The Ozone Layer

Ozone is generated in the stratosphere (12 - 50 km altitude) from molecular oxygen (O2) by UV photolysis.  This happens when UVC, the more energetic radiation below 280 nm in wavelength, breaks O2 into two oxygen atoms that then combine with other O2 molecules to form ozone (O3).  By this process solar UVC is entirely absorbed in the stratosphere, and does not reach earth's surface.  In its turn ozone absorbs UVB (280 - 315 nm) so that very little of that reaches the ground to cause sunburn, skin cancer, and other material damage, but also vitamin D formation and bacteriocidal benefits.  UVA (315 - > 400 nm) comes straight through.

Because O2 is uniformly available, the formation reaction by UVC is greatest where it is most intense, in the tropical stratosphere.  Ozone formed there flows outward toward the poles.  Ozone is destroyed in a variety of chemical reactions, being a strong oxidising agent, but though UVB absorption breaks up the O3 molecule that alone does not destroy it as the freed O is most likely to find another O2 first and reform O3.  In this sense O and O3 are equivalent, and simply referred to as free oxygen.  If free chlorine (Cl, ClO, OClO, etc.) is present it reacts with free oxygen to catalytically destroy ozone, whence the concern about CFCs.  Though entirely unreactive in the troposphere, they too are broken by UV photolysis to release free chlorine in the stratosphere.  The greatest damage requires a reaction surface, as provided by polar stratospheric clouds.  These occur over the Antarctic in winter, and sometimes over the Arctic, so major ozone destruction is confined to these areas.

Because of stratospheric circulation, the ozone holes are confined to the poles.  The much larger Antarctic ozone hole wobbles about as it rotates (West to East) and sometimes extends across the tip of South America, but when it is present (Southern Hemisphere springtime) there is an ozone ridge around it and New Zealand has its highest ozone column amounts.  When the ozone holes break up, ozone-depleted air is carried across mid-latitudes.  This happens in November or December over New Zealand, just when it doesn't need less ozone overhead.

Over 1 million tons of CFC were produced each year before they were banned.  About 1% annually were released into the atmosphere. 100,000 tons each year for about 25 years.  The inertness and lack of water solubility of CFCs mean they are not destroyed nor are they dissolved in rain water so they stay in the atmosphere for a very long time and diffuse up to the stratosphere. (55 years for CFC-11 and 140 years for CFC-12).  The dynamics of weather patterns also concentrate CFC's into the polar regions where ozone is thinnest.

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