11th Class Chemistry Environmental Chemistry

  • question_answer 53)   Sometime ago formation of polar stratospheric clouds was reported over Antarctica. Why were these formed? What happens when such clouds break up by warmth of sunlight?


      In Antarctica, the climatic conditions are quite different. In winters, there are special types of clouds called Polar stratospheric clouds. No ozone is generated during the long dark Antarctic winter. Meanwhile a heterogeneous reaction occurring on clouds of ice crystals at -\[85{}^\circ C\]produces species such as \[C{{l}_{2}}\]and\[HOCl\]. \[ClO+N{{O}_{2}}\to ClON{{O}_{2}}\] \[Cl+C{{H}_{4}}\to \overset{\centerdot }{\mathop{C}}\,{{H}_{3}}+HCl\] \[ClON{{O}_{2}}+{{H}_{2}}O\to HOCl+HN{{O}_{3}}\] \[ClON{{O}_{2}}+HCl\to C{{l}_{2}}+HN{{O}_{3}}\] When the sun reappears in September, these molecules decompose photo chemically to form Cl atoms. \[HOCl\xrightarrow{hv}{}^{\bullet }OH+C{{l}^{\bullet }}\] \[C{{l}_{2}}\xrightarrow{hv}2C{{l}^{\bullet }}\] The reactive chlorine atoms are thus rendered free to deplete ozone. Due to polar stratospheric clouds, a tight whirlpool of wind is formed in the stratosphere which surrounds Antarctica. It is called polar vortex. It is so rigid that it cuts-off Antarctica from the surrounding ozone rich air of the non-polar regions. As a result, the ozone hole remains unfilled. After the spring, when the intensity of sunlight increases and the vortex breaks down. The ozone rich air from surroundings immediately rushes to fill up the ozone hole.

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