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Ozone and its chemistry.

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The Ozone LayerThe ozone layer is not actually a layer but a zone withing the stratosphere where ozone is in higher concentrations compared to other regions of the atmosphere. The total amount of ozone is actually very small, but it serves an important purpose in regulating the amount of UVB that reaches the lithosphere. It is also important n stratospheric meteorology, where by the the process of absorbing UV radiation and the resulting heating effect, it determines the basic temperature structure of the region. Measurements of ozone are given in Dobson Units (DU) and 100 DU is equivalent to a 1mm thick layer of ozone. The average concentration of ozone at any point in the atmosphere is about 350 DU or 3.5mm. (Bonville, 1976)UV light has a wavelength between 10 and 400 nm. UV shorter than 180 nm does not reach the stratosphere, from 180-240 nm it is absorbed by O2 to create O3, from 240 to 290 nm is absorbed by O3 and does not reach the earths surface, 290-320 is partially absorbs by ozone and 320-400nm is almost entirely radiated to the ground.Ozone is created naturally in the stratosphere at an elevation of between 25 and 35Kms by the process of photo disacssociation , a process where high energy ultra violet light strikes molecular oxygen breaking it apart into 2 oxygen atoms. Being a highly reactive species, atomic oxygen will quickly react with an oxygen molecule to form ozone. The formation and destruction of ozone is a soley photochemical reaction involving the oxygen atom. This can be clearly seen in the Chapman Reaction Series. (World Meteorological Organization, 1976)Photo disaluson of molecular oxygen (O2) by ultraviolet light (hv). O2 + hv → O + O O + O2 + M → O3 + MIn the second part of the reaction M is given by any body typically N2 or H2. From here ozone is able to perform its function of limiting the input of UV into the lithosphere. O3 + hv → 02 + OWhere O is an oxygen atom that has been electronically excited to a higher state. This freed oxygen can then react to destroy ozone. This reaction accounts for 20% of all ozone destruction. 03 + O → 2O2The Chapman Series does not fully explain all the processes taking place and other gases are playing a roll in the regulation of ozone. These include Nox, H2O, CH4, Cl and Br, and under normal circumstances the creation and destruction of ozone and the formation of oxides are in a state of equilibrium. The destruction of ozone by CFC's and other man made chemicals, will if left unabated have a huge impact on life at the surface. Even small increases of UVB will have wide reaching consequences that will lead to an increase in skin cancers, a reduction in crop and timber yields, an increase in carcinoma in cattle and a decrease in plankton productivity which could lead to an entire collapse of marine systems. (World Meteorological Organization, 1976)Bonville. B.W., (1976), Environmental Aspects of Stratospheric Ozone, Environmental Sciences and Applications v4. p.1-19.Christopherson. R.W., (2009), Geosystems, Pearson Education Inc. p.507-521.World Meteorological Organization., (1976), Atmosphere Ozone – A Survey of the Current State of Knowledge of the Ozone Layer, Environmental Sciences and Applications v4. p.75-139.

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