Irrigation.ap.gov.in

depletion of the ozone layer
Overview of the ozone cycle
The cycle of ozone
Three forms (or allotropes) of oxygen are involved in the cycle of ozone and oxygen: oxygen atoms (O or atomic oxygen), oxygen gas (O2 or diatomic oxygen ), and ozone (O3 or triatomic oxygen). Ozone is formed in the stratosphere when oxygen molecules photodissociate after absorbing an ultraviolet photon whose wavelength is shorter than 240 nm. This produces two oxygen atoms. The atomic oxygen then combines with O2 to create O3. Ozone molecules absorb UV light between 310 and 200 nm, after which the ozone layer breaks down into a molecule of O2 and an oxygen atom. The oxygen atom then joins with an oxygen molecule to regenerate ozone. This is an ongoing process that ends when an oxygen atom "recombines" with an ozone molecule to make two molecules of O2: O3 + O 2 O2
Global monthly average total ozone.
Layers of the atmosphere (not to scale)
The total amount of ozone in the stratosphere is determined by a balance between photochemical production and recombination.
Ozone can be destroyed by a number of free radical catalysts, the most important are the hydroxyl radical (OH), nitric oxide radical (NO), atomic chlorine (Cl) and bromine (Br). All these sources are both natural and man, at present, most of OH and NO in the stratosphere is of natural origin, but human activity has significantly increased the levels of chlorine and bromine. These elements are found in certain stable organic compounds, particularly chlorofluorocarbons (CFCs), which can find their way into the stratosphere without being destroyed in the troposphere due to their low reactivity. Once in the stratosphere, Cl and Br atoms are released from parent compounds by the action of ultraviolet light, for example ("H" is Planck's constant,''is the frequency of electromagnetic radiation)
+ H CFCl3 CFCl2 + Cl
Cl and Br atoms can then destroy ozone molecules through a variety of catalytic cycles. In the simplest example of such a cycle, a chlorine atom reacts with an ozone molecule, taking an oxygen atom with it (training ClO) and leaving a normal oxygen molecule. The chlorine monoxide (eg, ClO) can react with a second molecule of ozone (ie O3) to give another chlorine atom and two oxygen molecules. The shortcut of chemical reactions in the gas phase is as follows:
Cl + O3 ClO + O2
Cl + O3 ClO + 2 O2
The overall effect is a reduction in the amount of ozone. More complex mechanisms have been discovered leading to the destruction of ozone in the lower stratosphere as well.
A single chlorine atom would continue to destroy the ozone layer (and thus a catalyst) to a maximum of two years (the time scale for transport to the lower troposphere), they are not reactions remove this cycle by forming reservoir species such as hydrogen chloride (HCl) and chlorine nitrate (ClONO2). On a per atom basis, bromine is more effective than chlorine at destroying ozone, but there is much less bromine in the atmosphere at the moment. As a result, chlorine and bromine contribute significantly to the global ozone depletion. Laboratory studies have shown that the iodine atoms of fluorine and participate in analogous catalytic cycles. However, in Earth's stratosphere, fluorine atoms react rapidly with water and methane to form strongly bound HF, while organic molecules which contain iodine react so rapidly in the lower atmosphere does not reach the stratosphere in large quantities. In addition, a single chlorine atom is capable of reacting with 100,000 ozone molecules. This fact, plus the amount of chlorine in the atmosphere by chlorofluorocarbons (CFCs) shows the annual CFC danger to the environment.
quantitative understanding of the processes of chemical ozone loss
In 2007, research on the breakdown of a key molecule in these chemicals that deplete the ozone layer by dichlorethyl.