, where there are hardly any Solar photons. Most of the harmful UV photons from the Sun fall in the UV-B band which is completely covered by the O3 absorption. The less harm- ful UV-A band is only partially covered by O3. See, e.g., Solar Radiation Spectrum from Wikipedia Source of absorption spectra: MPI-Mainz-UV-VIS Spectral Atlas of Gaseous Molecules Ozone is easily destroyed by the radicals NO, OH, Cl and Br which acts as catalysts (are not consumed in the process) and each radical can destroy more than 100,000 O3 molecules. The action of these free radicals and the formation of new O3 in the presence of sunlight, obviously results in a yearly cyclic variation in O3 concentrations in the Polar regions - that is a well known and natural cycle. What has changed, however, is the concentrations of Cl and Br due to human activity. These increases have caused a decline in the minimum (winter) concentrations of O3 in the Polar regions. These are the much cited ozone-holes. The depletion of the O3 layer seems to have slowed down, lately, and that is attributed to the international ban on CFCs. - Regner
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