As every year, September 16 was celebrated as the World Ozone Day as declared by United Nations General Assembly. The stratospheric ozone layer absorbs a portion of the radiation (UVB) from the sun, preventing it from reaching the surface of the earth. The UVB is very harmful which can cause skin cancer, cataracts, genetic disorder and crop damage etc.
Experiments on fish suggest that 90 to 95 percent of malignant melanomas (a form of skin cancer) may be due to UVA and UVB radiations. Plants are sensitive to UV radiation below 300 nm.
Ozone depletion may also affect the climate though not clearly understood. With its depletion, spectrum of incoming radiation will change and hence may produce new molecular species which may change the radiation emitting and absorbing properties of atmosphere. This may cause formation of more tropospheric ozone.
In the absence of ozone layer the radiations will directly flow to the earth surface and hence change the vertical distribution of molecules in the troposphere and stratosphere which in turn change the rate of vertical circulation.
Extensive use of chemicals like chlorofluorocarbons (CFCs), carbon tetrachloride, halons, methyl bromide etc. are responsible for depletion of ozone layer. These chemicals have atmospheric lifetimes long enough to allow them to be transported by wind to the stratosphere which catalytically destroy ozone.
Exposure to strong UV radiation breaks these molecules to release chlorine or bromine in atomic state. One chlorine atom can destroy over 100,000 ozone molecules. Atmospheric models indicate that the chlorine cycle plus the analogous bromine cycle account for more than 70 per cent of the ozone loss in the stratosphere. The remainder of the catalytic cycle for chlorine and ozone depends on OH radicals that exist throughout the atmosphere. OH reacts with ozone to form the intermediate HO2. More depletion means less protection and hence more UVB reaches the earth.
One percent loss of ozone could result in about a million extra human skin cancers per year worldwide if no protective measures are taken. Ozone reforms naturally, but not nearly as fast as it is destroyed.The hole in the layer over the Antarctic was discovered in the 1980s. It regularly tends to form in August, reaching a maximum size late September or early October before it fills again in December.
The size is dependent on weather conditions. This year, the hole began forming earlier than before (World Meteorological Organization).
Antarctica’s exceptionally cold winter temperatures (-850 to -900C) help break down ozone. Almost every year since it was discovered, the Antarctic ozone hole has grown. In 2000 the region of ozone depletion covered 29.8 million km2 (about the size of North America).
This phenomenon is now spreading to other parts of the world. Ozone depletion has been observed also over the North Pole.
In 1985, nations around the world in the Vienna Convention for the ozone layer developed a framework for co-operative activities to save the ozone layer.
As an international response in 1987 an international meeting in Montreal, Canada, produced the Montreal Protocol on phasing out most use of CFCs by 2000. CFC production in most developed countries has fallen sharply since 1989. The Montreal Protocol has been successful in reducing ozone depleting substances and reactive chlorine and bromine in the stratosphere. CFCs are being substituted by HCFCs (hydro chlorofluorocarbons) which release much less chlorine per molecule.
The 2019 ozone hole is indeed the smallest since recording of its size began in 1982 but the ozone is also influenced by temperature shifts and dynamics in the atmosphere through climate change. In 50 years or so, stratospheric ozone levels are expected to be back to normal. However the final recovery is expected to require several lifetimes.
(Dr Senapati is Dean Computer Application & Science, Biju Patnaik University of Technology and Professor of Chemistry, Trident Academy of Technology Bhubaneswar)