Stratospheric Ozone Depletion and Antarctic Ozone Hole
Stratospheric ozone plays a very significant role in the radiation balance of the Earth-atmosphere system and also protects life on the Earth’s surface from harmful UV radiation. Changes in stratospheric ozone levels can affect human health and ecosystem as well as the chemistry of the troposphere.
In Chapter 5, we have seen that the atmospheric ozone can be destroyed by a number of free radical catalysts, the most important of which are the hydroxyl radical (OH), the nitric oxide radical (NO), and atomic chlorine (Cl) and bromine (Br). All these have both natural and anthropogenic (man-made) sources. At the present time, most of the OH and NO in the stratosphere is of natural origin, but human activity has dramatically increased the high concentration of carbon dioxide, chlorine, and bromine. These elements are found in certain stable organic compounds, especially chlorofluorocarbons (CFCs), which may find their low reactivity. Once the Cl and Br atoms are liberated from the parent compounds by the action of UV light, it remains in the stratosphere for a longer period and goes on destroying ozone in this region.
A single chlorine atom would keep on destroying ozone for up to 2 years, the timescale required to transport back down to the troposphere, were it not for reactions that remove them from this cycle by forming reservoir species such as hydrogen chloride (HCL) and chlorine nitrate (ClONO2). On a per atom basis, bromine is even more efficient than chlorine at destroying ozone, but there is much less bromine present in the atmosphere. As a result, both chlorine and bromine contribute significantly to the overall ozone depletion. Laboratory studies have shown that fluorine and iodine atoms participate in analogous catalytic cycles. However, in the 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 that they do not reach the stratosphere in significant quantities.
KeywordsOzone Depletion Total Ozone Stratospheric Ozone Polar Vortex Lower Stratosphere
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