Why is the ozone hole concentrated over Antarctica?
ANANTHU KRISHNA, Kochi, Kerala
Ozone is a colourless gas. Chemically, it is very active and reacts readily with a number of substances. These reactions cause rubber to crack, hurt plant life, and damage people’s lung tissues. But ozone also absorbs harmful components of sunlight, “ultraviolet B”, or “UV-B, protecting living things below.
Ozone can be destroyed by a number of free radical catalysts, the most important of which are the hydroxyl radical (OH), nitric oxide radical (NO), chlorine atom (Cl) and bromine atom (Br). Human activity has dramatically increased the levels of chlorine and bromine in the atmosphere.
Each year for the past few decades during the Southern Hemisphere spring, chemical reactions involving chlorine and bromine cause ozone in the southern polar region to be destroyed rapidly and severely. This depleted region is known as the “ozone hole”. British scientists discovered this hole in 1985.
The hole in Antarctica occurs in the spring (September to December). It begins with this overall ozone thinning, but it is assisted by the presence of polar stratospheric clouds (PS clouds). During the extreme cold of winter, with no sun for six months, polar winds create a vortex which traps and chills the air; the temperature is below -80 Celsius. The ice in these PS clouds provides surfaces for the chemical reactions that destroy the ozone. This needs light to kick-start the reactions. By the end of spring warmer December temperatures break up the vortex and destroy the PS clouds. Sunlight starts creating ozone again and the hole begins to repair.
Every March to April during the Northern Hemisphere springtime similar, but less pronounced ozone hole forms above the Arctic. The natural circulation of wind, the polar vortex, is much less developed in the Northern Hemisphere above the Arctic.
S.P.S. JAIN, Former Member, Engineering, Indian Railways Greater Noida, Uttar Pradesh
