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| CORPORATE |  The Ozone Layer and Ozone Depletion A layer of ozone in the upper atmosphere filters out ultraviolet radiation from the sun before it reaches the earth's surface, preventing serious harm to humans, animals and plants. Certain man-made chemicals - mainly chlorofluorocarbons (CFCs) and halons, have ozone depletion potentials and are being phased out under the Montreal Protocol. Methyl bromide, a gas used to disinfect soil and agricultural products, also has ozone depleting potential. Ozone is created by the action of sunlight on oxygen high in the stratosphere where the air pressure is very low and sunlight very strong. Lower down in the stratosphere, ozone is naturally destroyed in reactions with other atmospheric gases and the ozone layer is a consequence of the creation and destruction processes. About 300,000,000 tonnes per day of ozone are involved in this cycle.1 Formation of ozone is controlled by sunlight and is not affected by any changes coming from the earth. This means that most ozone is formed over the equator and almost none over the Poles. The ozone in the stratosphere at the Poles was transported there on stratospheric winds. Ozone depletion occurs if the processes that destroy ozone are augmented, so that the balance between its formation and removal is disturbed. This makes the Polar stratosphere particularly vulnerable (because ozone is not formed there). A large number of chemical species are involved in the natural ozone removal processes: water, hydrogen peroxide, hypochlorous acid, nitric acid, methane, carbon monoxide, nitric oxide, nitrogen dioxide, nitrous oxide, chlorine, chlorine monoxide, hydrochloric acid and bromine monoxide all play a part. The removal process is also augmented by reactions that occur on the surface of particles; in turn stratospheric temperatures influence these. Because it is a consequence of an upset to the balance between formation and removal, ozone depletion is temporary. In the short-term, the lower ozone levels that result are regional and seasonal (like the Antarctic ozone hole) and, in the long-term, ozone depletion should cease when the balance is restored. Concerns that the earth's stratospheric ozone layer could be at risk from man-made substances were first raised during the early-to-mid-1970s. In the mid-1980s the Antarctic 'ozone hole' was discovered. In March 1985 the Vienna Convention for the Protection of the Ozone Layer was adopted. The Convention established the need to co-operate on monitoring, research and data exchanges. 1. Chapman S., A theory of upper atmospheric ozone, Mem. Roy. Met. Soc, 3, 103-125, 1930. |