Ozone (O₃) is produced naturally by the action of ultraviolet radiation on O₂ in the upper atmosphere states.
O₂ →hv O+ O
O+ O₂ →O₃
When ozone absorbs an ultraviolet photons, it's dissociates:
O₃ →hv O₂ + O
The resulting O atom can then the removal ozone in the reaction
O₃ + O → O₂ + O₂
This reaction is constitute the principal steps of the oxygen - ozone cycles that maintains an equilibrium concentration of ozone. If all the atmospheric O₃ molecule were condensed into a single layers at 1 atmosphere and 25⁰C, it would be cover the Earth to a depth of about 3 millimetres.
The stratosphere also contains naturally occurred species, such as the hydroxyl radicals and nitric oxide, that catalyse the destruction of ozone by reaction such as :-
X+ O₃ → XO + O₂
XO + O → X+ O₂
However, the main concerns about the loss of ozone centres on Cl and br atoms introduced artificially by industrial activities, which catalyse O₃ destruction very efficiently. Cl and Br carried into the stratosphere as the part of organohalogen molecules. RHal, which release the halogen atoms when the C- Hal bond is fragmenting by far - UV photons. The ozone - destroying potential of this molecule was pointed out in 1974 by Mario Molina and sherwood Rowland, whose won won the 1995 Noble prize in chemistry and together with Paul Crutzen for their work.
International action followed 13 years later in the form of the Montreal Protocol (1987) and given added impetus by the discovery of the "ozone hole" over Antarctica which is provided and dramatic evidence of the vulnerability of atmospheric ozone. This hole surprising even the scientists working on the problem. It's explanation required to additional chemistry, involving the polar stratospheric clouds that form in water. The ice crystals in this clouds adsorb molecule of the chlorine or bromine nitrate, ClONO₂ or BrONO₂, which form when stratospheric ClO or BrO combine with NO₂ once on the ice surface, this molecules react with water.
H₂O + XONO₂ → HOX + HNO₃
Where X = Cl or Br. They also react with co-adsorbed HCl or HBr formed by attacking of Cl⁻ or Br⁻ on the methane escaping from the troposphere.
HX + XONO₂ → X₂ + HNO₃
The niric acid, being as a hygroscopic, enters the ice crystals and the X₂ or XOX molecules are released during the dark polar winter. When the sunlight strengthens in the spring, these molecule photolyse, releasing at higher concentration of ozone - destroying radicals.
HOX hv → HO + X
X₂ hv → 2X.
To threaten the ozone layers, organohalogen molecules must survive their migration from the Earth's surface. Those contained H atoms are mostly broken down in the troposphere the lowest region of the atmosphere by reaction with HO radicals. Even so they may be problem if released in sufficient amounts. There's currently a major controversy over the use of bromoethane, CH₃Br, as agricultural fumigant. However the greatest potential for ozone destruction rest with molecules that lack of Hydrogen atom, the chlorofluorocarbons (CFCs) which is used in many industrial applications, and their brominated analogues. Which are used to extinguish fires. This compounds have no tropospheric sink and eventually reach the stratosphere unaltered. They are the mainly focus of the international regulatory regime worked out in 1987 and amended in 1990 and 1992. Most of the CFCs and halons had been phased out of production, and their atmospheric concentration are beginning to decline. The CFCs presented an additional problem as they are also potent greenhouse gases.
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